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Linux and USB 2.0

David Brownell
<dbrownell@users.sourceforge.net>

20 December 2002

This is a short writeup explaining what USB 2.0 changed and what's going on with it in Linux. It starts by talking about user visible changes (including usbfs information) followed by driver-visible ones. Finally it summarizes the current state of Linux USB 2.0 driver support in recent 2.4 and 2.5 kernels. If you want assistance getting this working on your hardware, try the linux-usb-users (or linux-usb-devel) mailing lists at http://www.linux-usb.org.

If you're a 2.4 user, use the latest 2.4.21-pre3 code, roughly matching the 2.5.54 EHCI code, which should be more stable and reliable on newer hardware and on CardBus. Some problems will still show up under load (notably on VIA), patches are under development; remember this is still classified as 'experimental'.

Most current distributions ship 2.4.18 kernels with EHCI code based on much older code, similar to what was in 2.5.2; the newer code supports more hardware, fixes bugs, is smaller, and has other goodness.

You should probably know that "USB" is an abbreviation of the Universal Serial Bus, which is widely used for peripherals in modern desktop systems. (It's not "universal" in the sense that you'd want it instead of HyperTransport!) PCs typically support one or more USB controllers (one per "USB bus") each of which can support up to 127 different devices. "Legacy free" PCs omit non-USB peripheral support (like RS-232 serial lines, and PS/2 ports). USB is asymmetric, even at the level of its cabling (so you can't hook things up incorrectly), and it supports "hotplugging" for all its peripherals (so you don't have to configure them by hand). Those features both help reduce end-user setup and configuration problems, which was a major goal for USB technology.


OS-Neutral Changes in USB 2.0

Today, most devices and systems support USB version 1.1, which supported two device speeds: low speed at 1.5 Mbit/sec, and full speed at 12 Mbit/sec. USB 2.0 is appearing in current product designs, and one of its main features is adding a new speed: high speed, at 480 Mbit/sec.

To put it another way: USB 1.1 was OK for low speed devices like mice and keyboards, and even for medium speed ones like Ethernet (10 Mbit/sec) adapters, or consumer electronics gadgets that only exchange a few megabytes of data (like many digital still cameras and MP3 players). But you need USB 2.0 to get reasonable speed for multiple large transfers as with some PC peripherals like disk drives (including MP3 jukeboxes :) or high resolution webcams (USB video), or get concurrent use from a bunch of 100BaseT networking adapters.

OK, it's faster! ... but what else will a Linux user notice about USB 2.0? We'll go from the outside in: starting from what you'll see with product boxes, and then working toward what you'll see with normal Linux user mode tools. (More technical details are in later sections.) One thing you won't notice is designed-in compatibility problems. Apart from some constraints on how you set up high speed devices, all your USB 1.1 hardware works just fine with USB 2.0 systems.

Updated USB Logos

One thing that you should watch for, and use to your advantage, is a new testing and branding program. Earlier USB devices sometimes had compatibility problems, and the tighter electrical requirements of USB 2.0 could have aggravated that issue. Instead, there are new USB 2.0 logos and a testing program that must be passed before devices are allowed to use them. That is, this isn't purely a marketing gimmick, there's actually some value wrapped up in this logo. Here are the new and old USB logos:

New USB 2.0 Logo
(high speed version, with red stripe)
Old USB 1.1 Logo (no compatibility testing done)

Be cautious about "high speed" devices that for any reason don't display that new logo. Those devices may not work very well, and it'll be your fault for encouraging vendors that sell nonconformant devices.

Hubs and Cabling

One big win for the USB 2.0 upgrade is that cables don't need to change. With the possible exception of some low quality cables that may not even handle USB 1.1 very well, you can keep using your existing USB cables. There was no need to switch to optical signal transmission, or anything similarly incompatible. There are new "Mini-B" cables, that can help eliminate the need for proprietary connectors on many small USB peripherals.

USB 2.0 hubs are special. Not only do they support high speed devices (older USB 1.1 hubs can't), but also because they all include "transaction translator" support that helps prevent full and low speed devices from wasting most of the USB bandwidth. See the next section, on backwards compatibility, for how this impacts end users with high speed buses whose devices may not all operate at high speed.

