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Linux Device Drivers, 2nd Edition

Linux Device Drivers, 2nd Edition

By Alessandro Rubini & Jonathan Corbet
2nd Edition June 2001
0-59600-008-1, Order Number: 0081
586 pages, $39.95

Chapter 15
Overview of Peripheral Buses

Contents:

The PCI Interface
A Look Back: ISA
PC/104 and PC/104+
Other PC Buses
SBus
NuBus
External Buses
Backward Compatibility
Quick Reference

Whereas Chapter 8, "Hardware Management" introduced the lowest levels of hardware control, this chapter provides an overview of the higher-level bus architectures. A bus is made up of both an electrical interface and a programming interface. In this chapter, we deal with the programming interface.

This chapter covers a number of bus architectures. However, the primary focus is on the kernel functions that access PCI peripherals, because these days the PCI bus is the most commonly used peripheral bus on desktops and bigger computers, and the one that is best supported by the kernel. ISA is still common for electronic hobbyists and is described later, although it is pretty much a bare-metal kind of bus and there isn't much to say in addition to what is covered in Chapter 8, "Hardware Management" and Chapter 9, "Interrupt Handling".

The PCI Interface

Although many computer users think of PCI (Peripheral Component Interconnect) as a way of laying out electrical wires, it is actually a complete set of specifications defining how different parts of a computer should interact.

The PCI specification covers most issues related to computer interfaces. We are not going to cover it all here; in this section we are mainly concerned with how a PCI driver can find its hardware and gain access to it. The probing techniques discussed in "Automatic and Manual Configuration" in Chapter 2, "Building and Running Modules", and "Autodetecting the IRQ Number" in Chapter 9, "Interrupt Handling" can be used with PCI devices, but the specification offers a preferable alternative to probing.

The PCI architecture was designed as a replacement for the ISA standard, with three main goals: to get better performance when transferring data between the computer and its peripherals, to be as platform independent as possible, and to simplify adding and removing peripherals to the system.

The PCI bus achieves better performance by using a higher clock rate than ISA; its clock runs at 25 or 33 MHz (its actual rate being a factor of the system clock), and 66-MHz and even 133-MHz implementations have recently been deployed as well. Moreover, it is equipped with a 32-bit data bus, and a 64-bit extension has been included in the specification (although only 64-bit platforms implement it). Platform independence is often a goal in the design of a computer bus, and it's an especially important feature of PCI because the PC world has always been dominated by processor-specific interface standards. PCI is currently used extensively on IA-32, Alpha, PowerPC, SPARC64, and IA-64 systems, and some other platforms as well.

What is most relevant to the driver writer, however, is the support for autodetection of interface boards. PCI devices are jumperless (unlike most older peripherals) and are automatically configured at boot time. The device driver, then, must be able to access configuration information in the device in order to complete initialization. This happens without the need to perform any probing.

PCI Addressing

Each PCI peripheral is identified by a busnumber, a device number, and a function number. The PCI specification permits a system to host up to 256 buses. Each bus hosts up to 32 devices, and each device can be a multifunction board (such as an audio device with an accompanying CD-ROM drive) with a maximum of eight functions. Each function can thus be identified at hardware level by a 16-bit address, or key. Device drivers written for Linux, though, don't need to deal with those binary addresses as they use a specific data structure, called pci_dev, to act on the devices. (We have already seen struct pci_dev, of course, in Chapter 13, "mmap and DMA".)

Most recent workstations feature at least two PCI buses. Plugging more than one bus in a single system is accomplished by means of bridges, special-purpose PCI peripherals whose task is joining two buses. The overall layout of a PCI system is organized as a tree, where each bus is connected to an upper-layer bus up to bus 0. The CardBus PC-card system is also connected to the PCI system via bridges. A typical PCI system is represented in Figure 15-1, where the various bridges are highlighted.

Figure 15-1

Figure 15-1. Layout of a Typical PCI System

The 16-bit hardware addresses associated with PCI peripherals, although mostly hidden in the struct pci_dev object, are still visible occasionally, especially when lists of devices are being used. One such situation is the output of lspci (part of the pciutils package, available with most distributions) and the layout of information in /proc/pci and /proc/bus/pci.[55] When the hardware address is displayed, it can either be shown as a 16-bit value, as two values (an 8-bit bus number and an 8-bit device and function number), or as three values (bus, device, and function); all the values are usually displayed in hexadecimal.

[55]Please note that the discussion, as usual, is based on the 2.4 version of the kernel, relegating backward compatibility issues to the end of the chapter.

For example, /proc/bus/pci/devices uses a single 16-bit field (to ease parsing and sorting), while /proc/bus/busnumbersplits the address into three fields. The following shows how those addresses appear, showing only the beginning of the output lines:

rudo% lspci | cut -d: -f1-2
00:00.0 Host bridge
00:01.0 PCI bridge
00:07.0 ISA bridge
00:07.1 IDE interface
00:07.3 Bridge
00:07.4 USB Controller
00:09.0 SCSI storage controller
00:0b.0 Multimedia video controller
01:05.0 VGA compatible controller
rudo% cat /proc/bus/pci/devices | cut -d\        -f1,3
0000    0
0008    0
0038    0
0039    0
003b    0
003c    b
0048    a
0058    b
0128    a

The two lists of devices are sorted in the same order, since lspci uses the /procfiles as its source of information. Taking the VGA video controller as an example, 0x128 means 01:05.0 when split into bus (eight bits), device (five bits) and function (three bits). The second field in the two listings shown shows the class of device and the interrupt number, respectively.

The hardware circuitry of each peripheral board answers queries pertaining to three address spaces: memory locations, I/O ports, and configuration registers. The first two address spaces are shared by all the devices on a PCI bus (i.e., when you access a memory location, all the devices see the bus cycle at the same time). The configuration space, on the other hand, exploits geographical addressing. Configuration transactions (i.e., bus accesses that insist on the configuration space) address only one slot at a time. Thus, there are no collisions at all with configuration access.

As far as the driver is concerned, memory and I/O regions are accessed in the usual ways via inb, readb, and so forth. Configuration transactions, on the other hand, are performed by calling specific kernel functions to access configuration registers. With regard to interrupts, every PCI slot has four interrupt pins, and each device function can use one of them without being concerned about how those pins are routed to the CPU. Such routing is the responsibility of the computer platform and is implemented outside of the PCI bus. Since the PCI specification requires interrupt lines to be shareable, even a processor with a limited number of IRQ lines, like the x86, can host many PCI interface boards (each with four interrupt pins).

The I/O space in a PCI bus uses a 32-bit address bus (leading to 4 GB of I/O ports), while the memory space can be accessed with either 32-bit or 64-bit addresses. However, 64-bit addresses are available only on a few platforms. Addresses are supposed to be unique to one device, but software may erroneously configure two devices to the same address, making it impossible to access either one; the problem never occurs unless a driver is willingly playing with registers it shouldn't touch. The good news is that every memory and I/O address region offered by the interface board can be remapped by means of configuration transactions. That is, the firmware initializes PCI hardware at system boot, mapping each region to a different address to avoid collisions.[56] The addresses to which these regions are currently mapped can be read from the configuration space, so the Linux driver can access its devices without probing. After reading the configuration registers the driver can safely access its hardware.

[56]Actually, that configuration is not restricted to the time the system boots; hot-pluggable devices, for example, cannot be available at boot time and appear later instead. The main point here is that the device driver need not change the address of I/O or memory regions.

The PCI configuration space consists of 256 bytes for each device function, and the layout of the configuration registers is standardized. Four bytes of the configuration space hold a unique function ID, so the driver can identify its device by looking for the specific ID for that peripheral.[57] In summary, each device board is geographically addressed to retrieve its configuration registers; the information in those registers can then be used to perform normal I/O access, without the need for further geographic addressing.

[57]You'll find the ID of any device in its own hardware manual. A list is included in the file pci.ids, part of the pciutils package and of the kernel sources; it doesn't pretend to be complete, but just lists the most renowned vendors and devices.

It should be clear from this description that the main innovation of the PCI interface standard over ISA is the configuration address space. Therefore, in addition to the usual driver code, a PCI driver needs the ability to access configuration space, in order to save itself from risky probing tasks.

