1 .. _usb-hostside-api:
2
3 ===========================
4 The Linux-USB Host Side API
5 ===========================
6
7 Introduction to USB on Linux
8 ============================
9
10 A Universal Serial Bus (USB) is used to connect a host, such as a PC or
11 workstation, to a number of peripheral devices. USB uses a tree
12 structure, with the host as the root (the system's master), hubs as
13 interior nodes, and peripherals as leaves (and slaves). Modern PCs
14 support several such trees of USB devices, usually
15 a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy
16 USB 2.0 (480 MBit/s) busses just in case.
17
18 That master/slave asymmetry was designed-in for a number of reasons, one
19 being ease of use. It is not physically possible to mistake upstream and
20 downstream or it does not matter with a type C plug (or they are built into the
21 peripheral). Also, the host software doesn't need to deal with
22 distributed auto-configuration since the pre-designated master node
23 manages all that.
24
25 Kernel developers added USB support to Linux early in the 2.2 kernel
26 series and have been developing it further since then. Besides support
27 for each new generation of USB, various host controllers gained support,
28 new drivers for peripherals have been added and advanced features for latency
29 measurement and improved power management introduced.
30
31 Linux can run inside USB devices as well as on the hosts that control
32 the devices. But USB device drivers running inside those peripherals
33 don't do the same things as the ones running inside hosts, so they've
34 been given a different name: *gadget drivers*. This document does not
35 cover gadget drivers.
36
37 USB Host-Side API Model
38 =======================
39
40 Host-side drivers for USB devices talk to the "usbcore" APIs. There are
41 two. One is intended for *general-purpose* drivers (exposed through
42 driver frameworks), and the other is for drivers that are *part of the
43 core*. Such core drivers include the *hub* driver (which manages trees
44 of USB devices) and several different kinds of *host controller
45 drivers*, which control individual busses.
46
47 The device model seen by USB drivers is relatively complex.
48
49 - USB supports four kinds of data transfers (control, bulk, interrupt,
50 and isochronous). Two of them (control and bulk) use bandwidth as
51 it's available, while the other two (interrupt and isochronous) are
52 scheduled to provide guaranteed bandwidth.
53
54 - The device description model includes one or more "configurations"
55 per device, only one of which is active at a time. Devices are supposed
56 to be capable of operating at lower than their top
57 speeds and may provide a BOS descriptor showing the lowest speed they
58 remain fully operational at.
59
60 - From USB 3.0 on configurations have one or more "functions", which
61 provide a common functionality and are grouped together for purposes
62 of power management.
63
64 - Configurations or functions have one or more "interfaces", each of which may have
65 "alternate settings". Interfaces may be standardized by USB "Class"
66 specifications, or may be specific to a vendor or device.
67
68 USB device drivers actually bind to interfaces, not devices. Think of
69 them as "interface drivers", though you may not see many devices
70 where the distinction is important. *Most USB devices are simple,
71 with only one function, one configuration, one interface, and one alternate
72 setting.*
73
74 - Interfaces have one or more "endpoints", each of which supports one
75 type and direction of data transfer such as "bulk out" or "interrupt
76 in". The entire configuration may have up to sixteen endpoints in
77 each direction, allocated as needed among all the interfaces.
78
79 - Data transfer on USB is packetized; each endpoint has a maximum
80 packet size. Drivers must often be aware of conventions such as
81 flagging the end of bulk transfers using "short" (including zero
82 length) packets.
83
84 - The Linux USB API supports synchronous calls for control and bulk
85 messages. It also supports asynchronous calls for all kinds of data
86 transfer, using request structures called "URBs" (USB Request
87 Blocks).
88
89 Accordingly, the USB Core API exposed to device drivers covers quite a
90 lot of territory. You'll probably need to consult the USB 3.0
91 specification, available online from www.usb.org at no cost, as well as
92 class or device specifications.
93
94 The only host-side drivers that actually touch hardware (reading/writing
95 registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs
96 provide the same functionality through the same API. In practice, that's
97 becoming more true, but there are still differences
98 that crop up especially with fault handling on the less common controllers.
99 Different controllers don't
100 necessarily report the same aspects of failures, and recovery from
101 faults (including software-induced ones like unlinking an URB) isn't yet
102 fully consistent. Device driver authors should make a point of doing
103 disconnect testing (while the device is active) with each different host
104 controller driver, to make sure drivers don't have bugs of their own as
105 well as to make sure they aren't relying on some HCD-specific behavior.
106
107 .. _usb_chapter9:
108
109 USB-Standard Types
110 ==================
111
112 In ``include/uapi/linux/usb/ch9.h`` you will find the USB data types defined
113 in chapter 9 of the USB specification. These data types are used throughout
114 USB, and in APIs including this host side API, gadget APIs, usb character
115 devices and debugfs interfaces. That file is itself included by
116 ``include/linux/usb/ch9.h``, which also contains declarations of a few
117 utility routines for manipulating these data types; the implementations
118 are in ``drivers/usb/common/common.c``.
119
120 .. kernel-doc:: drivers/usb/common/common.c
121 :export:
122
123 In addition, some functions useful for creating debugging output are
124 defined in ``drivers/usb/common/debug.c``.