There is also an evolution of USB 2.0 called USB "On The Go" (OTG) which supports some point-to-point peer style hookups, like a digital camera or PDA connecting to a USB printer, for devices (maybe portable) that are often used without a PC. That won't be discussed here, in part because USB OTG products don't exist yet. They're expected to start appearing later in 2002. For more information, see this information from Philips Semiconductors.

Host Controllers and Backwards Compatibility

You can connect USB 2.0 devices to USB 1.1 hosts and hubs, and they should work just fine ... but at 12 Mbit/sec, not at 480 Mbit/sec.

To get "high speed" behavior you'll need an updated host controller. It must support "USB 2.0 high speed", through the "EHCI" standard. Today you can get those as PCI cards (usually with a five port controller from NEC), and motherboards are starting to appear with such controllers built in. I've seen both NEC and VIA controllers on such motherboards, and PCI cards with controllers from NEC, Philips, and VIA. In the second half of 2002, it's become common that new motherboards include such support directly in their "south bridge", with chipsets from vendors such as ALI, ATI, Intel, NVidia, SiS, and VIA. (That's in alphabetical order; I'm not intending to slight any vendors.) See the section on Linux support (later) for more specific information.

You may also need newer hubs with USB 2.0 support. Probably the most important aspect of such hubs is how they support trees of devices that mix both high speed and full (or low) speed devices. The short version of the story is that to get high speed transfers out of a high-speed capable device, you must hook it up through an EHCI controller (that's using an EHCI driver!), and any hubs between the host and the device must only be USB 2.0 hubs.

That is, a tree of USB devices can start out with a high speed root hub, but if you plug in any USB 1.1 hubs to that tree, USB devices on branches below that hub will only operate at slower full/low (USB 1.1) speeds ... even if it's a USB 2.0 device. For now the lesson is that if you want to use a device at high speed, double check it after cabling things up. You can do this with "usbfs".


Changes Visible to Linux Users

Beyond those general issues, Linux users will notice a few changes if they poke around through what the kernel tells them about their hardware.

"usbfs" and /proc/bus/usb/devices

You may know "usbfs" through its original, and somewhat confusing name, of "usbdevfs". The name changed because it was completely unrelated to "devfs".

If you use "usbfs" (perhaps through scripts like usbtree or tools like usbview) you will notice a few minor changes. Most notably, the USB version for some devices will be "2.0", and those devices will report their speed as "480Mb/s" when they're running at high speed. (In compatibility mode, at full speed, they'll say "12Mb/s".) Here's what usbtree shows for some full speed devices hooked up to an USB 2.0 controller. Several are connected through a USB 2.0 hub, and there's a USB 1.1 hub (with no devices) hooked up to another root hub port.

/:  Bus 03.Port 1: Dev 1, Class=root_hub, Driver=ohci-hcd/2p, 12M
/:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=ohci-hcd/3p, 12M
        |__ Port 1: Dev 2, If 0, Class=hub, Driver=hub/4p, 12M
/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=ehci-hcd/5p, 480M
    |__ Port 2: Dev 2, If 0, Class=hub, Driver=hub/4p, 480M
        |__ Port 1: Dev 3, If 0, Class=>ifc, Driver=pegasus, 12M
        |__ Port 4: Dev 8, If 0, Class=hub, Driver=hub/3p, 12M
            |__ Port 1: Dev 9, If 0, Class=HID, Driver=hid, 12M

Some of the endpoint descriptors may change. Maximum packet sizes can be bigger, and polling intervals for periodic transfers will sometimes be measured in microseconds (like 250us), not milliseconds (like 2ms), and you may even see NAK rates for bulk endpoints. For example, this descriptor is for one USB 2.0 CD-RW when it's running at high speed:

T:  Bus=01 Lev=01 Prnt=01 Port=01 Cnt=01 Dev#=  2 Spd=480 MxCh= 0
D:  Ver= 2.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
P:  Vendor=05ab ProdID=0060 Rev= 2.10
S:  Manufacturer=In-System Design
S:  Product=USB Storage Adapter
S:  SerialNumber=2201010A0247C8AB
C:* #Ifs= 1 Cfg#= 2 Atr=c0 MxPwr= 98mA
I:  If#= 0 Alt= 0 #EPs= 3 Cls=08(stor.) Sub=06 Prot=50 Driver=usb-storage
E:  Ad=01(O) Atr=02(Bulk) MxPS= 512 Ivl=125us
E:  Ad=82(I) Atr=02(Bulk) MxPS= 512 Ivl=0ms
E:  Ad=83(I) Atr=03(Int.) MxPS=   2 Ivl=32ms