For the remainder of this chapter, we'll use the word device to refer to a device function, because each function in a multifunction board acts as an independent entity. When we refer to a device, we mean the tuple "bus number, device number, function number,'' which can be represented by a 16-bit number or two 8-bit numbers (usually called bus and devfn).

Boot Time

To see how PCI works, we'll start from system boot, since that's when the devices are configured.

When power is applied to a PCI device, the hardware remains inactive. In other words, the device will respond only to configuration transactions. At power on, the device has no memory and no I/O ports mapped in the computer's address space; every other device-specific feature, such as interrupt reporting, is disabled as well.

Fortunately, every PCI motherboard is equipped with PCI-aware firmware, called the BIOS, NVRAM, or PROM, depending on the platform. The firmware offers access to the device configuration address space by reading and writing registers in the PCI controller.

At system boot, the firmware (or the Linux kernel, if so configured) performs configuration transactions with every PCI peripheral in order to allocate a safe place for any address region it offers. By the time a device driver accesses the device, its memory and I/O regions have already been mapped into the processor's address space. The driver can change this default assignment, but it will never need to do that.

As suggested, the user can look at the PCI device list and the devices' configuration registers by reading /proc/bus/pci/devices and /proc/bus/pci/*/*. The former is a text file with (hexadecimal) device information, and the latter are binary files that report a snapshot of the configuration registers of each device, one file per device.

Configuration Registers and Initialization

As mentioned earlier, the layout of the configuration space is device independent. In this section, we look at the configuration registers that are used to identify the peripherals.

PCI devices feature a 256-byte address space. The first 64 bytes are standardized, while the rest are device dependent. Figure 15-2 shows the layout of the device-independent configuration space.

Figure 15-2

Figure 15-2. The standardized PCI configuration registers

As the figure shows, some of the PCI configuration registers are required and some are optional. Every PCI device must contain meaningful values in the required registers, whereas the contents of the optional registers depend on the actual capabilities of the peripheral. The optional fields are not used unless the contents of the required fields indicate that they are valid. Thus, the required fields assert the board's capabilities, including whether the other fields are usable or not.

It's interesting to note that the PCI registers are always little-endian. Although the standard is designed to be architecture independent, the PCI designers sometimes show a slight bias toward the PC environment. The driver writer should be careful about byte ordering when accessing multibyte configuration registers; code that works on the PC might not work on other platforms. The Linux developers have taken care of the byte-ordering problem (see the next section, "Accessing the Configuration Space"), but the issue must be kept in mind. If you ever need to convert data from host order to PCI order or vice versa, you can resort to the functions defined in <asm/byteorder.h>, introduced in Chapter 10, "Judicious Use of Data Types", knowing that PCI byte order is little-endian.

Describing all the configuration items is beyond the scope of this book. Usually, the technical documentation released with each device describes the supported registers. What we're interested in is how a driver can look for its device and how it can access the device's configuration space.

Three or five PCI registers identify a device: vendorID, deviceID, and class are the three that are always used. Every PCI manufacturer assigns proper values to these read-only registers, and the driver can use them to look for the device. Additionally, the fields subsystem vendorID and subsystem deviceID are sometimes set by the vendor to further differentiate similar devices.

Let's look at these registers in more detail.

vendorID

This 16-bit register identifies a hardware manufacturer. For instance, every Intel device is marked with the same vendor number, 0x8086. There is a global registry of such numbers, maintained by the PCI Special Interest Group, and manufacturers must apply to have a unique number assigned to them.

deviceID

This is another 16-bit register, selected by the manufacturer; no official registration is required for the device ID. This ID is usually paired with the vendor ID to make a unique 32-bit identifier for a hardware device. We'll use the word signature to refer to the vendor and device ID pair. A device driver usually relies on the signature to identify its device; you can find what value to look for in the hardware manual for the target device.

class

Every peripheral device belongs to a class. The class register is a 16-bit value whose top 8 bits identify the "base class'' (or group). For example, "ethernet'' and "token ring'' are two classes belonging to the "network'' group, while the "serial'' and "parallel'' classes belong to the "communication'' group. Some drivers can support several similar devices, each of them featuring a different signature but all belonging to the same class; these drivers can rely on the class register to identify their peripherals, as shown later.

subsystem vendorID
subsystem deviceID

These fields can be used for further identification of a device. If the chip in itself is a generic interface chip to a local (onboard) bus, it is often used in several completely different roles, and the driver must identify the actual device it is talking with. The subsystem identifiers are used to this aim.

Using those identifiers, you can detect and get hold of your device. With version 2.4 of the kernel, the concept of a PCI driver and a specialized initialization interface have been introduced. While that interface is the preferred one for new drivers, it is not available for older kernel versions. As an alternative to the PCI driver interface, the following headers, macros, and functions can be used by a PCI module to look for its hardware device. We chose to introduce this backward-compatible interface first because it is portable to all kernel versions we cover in this book. Moreover, it is somewhat more immediate by virtue of being less abstracted from direct hardware management.

#include <linux/config.h>

The driver needs to know if the PCI functions are available in the kernel. By including this header, the driver gains access to the CONFIG_ macros, including CONFIG_PCI, described next. But note that every source file that includes <linux/module.h> already includes this one as well.

CONFIG_PCI

This macro is defined if the kernel includes support for PCI calls. Not every computer includes a PCI bus, so the kernel developers chose to make PCI support a compile-time option to save memory when running Linux on non-PCI computers. If CONFIG_PCI is not enabled, every PCI function call is defined to return a failure status, so the driver may or may not use a preprocessor conditional to mask out PCI support. If the driver can only handle PCI devices (as opposed to both PCI and non-PCI device implementations), it should issue a compile-time error if the macro is undefined.

#include <linux/pci.h>

This header declares all the prototypes introduced in this section, as well as the symbolic names associated with PCI registers and bits; it should always be included. This header also defines symbolic values for the error codes returned by the functions.

int pci_present(void);

Because the PCI-related functions don't make sense on non-PCI computers, the pci_present function allows one to check if PCI functionality is available or not. The call is discouraged as of 2.4, because it now just checks if some PCI device is there. With 2.0, however, a driver had to call the function to avoid unpleasant errors when looking for its device. Recent kernels just report that no device is there, instead. The function returns a boolean value of true (nonzero) if the host is PCI aware.

struct pci_dev;

The data structure is used as a software object representing a PCI device. It is at the core of every PCI operation in the system.

struct pci_dev *pci_find_device (unsigned int vendor, unsigned int device, const struct pci_dev *from);

If CONFIG_PCI is defined and pci_present is true, this function is used to scan the list of installed devices looking for a device featuring a specific signature. The from argument is used to get hold of multiple devices with the same signature; the argument should point to the last device that has been found, so that the search can continue instead of restarting from the head of the list. To find the first device, from is specified as NULL. If no (further) device is found, NULL is returned.

struct pci_dev *pci_find_class (unsigned int class, const struct pci_dev *from);

This function is similar to the previous one, but it looks for devices belonging to a specific class (a 16-bit class: both the base class and subclass). It is rarely used nowadays except in very low-level PCI drivers. The from argument is used exactly like in pci_find_device.

int pci_enable_device(struct pci_dev *dev);

This function actually enables the device. It wakes up the device and in some cases also assigns its interrupt line and I/O regions. This happens, for example, with CardBus devices (which have been made completely equivalent to PCI at driver level).

struct pci_dev *pci_find_slot (unsigned int bus, unsigned int devfn);

This function returns a PCI device structure based on a bus/device pair. The devfn argument represents both the device and functionitems. Its use is extremely rare (drivers should not care about which slot their device is plugged into); it is listed here just for completeness.

Based on this information, initialization for a typical device driver that handles a single device type will look like the following code. The code is for a hypothetical device jail and is Just Another Instruction List:

#ifndef CONFIG_PCI
#  error "This driver needs PCI support to be available"
#endif

int jail_find_all_devices(void)
{
    struct pci_dev *dev = NULL;
    int found;

    if (!pci_present())
    return -ENODEV;

    for (found=0; found < JAIL_MAX_DEV;) {
        dev = pci_find_device(JAIL_VENDOR, JAIL_ID, dev);
        if (!dev) /* no more devices are there */
            break;
        /* do device-specific actions and count the device */
        found += jail_init_one(dev);
    }
    return  (index == 0) ? -ENODEV : 0;
}

The role of jail_init_one is very device specific and thus not shown here. There are, nonetheless, a few things to keep in mind when writing that function:

The function shown in the previous code excerpt returns 0 if it rejects the device and 1 if it accepts it (possibly based on the further probing just described).