Here's the descriptor for the same CD-RW when there's no EHCI driver, so it's running at full speed (may not be a good idea to burn CDs at this speed though):

T:  Bus=02 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12  MxCh= 0
D:  Ver= 2.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
P:  Vendor=05ab ProdID=0060 Rev= 2.10
S:  Manufacturer=In-System Design
S:  Product=USB Storage Adapter
S:  SerialNumber=2201010A0247C8AB
C:* #Ifs= 1 Cfg#= 2 Atr=c0 MxPwr= 98mA
I:  If#= 0 Alt= 0 #EPs= 3 Cls=08(stor.) Sub=06 Prot=50 Driver=usb-storage
E:  Ad=01(O) Atr=02(Bulk) MxPS=  64 Ivl=0ms
E:  Ad=82(I) Atr=02(Bulk) MxPS=  64 Ivl=0ms
E:  Ad=83(I) Atr=03(Int.) MxPS=   2 Ivl=32ms

Other than that "480Mb/s" speed rating, most of that per-device usbfs information will only be interesting to folk working with device drivers. That "480Mb/s" in the descriptor for your device will be a sign that you've cabled it up correctly (so it runs at high speed). But there are also differences you may notice if you look at the root hub support for each of your systems USB busses.

USB 1.1 "Companion Controllers"

Perhaps the most curious thing is that when you plug in a full (or low) speed device to a connector on your high speed USB controller, it will be connecting to a different bus than when you plug in a high speed device to that same USB "A" socket on your PC! In the CD-RW example above, the bus number changed. Sometimes the port number will also change. (OK, so maybe you wouldn't have noticed.)

That's because of how the controllers work. Here's how /proc/bus/usb/devices looks on one system with a five port NEC EHCI based controller, and nothing hooked up to it. I added some whitespace and highlighting so you can see what's going on a bit better:

T:  Bus=03 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12  MxCh= 3
B:  Alloc=  0/900 us ( 0%), #Int=  0, #Iso=  0
D:  Ver= 1.10 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=0000 ProdID=0000 Rev= 2.05
S:  Manufacturer=Linux 2.5.8 ohci-hcd
S:  Product=NEC Corporation USB
S:  SerialNumber=00:0b.0
C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   2 Ivl=255ms

T:  Bus=02 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12  MxCh= 2
B:  Alloc=  0/900 us ( 0%), #Int=  0, #Iso=  0
D:  Ver= 1.10 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=0000 ProdID=0000 Rev= 2.05
S:  Manufacturer=Linux 2.5.8 ohci-hcd
S:  Product=NEC Corporation USB (#2)
S:  SerialNumber=00:0b.1
C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   2 Ivl=255ms

T:  Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=480 MxCh= 5
B:  Alloc=  0/800 us ( 0%), #Int=  0, #Iso=  0
D:  Ver= 2.00 Cls=09(hub  ) Sub=00 Prot=01 MxPS= 8 #Cfgs=  1
P:  Vendor=0000 ProdID=0000 Rev= 2.05
S:  Manufacturer=Linux 2.5.8 ehci-hcd
S:  Product=NEC Corporation USB 2.0
S:  SerialNumber=00:0b.2
C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   2 Ivl=256ms

Yes, three USB buses on one controller card! (The PCI card is 00:0b but each bus has a different PCI function number.) And it looks like there's a total of ten ports (sum the "MxCh" entries saying how many children the root hubs have), not five ... how can that be?

The answer is that the two OHCI "companion controllers" are used along with the EHCI controller, and a silicon switch connects each port to only one controller at a time. When an EHCI driver runs, all ports start out connected to EHCI. When EHCI detects a full or low speed device on a port, that port is switched over to one of the companion controllers. High speed devices it keeps for itself ... so each port seems to connect to either EHCI or its companion controller (never both!) based on whether it runs at high speed or not. If there's no EHCI driver there to handle high speed devices, then everything gets treated as full or low speed since the switch won't connect things to the EHCI controller. (Companion controllers won't necessarily be OHCI; some are UHCI.)

So to fully use a USB 2.0 host controller you must still use an OHCI or UHCI host controller driver, as you've likely been doing already. And if that's the only driver you have, you can still use hardware that includes USB 2.0 support ... it just won't be as fast until you upgrade to an OS version with USB 2.0 support. In the big picture that's a great migration story for the core USB 2.0 technology: the hardware can upgrade, and the software follows later when it's convenient for end users. (Much like most vendors' stories for migrating from one generation of CPU, or instruction set, to the next one.)


USB 2.0 Hubs have "Transaction Translators"

OK, this section talks about what makes USB 2.0 tricky. And it's not all that user visible, so you can safely skip reading it if you're one of the many Linux users who just wants a top quality OS (or a Free one) and aren't very concerned about all the underlying technology.

If you're writing a device driver, you may want to know about this, since it can affect how you write your driver. At this writing, not all transaction translator features are used by Linux. Also, since those code paths aren't used much yet, a number of shortcuts have been taken, which limit throughput. (Someone will surely provide a patch to remove those shortcuts if/when they become troublesome.)

Split Transactions

USB 2.0 gets some of its backward compatibility through a technique called split transactions that's used in many other buses. Many disk interface technologies, for example, support concurrent requests to different devices (like USB does), and split their requests into a "start" phase, disconnecting, and reconnecting later for a "complete" phase. That lets the bus be used for other purposes while some slow I/O completes ... like a disk drive seeking and reading several sectors, or (in the USB case) like a 1.5 MBit/sec mouse delivering its motion event.

In the case of a USB 2.0 host talking to a USB 1.1 device, it does so either through a companion controller (so it's really acting as a USB 1.1 host), or through a transaction translator in a USB 2.0 hub. The host talks to the translator at high speed, and then the translator performs the full or low speed I/O to that USB 1.1 device. It buffers data in either direction as required, and later sends the results back to the host at high speed. A hub can have just one translator, or one per port. With just one translator, a hub can only use as much bandwidth as a USB 1.1 host can. With more translators, each port can use that much bandwidth.

So for example, each of six ports on the hub might have a full speed webcam, each of which is using ten megabits per second of I/O at the same time. With less capable hubs, only one port on that hub might be able to operate at that speed at a time, supporting only the bandwidth of a USB 1.1 bus.

Using Transaction Translators

From the perspective of a Linux USB 2.0 host, there are two ways it talks to USB 1.1 devices through transaction translators. Some requests do this almost transparently, like control requests and (full speed only) bulk data transfer. In those cases, the host must tell the EHCI controller that it's talking through a transaction translator, and maybe be ready to recover from some errors, but for most purposes there's control and bulk requests are completely "hands off". That means many USB 1.1 device drivers will continue to work just fine through a USB 2.0 hub ... many network adapters, modems, digital still cameras, MP3 players, disk drives, and similar devices won't need anything more.

Then there are the requests that need a lot of hands-on attention: periodic transfers like interrupt and (full speed only) isochronous ones. Because these are all reserved-bandwidth transacton types, they can't be automated as readily as control and bulk (which can opportunistically make progress whenever periodic transfers aren't active). The result is a lot of hub-related complexity in the EHCI driver, making sure that the periodic transfers are safely scheduled and that transaction translators don't get overcommited. Periodic transfers are already the most complex type in USB (though their API got simpler in the Linux kernel 2.5 series), so to some extent it's just one more consideration when scheduling,

For example, suppost a USB 1.1 periodic transfer is to be worked on every eight frames (milliseconds). That's going to go through a specific transaction translator; the transfer needs a time when that translator isn't in use, and when there's also bus bandwidth. The EHCI driver needs to schedule a "start split" (a bit in an 's-mask') sometime in one microframe, and a "complete split" (bits in a 'c-mask') in some later microframe, and ensure that between the start and completion no other request uses that translator. Isochronous transfers can be bigger, and are managed by EHCI using a different type of data structure, but in both cases you can see that there's a fair amount of housekeeping involved in tracking what requests are active.