The code excerpt shown is correct if the driver deals with only one kind of PCI device, identified by JAIL_VENDOR and JAIL_ID. If you need to support more vendor/device pairs, your best bet is using the technique introduced later in "Hardware Abstractions", unless you need to support older kernels than 2.4, in which case pci_find_class is your friend.

Using pci_find_class requires that jail_find_all_devices perform a little more work than in the example. The function should check the newly found device against a list of vendor/device pairs, possibly using dev->vendor and dev->device. Its core should look like this:

struct devid {unsigned short vendor, device} devlist[] = {
    {JAIL_VENDOR1, JAIL_DEVICE1},
    {JAIL_VENDOR2, JAIL_DEVICE2},
    /* ... */
    { 0, 0 }
};

    /* ... */

    for (found=0; found < JAIL_MAX_DEV;) {
        struct devid *idptr;
        dev = pci_find_class(JAIL_CLASS, dev);
        if (!dev) /* no more devices are there */
            break;
        for (idptr = devlist; idptr->vendor; idptr++) {
           if (dev->vendor != idptr->vendor) continue;
           if (dev->device != idptr->device) continue;
           break;
        }
	if (!idptr->vendor) continue; /* not one of ours */
        jail_init_one(dev); /* device-specific initialization */
	found++;
    }

Accessing the Configuration Space

After the driver has detected the device, it usually needs to read from or write to the three address spaces: memory, port, and configuration. In particular, accessing the configuration space is vital to the driver because it is the only way it can find out where the device is mapped in memory and in the I/O space.

Because the microprocessor has no way to access the configuration space directly, the computer vendor has to provide a way to do it. To access configuration space, the CPU must write and read registers in the PCI controller, but the exact implementation is vendor dependent and not relevant to this discussion because Linux offers a standard interface to access the configuration space.

As far as the driver is concerned, the configuration space can be accessed through 8-bit, 16-bit, or 32-bit data transfers. The relevant functions are prototyped in <linux/pci.h>:

int pci_read_config_byte(struct pci_dev *dev, int where, u8 *ptr);
int pci_read_config_word(struct pci_dev *dev, int where, u16 *ptr);
int pci_read_config_dword(struct pci_dev *dev, int where, u32 *ptr);

Read one, two, or four bytes from the configuration space of the device identified by dev. The where argument is the byte offset from the beginning of the configuration space. The value fetched from the configuration space is returned through ptr, and the return value of the functions is an error code. The word and dword functions convert the value just read from little-endian to the native byte order of the processor, so you need not deal with byte ordering.

int pci_write_config_byte (struct pci_dev *dev, int where, u8 val);
int pci_write_config_word (struct pci_dev *dev, int where, u16 val);
int pci_write_config_dword (struct pci_dev *dev, int where, u32 val);

Write one, two, or four bytes to the configuration space. The device is identified by dev as usual, and the value being written is passed as val. The word and dword functions convert the value to little-endian before writing to the peripheral device.

The preferred way to read the configuration variables you need is using the fields of the struct pci_dev that refers to your device. Nonetheless, you'll need the functions just listed if you need to write and read back a configuration variable. Also, you'll need the pci_read_ functions if you want to keep backward compatibility with kernels older than 2.4.[58]

[58]The field names in struct pci_dev changed from version 2.2 and 2.4 because the first layout proved suboptimal. As for 2.0, there was no pci_dev structure, and the one you use is a light emulation offered by the pci-compat.hheader.

The best way to address the configuration variables using the pci_read_ functions is by means of the symbolic names defined in <linux/pci.h>. For example, the following two-line function retrieves the revision ID of a device by passing the symbolic name for where to pci_read_config_byte:

unsigned char jail_get_revision(unsigned char bus, unsigned char fn)
{
    unsigned char *revision;

    pci_read_config_byte(bus, fn, PCI_REVISION_ID, &revision);
    return revision;
}

As suggested, when accessing multibyte values as single bytes the programmer must remember to watch out for byte-order problems.

Looking at a configuration snapshot

If you want to browse the configuration space of the PCI devices on your system, you can proceed in one of two ways. The easier path is using the resources that Linux already offers via /proc/bus/pci, although these were not available in version 2.0 of the kernel. The alternative that we follow here is, instead, writing some code of our own to perform the task; such code is both portable across all known 2.x kernel releases and a good way to look at the tools in action. The source file pci/pcidata.c is included in the sample code provided on the O'Reilly FTP site.

This module creates a dynamic /proc/pcidata file that contains a binary snapshot of the configuration space for your PCI devices. The snapshot is updated every time the file is read. The size of /proc/pcidata is limited to PAGE_SIZE bytes (to avoid dealing with multipage /proc files, as introduced in "Using the /proc Filesystem" in Chapter 4, "Debugging Techniques"). Thus, it lists only the configuration memory for the first PAGE_SIZE/256 devices, which means 16 or 32 devices according to the platform you are running on. We chose to make /proc/pcidata a binary file to keep the code simple, instead of making it a text file like most /proc files. Note that the files in /proc/bus/pci are binary as well.

Another limitation of pcidata is that it scans only the first PCI bus on the system. If your computer includes bridges to other PCI buses, pcidata ignores them. This should not be an issue for sample code not meant to be of real use.

Devices appear in /proc/pcidata in the same order used by /proc/bus/pci/devices (but in the opposite order from the one used by /proc/pci in version 2.0).

For example, our frame grabber appears fifth in /proc/pcidata and (currently) has the following configuration registers:

morgana% dd bs=256 skip=4 count=1 if=/proc/pcidata | od -Ax -t x1
1+0 records in
1+0 records out
000000 86 80 23 12 06 00 00 02 00 00 00 04 00 20 00 00
000010 00 00 00 f1 00 00 00 00 00 00 00 00 00 00 00 00
000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000030 00 00 00 00 00 00 00 00 00 00 00 00 0a 01 00 00
000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
000100

The numbers in this dump represent the PCI registers. Using Figure 15-2 as a reference, you can look at the meaning of the numbers shown. Alternatively, you can use the pcidump program, also found on the FTP site, which formats and labels the output listing.

The pcidump code is not worth including here because the program is simply a long table, plus 10 lines of code that scan the table. Instead, let's look at some selected output lines:

morgana% dd bs=256 skip=4 count=1 if=/proc/pcidata | ./pcidump
1+0 records in
1+0 records out
        Compulsory registers:
Vendor id: 8086
Device id: 1223
I/O space enabled: n
Memory enabled: y
Master enabled: y
Revision id (decimal): 0
Programmer Interface: 00
Class of device: 0400
Header type: 00
Multi function device: n
        Optional registers:
Base Address 0: f1000000
Base Address 0 Is I/O: n
Base Address 0 is 64-bits: n
Base Address 0 is below-1M: n
Base Address 0 is prefetchable: n
Does generate interrupts: y
Interrupt line (decimal): 10
Interrupt pin (decimal): 1

pcidata and pcidump, used with grep, can be useful tools for debugging a driver's initialization code, even though their task is in part already available in the pciutils package, included in all recent Linux distributions. Note that, unlike other sample code accompanying the book, the pcidata.cmodule is subject to the GPL because we took the PCI scanning loop from the kernel sources. This shouldn't matter to you as a driver writer, because we've included the module in the source files only as a support utility, not as a template to be reused in new drivers.

Accessing the I/O and Memory Spaces

A PCI device implements up to six I/O address regions. Each region consists of either memory or I/O locations. Most devices implement their I/O registers in memory regions, because it's generally a saner approach (as explained in "I/O Ports and I/O Memory", in Chapter 8, "Hardware Management"). However, unlike normal memory, I/O registers should not be cached by the CPU because each access can have side effects. The PCI device that implements I/O registers as a memory region marks the difference by setting a "memory-is-prefetchable'' bit in its configuration register.[59] If the memory region is marked as prefetchable, the CPU can cache its contents and do all sorts of optimization with it; nonprefetchable memory access, on the other hand, can't be optimized because each access can have side effects, exactly like I/O ports usually have. Peripherals that map their control registers to a memory address range declare that range as nonprefetchable, whereas something like video memory on PCI boards is prefetchable. In this section, we use the word region to refer to a generic I/O address space, either memory-mapped or port-mapped.

[59]The information lives in one of the low-order bits of the base address PCI registers. The bits are defined in <linux/pci.h>.

An interface board reports the size and current location of its regions using configuration registers -- the six 32-bit registers shown in Figure 15-2, whose symbolic names are PCI_BASE_ADDRESS_0 through PCI_BASE_ADDRESS_5. Since the I/O space defined by PCI is a 32-bit address space, it makes sense to use the same configuration interface for memory and I/O. If the device uses a 64-bit address bus, it can declare regions in the 64-bit memory space by using two consecutive PCI_BASE_ADDRESS registers for each region, low bits first. It is possible for one device to offer both 32-bit regions and 64-bit regions.

PCI I/O resources in Linux 2.4

In Linux 2.4, the I/O regions of PCI devices have been integrated in the generic resource management. For this reason, you don't need to access the configuration variables in order to know where your device is mapped in memory or I/O space. The preferred interface for getting region information consists of the following functions:

unsigned long pci_resource_start(struct pci_dev *dev, int bar);

The function returns the first address (memory address or I/O port number) associated with one of the six PCI I/O regions. The region is selected by the integer bar (the base address register), ranging from 0 to 5, inclusive.

unsigned long pci_resource_end(struct pci_dev *dev, int bar);

The function returns the last address that is part of the I/O region number bar. Note that this is the last usable address, not the first address after the region.

unsigned long pci_resource_flags(struct pci_dev *dev, int bar);

This function returns the flags associated with this resource.

Resource flags are used to define some features of the individual resource. For PCI resources associated with PCI I/O regions, the information is extracted from the base address registers, but can come from elsewhere for resources not associated with PCI devices.

All resource flags are defined in <linux/ioport.h>; the most important of them are listed here.

IORESOURCE_IO
IORESOURCE_MEM

If the associated I/O region exists, one and only one of these flags is set.

IORESOURCE_PREFETCH
IORESOURCE_READONLY

The flags tell whether a memory region is prefetchable and/or write protected. The latter flag is never set for PCI resources.

By making use of the pci_resource_ functions, a device driver can completely ignore the underlying PCI registers, since the system already used them to structure resource information.

Peeking at the base address registers

By avoiding direct access to the PCI registers, you gain a better hardware abstraction and forward portability but can get no backward portability. If you want your device driver to work with Linux versions older than 2.4, you can't use the beautiful resource interface and must access the PCI registers directly.

In this section we look at how base address registers behave and how they can be accessed. All of this is obviously superfluous if you can exploit resource management as shown previously.

We won't go into much detail here about the base address registers, because if you're going to write a PCI driver, you will need the hardware manual for the device anyway. In particular, we are not going to use either the prefetchable bit or the two "type'' bits of the registers, and we'll limit the discussion to 32-bit peripherals. It's nonetheless interesting to see how things are usually implemented and how Linux drivers deal with PCI memory.

The PCI specs state that manufacturers must map each valid region to a configurable address. This means that the device must be equipped with a programmable 32-bit address decoder for each region it implements, and a 64-bit programmable decoder must be present in any board that exploits the 64-bit PCI extension.

The actual implementation and use of a programmable decoder is simplified by the fact that usually the number of bytes in a region is a power of two, for example, 32 bytes, 4 KB, or 2 MB. Moreover, it wouldn't make much sense to map a region to an unaligned address; 1 MB regions naturally align at an address that is a multiple of 1 MB, and 32-byte regions at a multiple of 32. The PCI specification exploits this alignment; it states that the address decoder must look only at the high bits of the address bus and that only the high bits are programmable. This convention also means that the size of any region must be a power of two.

Mapping a PCI region in the physical address space is thus performed by setting a suitable value in the high bits of a configuration register. For example, a 1-MB region, which has 20 bits of address space, is remapped by setting the high 12 bits of the register; thus, to make the board respond to the 64-MB to 65-MB address range, you can write to the register any address in the 0x040xxxxx range. In practice, only very high addresses are used to map PCI regions.

This "partial decoding'' technique has the additional advantage that the software can determine the size of a PCI region by checking the number of nonprogrammable bits in the configuration register. To this end, the PCI standard states that unused bits must always read as 0. By imposing a minimum size of 8 bytes for I/O regions and 16 bytes for memory regions, the standard can fit some extra information into the low bits of the base address registers:

It's apparent from whence information for the resource flags comes.

Detecting the size of a PCI region is simplified by using several bit masks defined in <linux/pci.h>: the PCI_BASE_ADDRESS_SPACE bit mask is set to PCI_BASE_ADDRESS_SPACE_MEMORY if this is a memory region, and to PCI_BASE_ADDRESS_SPACE_IO if it is an I/O region. To know the actual address where a memory region is mapped, you can AND the PCI register with PCI_BASE_ADDRESS_MEM_MASK to discard the low bits listed earlier. Use PCI_BASE_ADDRESS_IO_MASK for I/O regions. Please note that PCI regions may be allocated in any order by device manufacturers; it's not uncommon to find devices that use the first and third regions, leaving the second unused.

Typical code for reporting the current location and size of the PCI regions looks like the following. This code is part of the pciregions module, distributed in the same directory as pcidata; the module creates a /proc/pciregions file, using the code shown earlier to generate data. The program writes a value of all 1s to the configuration register and reads it back to know how many bits of the registers can be programmed. Note that while the program probes the configuration register, the device is actually remapped to the top of the physical address space, which is why interrupt reporting is disabled during the probe (to prevent a driver from accessing the region while it is mapped to the wrong place).

Despite the PCI specs stating that the I/O address space is 32 bits wide, a few manufacturers, clearly x86 biased, pretend that it is 64 KB and do not implement all 32 bits of the base address register. That's why the following code (and the kernel proper) ignores high bits of the address mask for I/O regions.

 
static u32 addresses[] = {
    PCI_BASE_ADDRESS_0,
    PCI_BASE_ADDRESS_1,
    PCI_BASE_ADDRESS_2,
    PCI_BASE_ADDRESS_3,
    PCI_BASE_ADDRESS_4,
    PCI_BASE_ADDRESS_5,
    0
};

int pciregions_read_proc(char *buf, char **start, off_t offset,
                   int len, int *eof, void *data)
{
    /* this macro helps in keeping the following lines short */
#define PRINTF(fmt, args...) sprintf(buf+len, fmt, ## args)
    len=0;

    /* Loop through the devices (code not printed in the book) */

        /* Print the address regions of this device */
        for (i=0; addresses[i]; i++) {
            u32 curr, mask, size;
            char *type;

            pci_read_config_dword(dev, addresses[i],&curr);
            cli();
            pci_write_config_dword(dev, addresses[i],~0);
            pci_read_config_dword(dev, addresses[i],&mask);
            pci_write_config_dword(dev, addresses[i],curr);
            sti();

            if (!mask)
		continue; /* there may be other regions */

	    /*
	     * apply the I/O or memory mask to current position.
	     * note that I/O is limited to 0xffff, and 64-bit is not
	     * supported by this simple implementation
	     */
	    if (curr & PCI_BASE_ADDRESS_SPACE_IO) {
		curr &= PCI_BASE_ADDRESS_IO_MASK;
	    } else {
		curr &= PCI_BASE_ADDRESS_MEM_MASK;
	    }
		
            len += PRINTF("\tregion %i: mask 0x%08lx, now at 0x%08lx\n",
                        i, (unsigned long)mask,
                           (unsigned long)curr);
            /* extract the type, and the programmable bits */
            if (mask & PCI_BASE_ADDRESS_SPACE_IO) {
                type = "I/O"; mask &= PCI_BASE_ADDRESS_IO_MASK;
		size = (~mask + 1) & 0xffff; /* Bleah */
            } else {
                type = "mem"; mask &= PCI_BASE_ADDRESS_MEM_MASK;
		size = ~mask + 1;
            }
            len += PRINTF("\tregion %i: type %s, size %i (%i%s)\n", i,
                          type, size,
			  (size & 0xfffff) == 0 ? size >> 20 :
			    (size & 0x3ff) == 0 ? size >> 10 : size,
			  (size & 0xfffff) == 0 ? "MB" :
			    (size & 0x3ff) == 0 ? "KB" : "B");
	    if (len > PAGE_SIZE / 2) {
		len += PRINTF("... more info skipped ...\n");
		*eof = 1; return len;
	    }
        }
    return len;
}

Here, for example, is what /proc/pciregionsreports for our frame grabber:

Bus 0, device 13, fun  0 (id 8086-1223)
        region 0: mask 0xfffff000, now at 0xf1000000
        region 0: type mem, size 4096 (4KB)

It's interesting to note that the memory size reported by the program just listed can be overstated. For instance, /proc/pciregions reported that a video device had 16 MB of memory when it actually had only 1. This lie is acceptable because the size information is used only by the firmware to allocate address ranges; region oversizing is not a problem for the driver writer who knows the internals of the device and can correctly deal with the address range assigned by the firmware. In this case, device RAM could be added later without the need to change the behavior of PCI registers while upgrading the RAM.