Driver-Visible Changes in USB 2.0

The most visible change was the addition of "high speed" devices and transfers. In a Linux USB device driver, you can tell if the device is high speed (and thus whether its transfers will be at high speed!) by testing whether dev->speed == USB_SPEED_HIGH. You've been able to do that since about the 2.4.10 kernel, but you probably didn't have high speed devices to work with then.

The rules for full and low speed transfers have not changed, but some rules changed for high speed transfers. (You may have noticed some of those changes when comparing the high speed and full speed usbfs information shown above for the CD-RW.) In a few cases drivers need to have code that knows which rules apply, but mostly the changes will be transparent to correctly written drivers. Those changes include:

  • Maximum packet sizes for control transfers are fixed at 64 bytes, and for bulk transfers they are fixed at 512 bytes. For full and low speed, those limits are smaller and can also vary between devices.
  • Maximum packet sizes for interrupt transfers can now be up to 1024 bytes. (For full and low speed, those limits are much smaller.) Likewise for ISO transfers (which previously had a curious 1023 byte limit).
  • There are now eight "microframes" per frame. A useful number to keep in mind is that thirteen bulk packets of 512 bytes each can be exchanged in one microframe. (At full speed, nineteen bulk packets of 64 packets could be exchanged in one frame. High speed transfers can be more than forty times faster than full speed ones!)
  • Periodic transfers can only reserve up to eighty percent of the USB bandwidth (per microframe). In USB 1.1, up to ninety percent was available.
  • Periodic transfer periods are measured in microframes, not frames. Both ISO and interrupt transfer intervals now use a log2 encoding, where previously only ISO transfers used that encoding. (Interrupt transfers can now have larger or smaller periods.) Drivers will need to interpret endpoint->bInterval differently for high speed devices, and also set urb->interval accordingly.
  • Periodic transfers can support a "high bandwidth" mode, where up to 3 packets (3KB at most) per microframe can be transferred. This shows up in a new field in endpoint descriptors. Drivers will need to interpret usb_maxpacket() differently for high speed, since that holds the new bit field as well as the original one. (With USB 1.1 devices, periodic transfers only involve one packet each period.)
  • EHCI provides less detailed error indicators than OHCI or UHCI. This means that drivers for USB 1.1 devices should retest for good behavior while connecting through a transaction translator in a USB 2.0 hub. Try things like unplugging devices while they're actively in use. It's likely they'll see EPIPE (endpoint stall) errors in some cases where previously they saw ETIMEDOUT (ohci) or EILSEQ (uhci). Unlike true endpoint stalls, usb_clear_halt() won't be able to talk to the device. (Your driver does have code to recover correctly from halted endpoints, doesn't it? It should!)

There are also protocol changes that should be invisible to most device drivers ... except in some cases for the host or device controller drivers, or the hub driver:

  • There's a new "other speed configuration" descriptor for devices that can work at both full and high speed. When a device is working at full speed, it would usually have different descriptors for high speed operation ... see the CD-RW example above. You don't need to re-enumerate at the other speed to see those descriptors.
  • "Split transactions" use a new feature of USB 2.0 hubs, the "transaction translator, which makes more efficient use of USB bandwidth. (All transactions run at high speed. Hubs build in buffering and logic for full or low speed speed operations.)
  • NYET transactions on the wire, used to detect that split transactions are not yet complete.
  • PING transactions on the wire help flow control to high speed bulk or control endpoints.
  • DATA2 and MDATA transactions on the wire are used with isochronous transfers.
  • The hub descriptor defines a few new fields, and uses the protocol number to say whether each port has its own transaction translator.

When Linux start to be embedded more widely inside USB devices (not just USB hosts) the USB "function" or "target" drivers will need a new kind of USB programming interface. That will also support USB 2.0; higher speed targets will benefit from the capabilities of Linux more than most USB 1.1 targets (even including Linux-based PDAs). The current Linux-USB programming interface has a host side model: one master talking to many targets (like a web client talking to many servers). A new target side model is in the works, for the 2.5 series. While it should resemble the current programming interfaces (at least in terms of submitting URBs) it must treat USB busses very differently from a host side API. That's because implementing the target function only involves talking to a single USB Host (like a web server only responding to one client at a time). Also, target side drivers can never initiate control requests, they can only respond to them.