Such overstating, when present, is reflected in the resource interface, and pci_resource_size will report the overstated size.

PCI Interrupts

As far as interrupts are concerned, PCI is easy to handle. By the time Linux boots, the computer's firmware has already assigned a unique interrupt number to the device, and the driver just needs to use it. The interrupt number is stored in configuration register 60 (PCI_INTERRUPT_LINE), which is one byte wide. This allows for as many as 256 interrupt lines, but the actual limit depends on the CPU being used. The driver doesn't need to bother checking the interrupt number, because the value found in PCI_INTERRUPT_LINE is guaranteed to be the right one.

If the device doesn't support interrupts, register 61 (PCI_INTERRUPT_PIN) is 0; otherwise, it's nonzero. However, since the driver knows if its device is interrupt driven or not, it doesn't usually need to read PCI_INTERRUPT_PIN.

Thus, PCI-specific code for dealing with interrupts just needs to read the configuration byte to obtain the interrupt number that is saved in a local variable, as shown in the following code. Otherwise, the information in Chapter 9, "Interrupt Handling" applies.

result = pci_read_config_byte(dev, PCI_INTERRUPT_LINE, &myirq);
if (result) { /* deal with error */ }

The rest of this section provides additional information for the curious reader, but isn't needed for writing drivers.

A PCI connector has four interrupt pins, and peripheral boards can use any or all of them. Each pin is individually routed to the motherboard's interrupt controller, so interrupts can be shared without any electrical problems. The interrupt controller is then responsible for mapping the interrupt wires (pins) to the processor's hardware; this platform-dependent operation is left to the controller in order to achieve platform independence in the bus itself.

The read-only configuration register located at PCI_INTERRUPT_PIN is used to tell the computer which single pin is actually used. It's worth remembering that each device board can host up to eight devices; each device uses a single interrupt pin and reports it in its own configuration register. Different devices on the same device board can use different interrupt pins or share the same one.

The PCI_INTERRUPT_LINE register, on the other hand, is read/write. When the computer is booted, the firmware scans its PCI devices and sets the register for each device according to how the interrupt pin is routed for its PCI slot. The value is assigned by the firmware because only the firmware knows how the motherboard routes the different interrupt pins to the processor. For the device driver, however, the PCI_INTERRUPT_LINE register is read-only. Interestingly, recent versions of the Linux kernel under some circumstances can assign interrupt lines without resorting to the BIOS.

Handling Hot-Pluggable Devices

During the 2.3 development cycle, the kernel developers overhauled the PCI programming interface in order to simplify things and support hot-pluggable devices, that is, those devices that can be added to or removed from the system while the system runs (such as CardBus devices). The material introduced in this section is not available in 2.2 and earlier kernels, but is the preferred way to go for newer drivers.

The basic idea being exploited is that whenever a new device appears during the system's lifetime, all available device drivers must check whether the new device is theirs or not. Therefore, instead of using the classic init and cleanupentry points for the driver, the hot-plug-aware device driver must register an object with the kernel, and the probefunction for the object will be asked to check any device in the system to take hold of it or leave it alone.

This approach has no downside: the usual case of a static device list is handled by scanning the device list once for each device at system boot; modularized drivers will just unload as usual if no device is there, and an external process devoted to monitoring the bus will arrange for them to be loaded if the need arises. This is exactly how the PCMCIA subsystem has always worked, and having it integrated in the kernel proper allows for more coherent handling of similar issues with different hardware environments.

While you may object that hot-pluggable PCI is not common these days, the new driver-object technique proves very useful even for non-hot-plug drivers that must handle a number of alternative devices. The initialization code is simplified and streamlined because it just needs to check the current device against a list of known devices, instead of actively searching the PCI bus by looping once around pci_find_class or looping several times around pci_find_device.

But let's show some code. The design is built around struct pci_driver, defined in <linux/pci.h> as usual. The structure defines the operations it implements, and also includes a list of devices it supports (in order to avoid unneeded calls to its code). In short, here's how initialization and cleanup are handled, for a hypothetical "hot plug PCI module'' (HPPM):

struct pci_driver hppm_driver = { /* .... */ };

int hppm_init_module(void)
{
    return pci_module_init(&hppm_driver);
}

int hppm_cleanup_module(void)
{
    pci_unregister_driver(&hppm_driver);
}

module_init(hppm);
module_exit(hppm);

That's all. It's incredibly easy. The hidden magic is split between the implementation of pci_module_init and the internals of the driver structure. We'd better follow a top-down path and start by introducing the relevant functions:

int pci_register_driver(struct pci_driver *drv);

This function inserts the driver in a linked list that is maintained by the system. That's how compiled-in device drivers perform their initialization; it is not used directly by modularized code. The return value is a count of devices being handled by the driver.

int pci_module_init(struct pci_driver *drv);

This function is a wrapper over the previous one and is meant to be called by modularized initialization code. It returns 0 for success and -ENODEV if no device has been found. This is meant to prevent a module from staying in memory if no device is currently there (expecting the module to be auto-loaded later if a matching device appears). Since this function is defined as inline, its behavior actually changes depending on whether MODULE is defined or not; it can thus be used as a drop-in replacement for pci_register_driver even for nonmodularized code.

void pci_unregister_driver(struct pci_driver *drv);

This function removes the driver from the linked list of known drivers.

void pci_insert_device(struct pci_dev *dev, struct pci_bus *bus);
void pci_remove_device(struct pci_dev *dev);

These two functions implement the flip side of the hot-plug system; they are called by the event handlers associated with plug/unplug events reported by a bus. The dev structure is used to scan the list of registered drivers. There is no need for device drivers to call them, and they are listed here to help give a complete view of the design around PCI drivers.

struct pci_driver *pci_dev_driver(const struct pci_dev *dev);

This is a utility function to look up the driver associated with a device (if any). It's used by /proc/bus support functions and is not meant to be called by device drivers.

The pci_driver structure

The pci_driver data structure is the core of hot-plug support, and we'll describe it in detail to complete the whole picture. The structure is pretty small, being made of just a few methods and a device ID list.

struct list_head node;

Used to manage a list of drivers. It's an example of generic lists, which were introduced in "Linked Lists" in Chapter 10, "Judicious Use of Data Types"; it's not meant to be used by device drivers.

char *name;

The name of the driver; it has informational value.

const struct pci_device_id *id_table;

An array that lists which devices are supported by this driver. The probe method will be called only for devices that match one of the items in the array. If the field is specified as NULL, the probe function will be called for every device in the system. If the field is not NULL, the last item in the array must be set to 0.

int (*probe)(struct pci_dev *dev, const struct pci_device_id *id);

The function must initialize the device it is passed and return 0 in case of success or a negative error code (actually, the error code is not currently used, but it's safe to return an errno value anyway instead of just -1).

void (*remove)(struct pci_dev *dev);

The remove method is used to tell the device driver that it should shut down the device and stop dealing with it, releasing any associated storage. The function is called either when the device is removed from the system or when the driver calls pci_unregister_driver in order to be unloaded from the system. Unlike probe, this method is specific to one PCI device, not to the whole set handled by this driver; the specific device is passed as an argument.

int (*suspend)(struct pci_dev *dev, u32 state);
int (*resume)(struct pci_dev *dev);

These are the power-management functions for PCI devices. If the device driver supports power-management features, these two methods should be implemented to shut down and reactivate the device; they are called by higher layers at proper times.