Linux Status

People have been using USB 2.0 with usb-storage devices from Linux hosts since June 2001. But it was only towards the end of that year, a short while before Linus created the 2.5 development branch, that other USB 2.0 devices (notably, hubs) began to be available. So while some changes for USB 2.0 were rolling into 2.4 kernels through the year 2001, largely done by about 2.4.10, the most tricky stuff (the ehci-hcd driver) stayed separate until the 2.5 kernel branched. Linux distributions have begun to include this support.

Host Controller Driver Support

At this writing the "ehci-hcd" driver is not as well proven as the OHCI or UHCI drivers, so it's still labeled experimental, but it's just fine for many purposes.

  • The Linux 2.5 kernel has the most up-to-date USB 2.0 support, as well as quite a number of other USB updates and cleanups. At this writing 2.5.52 is the current version, with ehci patches pending.
  • The 2.4.19 kernel is the first to bundle a 2.4 based version of the ehci-hcd driver. Expect that 2.4 version to be less current than the 2.5 version. At this writing, 2.4.21-pre1 EHCI matches what's found in the 2.5.39 kernel, and it has some known issues.
  • Recent Linux distributions (like RedHat or Mandrake) include a 2.4.18 based version that's less up-to-date than the current 2.4 version. (Their versions are similar to what went into the 2.5.2 kernel.)

In terms of functionality, the latest driver:

  • Supports all four USB transfer types at high speed: control, bulk, interrupt, and isochronous. However, scheduling for interrupt transfers currently takes shortcuts which prevent them from using much bandwidth. (Or handling many devices.) And nobody's really done much yet with the isochronous code, which means any bugs wouldn't be known.
  • Has partial support for split transactions (full and low speed transfers) through USB 2.0 hubs:
    • Control and bulk transfers work, though older versions may not recover from some errors appropriately. That means: that while you should be able to use many USB 1.1 devices like disks and still cameras through such hubs, you might also run into some failure modes that can't yet be recovered. (And some device drivers might also need updates to handle new failure modes, as noted above.)
    • Full and low speed interrupt transactions work, taking the same scheduling shortcuts as high speed interrupt transfers take. So most keyboards and mice should work on USB 2.0 hubs, if you're using one of the more recent kernels. (If you're using a 2.4 kernel, you'll want that 2.5.39 update.)
    • You can't use full speed isochronous transfers through USB 2.0 hubs. That means: don't hook up USB 1.1 webcams, speakers, etc. to high speed buses, they'll enumerate but you won't be able to use them otherwise. Eventually of course a dozen USB 1.1 webcams will easily coexist at full data rate on a single USB 2.0 bus, using USB 2.0 hubs.
  • Only the newest kernels support BIOS handshaking (transferring "ownership" of EHCI from BIOS/SMM to Linux, as is done for OHCI or UHCI). That's because it depends on EHCI hardware features that are not yet generally available, used by BIOS implementations that talk to EHCI controllers integrated on motherboard "South Bridge" chipsets. (Though it's not clear when BIOS code will bother to talk to EHCI; it can use the companion controllers for early stages of booting.)
  • Works with a variety of EHCI silicon implementations. NEC led the market by over a year, but now implementations from Intel (ICH4), Philips and VIA (vt6202, vt8235) also work with that driver. Other implementations (ALI, ATI, InSilicon, NVidia, SiS, ...) are anticipated to work without needing significant changes. There are current reports of problems with VT-6202 support. For new hardware, try the latest 2.5 kernel code. For example, patches that help with VIA VT8235 southbridges (used with KT333 and KT400 chipsets) are included in the 2.5.35 kernel. Similar fixes may well help problems with other hardware.
  • Should tolerate more abuse on CardBus adapters. Before the 2.5.29 kernel, you needed to manually "rmmod" ehci-hcd (and often ohci-hcd) to cope with a sequencing bug in the CardBus shutdown. As of 2.5.29 you should be able to do rude things like eject the card without any preparation. (Though be careful, there seem to be PCI or USB driver resources that aren't fully cleaned up. Nonfatal oopses have been observed.)