The PCI driver object is quite straightforward and a pleasure to use. We think there's little to add to the field enumeration, because normal hardware-handling code fits well in this abstraction without the need to tweak it in any way.

The only missing piece left to describe is the struct pci_device_id object. The structure includes several ID fields, and the actual device that needs to be driven is matched against all of the fields. Any field can be set to PCI_ANY_ID to tell the system to effectively ignore it.

unsigned int vendor, device;

The vendor and device IDs of the device this driver is interested in. The values are matched against registers 0x00 and 0x02 of the PCI configuration space.

unsigned int subvendor, subdevice;

The sub-IDs, matched against registers 0x2C and 0x2E of the PCI configuration space. They are used in matching the device because sometimes a vendor/device ID pair identifies a group of devices and the driver can only work with a few items in the group.

unsigned int class, class_mask;

If the device driver wants to deal with an entire class or a subset thereof, it can set the previous fields to PCI_ANY_ID and use class identifiers instead. The class_mask is present to allow both for drivers that want to deal with a base class and for drivers that are only interested in a subclass. If device selection is performed using vendor/device identifiers, both these fields must be set to 0 (not to PCI_ANY_ID, since the check is performed through a logical AND with the mask field).

unsigned long driver_data;

A field left for use by the device driver. It can, for example, differentiate between the various devices at compilation time, avoiding tedious arrays of conditional tests at runtime.

It's interesting to note that the pci_device_id data structure is just a hint to the system; the actual device driver is still free to return 0 from its probe method, thus refusing the device even if it matched the array of device identifiers. Thus if, for example, there exist several devices with the same signature, the driver can look for further information before choosing whether it is able to drive the peripheral or not.

Hardware Abstractions

We complete the discussion of PCI by taking a quick look at how the system handles the plethora of PCI controllers available on the marketplace. This is just an informative section, meant to show to the curious reader how the object-oriented layout of the kernel extends down to the lowest levels.

The mechanism used to implement hardware abstraction is the usual structure containing methods. It's a powerful technique that adds just the minimal overhead of dereferencing a pointer to the normal overhead of a function call. In the case of PCI management, the only hardware-dependent operations are the ones that read and write configuration registers, because everything else in the PCI world is accomplished by directly reading and writing the I/O and memory address spaces, and those are under direct control of the CPU.

The relevant structure for hardware abstraction, thus, includes only six fields:

struct pci_ops {
    int (*read_byte)(struct pci_dev *, int where, u8 *val);
    int (*read_word)(struct pci_dev *, int where, u16 *val);
    int (*read_dword)(struct pci_dev *, int where, u32 *val);
    int (*write_byte)(struct pci_dev *, int where, u8 val);
    int (*write_word)(struct pci_dev *, int where, u16 val);
    int (*write_dword)(struct pci_dev *, int where, u32 val);
};

The structure is defined in <linux/pci.h> and used by drivers/pci/pci.c, where the actual public functions are defined.

The six functions that act on the PCI configuration space have more overhead than dereferencing a pointer, because they use cascading pointers due to the high object-orientedness of the code, but the overhead is not an issue in operations that are performed quite rarely and never in speed-critical paths. The actual implementation of pci_read_config_byte(dev), for instance, expands to:

dev->bus->ops->read_byte();

The various PCI buses in the system are detected at system boot, and that's when the struct pci_bus items are created and associated with their features, including the ops field.

Implementing hardware abstraction via "hardware operations'' data structures is typical in the Linux kernel. One important example is the struct alpha_machine_vector data structure. It is defined in <asm-alpha/machvec.h> and it takes care of everything that may change across different Alpha-based computers.

A Look Back: ISA

The ISA bus is quite old in design and is a notoriously poor performer, but it still holds a good part of the market for extension devices. If speed is not important and you want to support old motherboards, an ISA implementation is preferable to PCI. An additional advantage of this old standard is that if you are an electronic hobbyist, you can easily build your own ISA devices, something definitely not possible with PCI.

On the other hand, a great disadvantage of ISA is that it's tightly bound to the PC architecture; the interface bus has all the limitations of the 80286 processor and causes endless pain to system programmers. The other great problem with the ISA design (inherited from the original IBM PC) is the lack of geographical addressing, which has led to many problems and lengthy unplug-rejumper-plug-test cycles to add new devices. It's interesting to note that even the oldest Apple II computers were already exploiting geographical addressing, and they featured jumperless expansion boards.

Despite its great disadvantages, ISA is still used in several unexpected places. For example, the VR41xx series of MIPS processors used in several palmtops features an ISA-compatible expansion bus, strange as it seems. The reason behind these unexpected uses of ISA is the extreme low cost of some legacy hardware, like 8390-based Ethernet cards, so a CPU with ISA electrical signaling can easily exploit the awful but cheap PC devices.

Hardware Resources

An ISA device can be equipped with I/O ports, memory areas, and interrupt lines.

Even though the x86 processors support 64 kilobytes of I/O port memory (i.e., the processor asserts 16 address lines), some old PC hardware decodes only the lowest 10 address lines. This limits the usable address space to 1024 ports, because any address in the range 1 KB to 64 KB will be mistaken for a low address by any device that decodes only the low address lines. Some peripherals circumvent this limitation by mapping only one port into the low kilobyte and using the high address lines to select between different device registers. For example, a device mapped at 0x340 can safely use port 0x740, 0xB40, and so on.

If the availability of I/O ports is limited, memory access is still worse. An ISA device can use only the memory range between 640 KB and 1 MB and between 15 MB and 16 MB. The 640-KB to 1-MB range is used by the PC BIOS, by VGA-compatible video boards, and by various other devices, leaving little space available for new devices. Memory at 15 MB, on the other hand, is not directly supported by Linux, and hacking the kernel to support it is a waste of programming time nowadays.

The third resource available to ISA device boards is interrupt lines. A limited number of interrupt lines are routed to the ISA bus, and they are shared by all the interface boards. As a result, if devices aren't properly configured, they can find themselves using the same interrupt lines.

Although the original ISA specification doesn't allow interrupt sharing across devices, most device boards allow it.[60] Interrupt sharing at the software level is described in "Interrupt Sharing", in Chapter 9, "Interrupt Handling".

[60]The problem with interrupt sharing is a matter of electrical engineering: if a device drives the signal line inactive -- by applying a low-impedance voltage level -- the interrupt can't be shared. If, on the other hand, the device uses a pull-up resistor to the inactive logic level, then sharing is possible. This is nowadays the norm. However, there's still a potential risk of losing interrupt events since ISA interrupts are edge triggered instead of level triggered. Edge-triggered interrupts are easier to implement in hardware but don't lend themselves to safe sharing.

ISA Programming

As far as programming is concerned, there's no specific aid in the kernel or the BIOS to ease access to ISA devices (like there is, for example, for PCI). The only facilities you can use are the registries of I/O ports and IRQ lines, described in "Using Resources" (Chapter 2, "Building and Running Modules") and "Installing an Interrupt Handler" (Chapter 9, "Interrupt Handling").

The programming techniques shown throughout the first part of this book apply to ISA devices; the driver can probe for I/O ports, and the interrupt line must be autodetected with one of the techniques shown in "Autodetecting the IRQ Number", in Chapter 9, "Interrupt Handling".

The helper functions isa_readb and friends have been briefly introduced in "Using I/O Memory" in Chapter 8, "Hardware Management" and there's nothing more to say about them.

The Plug-and-Play Specification

Some new ISA device boards follow peculiar design rules and require a special initialization sequence intended to simplify installation and configuration of add-on interface boards. The specification for the design of these boards is called Plug and Play(PnP) and consists of a cumbersome rule set for building and configuring jumperless ISA devices. PnP devices implement relocatable I/O regions; the PC's BIOS is responsible for the relocation -- reminiscent of PCI.