You'll find that most PCI USB 2.0 add-in cards (and CardBus/PCMCIA ones) use the NEC silicon. (Some of those will likely start to use an updated and faster "1.0" version that NEC announced.) The Adaptec card that does both USB 2.0 and FireWire has been a problem for some people too, perhaps because of the built-in PCI bridge.

Some folk have problems with the CardBus versions, because Linux hasn't always managed to get the IRQs assigned correctly on their particular laptop make/model. Once such BIOS/PCI level problems get resolved, the ehci-hcd driver works with those CardBus configurations too. It may be very important to use an external power supply for the CardBus adapter. It seems like CardBus can't supply all of the power needed by many devices.

If you're using a CardBus adapter with an older driver version, you may need to manually remove the driver (with rmmod) before a CardBus eject (including cardctl eject) or system shutdown; the CardBus shutdown doesn't shut down drivers when it should, so that's the only "clean shutdown" solution..

See the linux/Documentation/usb/ehci.txt file in your Linux kernel source distribution for some more information. As always with drivers still in development, if you have any problems, try the latest code (preferably in the 2.5 kernel, otherwise in the latest 2.4 code) to see if the problem has been fixed yet. And if not, report problems to the linux-usb-devel mailing list.

Device Driver Support

High speed storage devices, and to some extent hubs, seem to be all the USB 2.0 devices that most folk have so far. High speed scanners and printers are available, and there are early reports of success using them. (If you're interested in what USB 2.0 devices are available on the market, http://usb-20.com is a website specializing in selling you such devices. A site that's been established a bit longer is http://www.usbstuff.com, which also includes USB 1.1 products.)

Most full and Low speed (USB 1.1) devices will work when connected to USB 2.0 root hubs, using their current drivers, since they'll be handled by the companion controllers. Many of them should also work when connected to USB 2.0 external hubs, subject to the restrictions desribed earlier. (Which, for the latest code, amounts to no webcam or audio support yet.)

Device Driver Comments
USB 2.0 hubs hub (usbcore) USB 2.0 hubs work, but haven't been widely stressed (especially in terms of faults happening during transaction translation). Warning: don't unplug these hubs with the 2.4.19 code ... a patch is of course available!
Disks, CD-RW, etc usb-storage Linux USB 2.0 support seems to work pretty well for the usb-storage devices that now exist, though it's slower on 2.4 since the usb-storage driver does not queue its USB requests. (Where "slower" still tops 10 MByte/sec on most hardware.) Starting with about 2.5.45, storage uses the new usb_sg_submit() call to queue all I/Os in a scatterlist at once, so nothing should be in the way of letting your hardware achieve 20+ MByte/sec (except maybe bugs that aren't yet fixed).
BackPack USB Drives usb-storage Micro-Solutions has provided software to set up their devices with hotplugging, so that you can easily connect theeir drives to Linux. Available in several formats, including RPM.
Archos MP3 Jukebox usb-storage Older kernels may need a tweak to usb-storage error handling, or a workaround: start the Archos and let it boot up before you load or hotplug the usb-storage module..
D-Link DUB-E100 driver needed This 10/100 Ethernet adapter should give much better throughput than similar full-speed products. One would expect to be able to saturate more than one 100BaseT LAN from the same USB 2.0 controller.
IOGear Host-to-Host Link usbnet (patch needed) Press Release
Orange iBOT2 WebCam ovfx2, evolved from ov511 This is the first USB 2.0 webcam announced.
Cypress EZ-USB FX2 custom ... depends on what your firmware makes it do These are 8051 microcontrollers with support for high speed transfers, designed for use inside USB devices and with custom firmware. The fxload software lets you download that firmware. For the start of one interesting Linux-based example (hosted at sf.net), see the SR-1 software radio.

NetChip device controllers have also worked nicely with Linux. Their new PCI based device controller should work quite nicely as part of USB "gadgets" which embed Linux.

Regardless of what type of high speed device you use, USB hotplugging works the same as it always has on Linux.

User mode device drivers should be able to use the usbfs APIs as usual. They may be able to tell that they're talking to USB 2.0 devices by details in the device descriptors (such as 512 byte bulk packets); the Java USB APIs expose such information more directly.


References

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