In short, the goal of PnP is to obtain the same flexibility found in PCI devices without changing the underlying electrical interface (the ISA bus). To this end, the specs define a set of device-independent configuration registers and a way to geographically address the interface boards, even though the physical bus doesn't carry per-board (geographical) wiring -- every ISA signal line connects to every available slot.

Geographical addressing works by assigning a small integer, called the Card Select Number (CSN), to each PnP peripheral in the computer. Each PnP device features a unique serial identifier, 64 bits wide, that is hardwired into the peripheral board. CSN assignment uses the unique serial number to identify the PnP devices. But the CSNs can be assigned safely only at boot time, which requires the BIOS to be PnP aware. For this reason, old computers require the user to obtain and insert a specific configuration diskette even if the device is PnP capable.

Interface boards following the PnP specs are complicated at the hardware level. They are much more elaborate than PCI boards and require complex software. It's not unusual to have difficulty installing these devices, and even if the installation goes well, you still face the performance constraints and the limited I/O space of the ISA bus. It's much better in our opinion to install PCI devices whenever possible and enjoy the new technology instead.

If you are interested in the PnP configuration software, you can browse drivers/net/3c509.c, whose probing function deals with PnP devices. Linux 2.1.33 added some initial support for PnP as well, in the directory drivers/pnp.

PC/104 and PC/104+

In the industrial world, two bus architectures are quite fashionable currently: PC/104 and PC/104+. Both are standard in PC-class single-board computers.

Both standards refer to specific form factors for printed circuit boards as well as electrical/mechanical specifications for board interconnections. The practical advantage of these buses is that they allow circuit boards to be stacked vertically using a plug-and-socket kind of connector on one side of the device.

The electrical and logical layout of the two buses is identical to ISA (PC/104) and PCI (PC/104+), so software won't notice any difference between the usual desktop buses and these two.

Other PC Buses

PCI and ISA are the most commonly used peripheral interfaces in the PC world, but they aren't the only ones. Here's a summary of the features of other buses found in the PC market.

MCA

Micro Channel Architecture (MCA) is an IBM standard used in PS/2 computers and some laptops. The main problem with Micro Channel is the lack of documentation, which has resulted in a lack of Linux support for MCA up until recently.

At the hardware level, Micro Channel has more features than ISA. It supports multimaster DMA, 32-bit address and data lines, shared interrupt lines, and geographical addressing to access per-board configuration registers. Such registers are called Programmable Option Select, or POS, but they don't have all the features of the PCI registers. Linux support for Micro Channel includes functions that are exported to modules.

A device driver can read the integer value MCA_bus to see if it is running on a Micro Channel computer, similar to how it uses pci_present if it's interested in PCI support. If the symbol is a preprocessor macro, the macro MCA_bus__is_a_macro is defined as well. If MCA_bus__is_a_macro is undefined, then MCA_bus is an integer variable exported to modularized code. Both MCA_BUS and MCA_bus__is_a_macro are defined in <asm/processor.h>.

EISA

The Extended ISA (EISA) bus is a 32-bit extension to ISA, with a compatible interface connector; ISA device boards can be plugged into an EISA connector. The additional wires are routed under the ISA contacts.

Like PCI and MCA, the EISA bus is designed to host jumperless devices, and it has the same features as MCA: 32-bit address and data lines, multimaster DMA, and shared interrupt lines. EISA devices are configured by software, but they don't need any particular operating system support. EISA drivers already exist in the Linux kernel for Ethernet devices and SCSI controllers.

An EISA driver checks the value EISA_bus to determine if the host computer carries an EISA bus. Like MCA_bus, EISA_bus is either a macro or a variable, depending on whether EISA_bus__is_a_macro is defined. Both symbols are defined in <asm/processor.h>.

As far as the driver is concerned, there is no special support for EISA in the kernel, and the programmer must deal with ISA extensions by himself. The driver uses standard EISA I/O operations to access the EISA registers. The drivers that are already in the kernel can be used as sample code.

VLB

Another extension to ISA is the VESA Local Bus (VLB) interface bus, which extends the ISA connectors by adding a third lengthwise slot. A device can just plug into this extra connector (without plugging in the two associated ISA connectors), because the VLB slot duplicates all important signals from the ISA connectors. Such "standalone'' VLB peripherals not using the ISA slot are rare, because most devices need to reach the back panel so that their external connectors are available.

The VESA bus is much more limited in its capabilities than the EISA, MCA, and PCI buses and is disappearing from the market. No special kernel support exists for VLB. However, both the Lance Ethernet driver and the IDE disk driver in Linux 2.0 can deal with VLB versions of their devices.

SBus

While most computers nowadays are equipped with a PCI or ISA interface bus, most not-so-recent SPARC-based workstations use SBus to connect their peripherals.

SBus is quite an advanced design, although it has been around for a long time. It is meant to be processor independent (even though only SPARC computers use it) and is optimized for I/O peripheral boards. In other words, you can't plug additional RAM into SBus slots (RAM expansion boards have long been forgotten even in the ISA world, and PCI does not support them either). This optimization is meant to simplify the design of both hardware devices and system software, at the expense of some additional complexity in the motherboard.

This I/O bias of the bus results in peripherals using virtual addresses to transfer data, thus bypassing the need to allocate a contiguous DMA buffer. The motherboard is responsible for decoding the virtual addresses and mapping them to physical addresses. This requires attaching an MMU (memory management unit) to the bus; the chipset in charge of the task is called IOMMU. Although somehow more complex than using physical addresses on the interface bus, this design is greatly simplified by the fact that SPARC processors have always been designed by keeping the MMU core separate from the CPU core (either physically or at least conceptually). Actually, this design choice is shared by other smart processor designs and is beneficial overall. Another feature of this bus is that device boards exploit massive geographical addressing, so there's no need to implement an address decoder in every peripheral or to deal with address conflicts.

SBus peripherals use the Forth language in their PROMs to initialize themselves. Forth was chosen because the interpreter is lightweight and therefore can be easily implemented in the firmware of any computer system. In addition, the SBus specification outlines the boot process, so that compliant I/O devices fit easily into the system and are recognized at system boot. This was a great step to support multi-platform devices; it's a completely different world from the PC-centric ISA stuff we were used to. However, it didn't succeed for a variety of commercial reasons.

Although current kernel versions offer quite full-featured support for SBus devices, the bus is so little used nowadays that it's not worth covering in detail here. Interested readers can look at source files in arch/sparc/kernel and arch/sparc/mm.

NuBus

Another interesting but forgotten interface bus is NuBus. It is found on older Mac computers (those with the M68k family of CPUs).

All of the bus is memory-mapped (like everything with the M68k), and the devices are only geographically addressed. This is good and typical of Apple, as the much older Apple II already had a similar bus layout. What is bad is that it's almost impossible to find documentation on NuBus, due to the close-everything policy Apple has always followed with its Mac computers (and unlike the previous Apple II, whose source code and schematics were available at little cost).

The file drivers/nubus/nubus.c includes almost everything we know about this bus, and it's interesting reading; it shows how much hard reverse engineering developers had to do.

External Buses

One of the most recent entries in the field of interface buses is the whole class of external buses. This includes USB, FireWire, and IEEE1284 (parallel-port-based external bus). These interfaces are somewhat similar to older and not-so-external technology such as PCMCIA/CardBUS and even SCSI.

Conceptually, these buses are neither full-featured interface buses (like PCI is) nor dumb communication channels (like the serial ports are). It's hard to classify the software that is needed to exploit their features, as it's usually split into two levels: the driver for the hardware controller (like drivers for PCI SCSI adaptors or PCI controllers introduced earlier in "The PCI Interface") and the driver for the specific "client'' device (like sd.c handles generic SCSI disks and so-called PCI drivers deal with cards plugged in the bus).

But there's another problem with these new buses. With the exception of USB, their support is either not mature or is somehow in need of a revision (the latter condition applies especially to the SCSI kernel subsystem, which is reported to be far from optimal by several of the best kernel hackers).

USB

USB, the Universal Serial Bus, is the only external bus that is currently mature enough to deserve some discussion. Topologically, a USB subsystem is not laid out as a bus; it is rather a tree built out of several point-to-point links. The links are four-wire cables (ground, power, and two signal wires) that connect a device and a hub (just like twisted pair Ethernet). Usually, PC-class computers are equipped with a "root hub'' and offer two plugs for external connections. You can connect either devices or additional hubs to the plugs.

The bus is nothing exciting at the technological level, as it's a single-master implementation in which the host computer polls the various devices. Despite this intrinsic limit of the bus, it has interesting features, such as the ability for a device to request a fixed bandwidth for its data transfers in order to reliably support video and audio I/O. Another important feature of USB is that it acts merely as a communication channel between the device and the host, without requiring specific meaning or structure in the data it delivers.[61]

[61]Actually, some structuring is there, but it mostly reduces to the requirement for the communication to fit into one of a few predefined classes: a keyboard won't allocate bandwidth, for example, while a camera will.

This is unlike SCSI communication and like standard serial media.

These features, together with the inherent hot-plug capability of the design, make USB a handy low-cost mechanism to connect (and disconnect) several devices to the computer without the need to shut the system down, open the cover, and swear over screws and wires. USB is becoming popular in the PC market but remains unsuitable for high-speed devices because its maximum transfer rate is 12 Mb per second.

USB is supported by version 2.2.18 (and later) and 2.4.x of the Linux kernel. The USB controller in any computer belongs to one of two kinds, and both drivers are part of the standard kernel.

Writing a USB Driver

As far as "client'' device drivers are concerned, the approach to USB is similar to the pci_driver layout: the device driver registers its driver object with the USB subsystem, and it later uses vendor and device identifiers to identify insertion of its hardware.

The relevant data structure is struct usb_driver, and its typical use is as follows:

#include <linux/usb.h>

static struct usb_driver sample_usb_driver = {
        name:        "sample",
        probe:       sample_probe,
        disconnect:  sample_disconnect,
};

int init_module(void)
{
    /* just register it; returns 0 or error code */
    return usb_register(&sample_usb_driver);
}

void cleanup_module(void)
{
    usb_deregister(&sample_usb_driver);
}

The probe function declared in the data structure is called by the USB kernel subsystem whenever a new device is connected to the system (or when the driver is loaded if any unclaimed devices are already connected to the bus).

Each device identifies itself by providing the system with vendor, device, and class identifiers, similar to what PCI devices do. The task of sample_probe, therefore, is looking into the information it receives and claiming ownership of the device if suitable.

To claim ownership, the function returns a non-NULL pointer that will be used to identify the device. This will usually be a pointer to the device-specific data structure that is at the core of the device driver as a whole.

To exchange information with the device, you'll then need to tell the USB subsystem how to communicate. This task is performed by filling a struct urb (for USB request block) and by passing it to usb_submit_urb. This step is usually performed by the open method associated with the device special file, or an equivalent function.

Note that not every USB driver needs to implement its own device special files by requesting a major number and so on. Devices that fall within a class for which the kernel offers generalized support won't have their own device files and will report their information through other means.

An example of generalized management is input handling. If your USB device is an input device (such as a graphic tablet), you won't allocate a major number but rather will register your hardware by calling input_register_device. In this case, the open callback of your input device is in charge of establishing communication by calling usb_submit_urb.

A USB input driver, therefore, must rely on several other system blocks, and most of them can be modules as well. The module-stacking architecture for USB input device drivers is shown in Figure 15-3.

Figure 15-3

Figure 15-3. Modules involved in USB input management

You'll find a complete USB device driver in the sample files available on the O'Reilly FTP site. It is a very simplified keyboard and mouse driver that shows how to lay out a complete USB driver. To keep it simple, it doesn't use the input subsystem to report events but rather posts messages about them using printk. You'll need at least a USB keyboard or a USB mouse to test the driver.

There's quite a lot of documentation on USB available currently, including two articles by one of your authors, whose style and technical level resembles that of Linux Device Drivers. These articles even include a more complete USB sample device driver that uses the input kernel subsystem and can be run by alternative means if you have no USB devices handy. You can find them at http://www.linux.it/kerneldocs.

Backward Compatibility

The current implementation of PCI support in the kernel was not available with version 2.0 of the kernel. With 2.0 the support API was much more raw, because it lacked the various objects that have been described in this chapter.

The six functions to access the configuration space received as arguments the 16-bit low-level key to the PCI device instead of using a pointer to struct pci_dev. Also, you had to include <asm/pcibios.h> before being able to read or write to the configuration space.

Fortunately, dealing with the difference is not a big problem, and if you include sysdep.h you'll be able to use 2.4 semantics even when compiling under 2.0. PCI support for version 2.0 is available in the header pci-compat.h, automatically included by sysdep.h when you compile under 2.0. The header, as distributed, implements the most important functions used to work with the PCI bus.

If you use pci-compat.h to develop drivers that work all the way from 2.0 through 2.4, you must call pci_release_device when you are done with a pci_dev item. This happens because the fake pci_dev structures created by the header are allocated with kmalloc, whereas the real structures of 2.2 and 2.4 are static resources in the kernel proper. The extra function is defined to do nothing by sysdep.h whenever compiling for 2.2 or 2.4, so it does no harm. Feel free to look at pciregions.cor pcidata.c to see portable code in action.

Another relevant difference in 2.0 is /procsupport for PCI. There was no /proc/bus/pci file hierarchy (and no /proc/bus at all, actually), only a single /proc/pci file. It was meant more for human reading than for machine reading, and it was not very readable anyway. Under 2.2 it was possible to select a "backward-compatible /proc/pci'' at compile time, but the obsolete file was completely removed in version 2.4.

The concept of hot-pluggable PCI drivers (and struct pci_driver) is new as of version 2.4. We do not offer backward-compatible macros to use the feature on older kernels.

Quick Reference

This section, as usual, summarizes the symbols introduced in the chapter.

#include <linux/config.h>
CONFIG_PCI

This macro should be used to conditionally compile PCI-related code. When a PCI module is loaded to a non-PCI kernel, insmod complains about several symbols being unresolved.

#include <linux/pci.h>

This header includes symbolic names for the PCI registers and several vendor and device ID values.

int pci_present(void);

This function returns a boolean value that tells whether the computer we're running on has PCI capabilities or not.

struct pci_dev;
struct pci_bus;
struct pci_driver;
struct pci_device_id;

These structures represent the objects involved in PCI management. The concept of pci_driver is new as of Linux 2.4, and struct pci_device_id is central to it.

struct pci_dev *pci_find_device(unsigned int vendor, unsigned int device, struct pci_dev *from);
struct pci_dev *pci_find_class(unsigned int class, struct pci_dev *from);

These functions are used to look up the device list looking for devices with a specific signature or belonging to a specific class. The return value is NULL if none is found. from is used to continue a search; it must be NULL the first time you call either function, and it must point to the device just found if you are searching for more devices.

int pci_read_config_byte(struct pci_dev *dev, int where, u8 *val);
int pci_read_config_word(struct pci_dev *dev, int where, u16 *val);
int pci_read_config_dword(struct pci_dev *dev, int where, u32 *val);
int pci_write_config_byte (struct pci_dev *dev, int where, u8 *val);
int pci_write_config_word (struct pci_dev *dev, int where, u16 *val);
int pci_write_config_dword (struct pci_dev *dev, int where, u32 *val);

These functions are used to read or write a PCI configuration register. Although the Linux kernel takes care of byte ordering, the programmer must be careful about byte ordering when assembling multibyte values from individual bytes. The PCI bus is little-endian.

int pci_register_driver(struct pci_driver *drv);
int pci_module_init(struct pci_driver *drv);
void pci_unregister_driver(struct pci_driver *drv);

These functions support the concept of a PCI driver. Whereas compiled-in code uses pci_register_driver (which returns the number of devices that are managed by this driver), modularized code should call pci_module_init instead (which returns 0 if one or more devices are there and -ENODEV if no suitable device is plugged into the system).

#include <linux/usb.h>
#include <linux/input.h>

The former header is where everything related to USB resides and must be included by USB device drivers. The latter defines the core of the input subsystem. Neither of them is available in Linux 2.0.

struct usb_driver;
int usb_register(struct usb_driver *d);
void usb_deregister(struct usb_driver *d);

usb_driver is the main building block of USB device drivers. It must be registered and unregistered at module load and unload time.


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