125
126 .. _usb_header:
127
128 Host-Side Data Types and Macros
129 ===============================
130
131 The host side API exposes several layers to drivers, some of which are
132 more necessary than others. These support lifecycle models for host side
133 drivers and devices, and support passing buffers through usbcore to some
134 HCD that performs the I/O for the device driver.
135
136 .. kernel-doc:: include/linux/usb.h
137 :internal:
138
139 USB Core APIs
140 =============
141
142 There are two basic I/O models in the USB API. The most elemental one is
143 asynchronous: drivers submit requests in the form of an URB, and the
144 URB's completion callback handles the next step. All USB transfer types
145 support that model, although there are special cases for control URBs
146 (which always have setup and status stages, but may not have a data
147 stage) and isochronous URBs (which allow large packets and include
148 per-packet fault reports). Built on top of that is synchronous API
149 support, where a driver calls a routine that allocates one or more URBs,
150 submits them, and waits until they complete. There are synchronous
151 wrappers for single-buffer control and bulk transfers (which are awkward
152 to use in some driver disconnect scenarios), and for scatterlist based
153 streaming i/o (bulk or interrupt).
154
155 USB drivers need to provide buffers that can be used for DMA, although
156 they don't necessarily need to provide the DMA mapping themselves. There
157 are APIs to use used when allocating DMA buffers, which can prevent use
158 of bounce buffers on some systems. In some cases, drivers may be able to
159 rely on 64bit DMA to eliminate another kind of bounce buffer.
160
161 .. kernel-doc:: drivers/usb/core/urb.c
162 :export:
163
164 .. kernel-doc:: drivers/usb/core/message.c
165 :export:
166
167 .. kernel-doc:: drivers/usb/core/file.c
168 :export:
169
170 .. kernel-doc:: drivers/usb/core/driver.c
171 :export:
172
173 .. kernel-doc:: drivers/usb/core/usb.c
174 :export:
175
176 .. kernel-doc:: drivers/usb/core/hub.c
177 :export:
178
179 Host Controller APIs
180 ====================
181
182 These APIs are only for use by host controller drivers, most of which
183 implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI
184 was one of the first interfaces, designed by Intel and also used by VIA;
185 it doesn't do much in hardware. OHCI was designed later, to have the
186 hardware do more work (bigger transfers, tracking protocol state, and so
187 on). EHCI was designed with USB 2.0; its design has features that
188 resemble OHCI (hardware does much more work) as well as UHCI (some parts
189 of ISO support, TD list processing). XHCI was designed with USB 3.0. It
190 continues to shift support for functionality into hardware.
191
192 There are host controllers other than the "big three", although most PCI
193 based controllers (and a few non-PCI based ones) use one of those
194 interfaces. Not all host controllers use DMA; some use PIO, and there is
195 also a simulator and a virtual host controller to pipe USB over the network.
196
197 The same basic APIs are available to drivers for all those controllers.
198 For historical reasons they are in two layers: :c:type:`struct
199 usb_bus <usb_bus>` is a rather thin layer that became available
200 in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>`
201 is a more featureful layer
202 that lets HCDs share common code, to shrink driver size and
203 significantly reduce hcd-specific behaviors.
204
205 .. kernel-doc:: drivers/usb/core/hcd.c
206 :export:
207
208 .. kernel-doc:: drivers/usb/core/hcd-pci.c
209 :export:
210
211 .. kernel-doc:: drivers/usb/core/buffer.c
212 :internal:
213
214 The USB character device nodes
215 ==============================
216
217 This chapter presents the Linux character device nodes. You may prefer
218 to avoid writing new kernel code for your USB driver. User mode device
219 drivers are usually packaged as applications or libraries, and may use
220 character devices through some programming library that wraps it.
221 Such libraries include:
222
223 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and
224 - `jUSB <http://jUSB.sourceforge.net>`__ for Java.
225
226 Some old information about it can be seen at the "USB Device Filesystem"
227 section of the USB Guide. The latest copy of the USB Guide can be found
228 at http://www.linux-usb.org/
229
230 .. note::
231
232 - They were used to be implemented via *usbfs*, but this is not part of
233 the sysfs debug interface.
234
235 - This particular documentation is incomplete, especially with respect
236 to the asynchronous mode. As of kernel 2.5.66 the code and this
237 (new) documentation need to be cross-reviewed.
238
239 What files are in "devtmpfs"?
240 -----------------------------
241
242 Conventionally mounted at ``/dev/bus/usb/``, usbfs features include:
243
244 - ``/dev/bus/usb/BBB/DDD`` ... magic files exposing the each device's
245 configuration descriptors, and supporting a series of ioctls for
246 making device requests, including I/O to devices. (Purely for access
247 by programs.)
248
249 Each bus is given a number (``BBB``) based on when it was enumerated; within
250 each bus, each device is given a similar number (``DDD``). Those ``BBB/DDD``
251 paths are not "stable" identifiers; expect them to change even if you
252 always leave the devices plugged in to the same hub port. *Don't even
253 think of saving these in application configuration files.* Stable
254 identifiers are available, for user mode applications that want to use
255 them. HID and networking devices expose these stable IDs, so that for
256 example you can be sure that you told the right UPS to power down its
257 second server. Pleast note that it doesn't (yet) expose those IDs.
258
259 /dev/bus/usb/BBB/DDD
260 --------------------
261
262 Use these files in one of these basic ways:
263
264 - *They can be read,* producing first the device descriptor (18 bytes) and
265 then the descriptors for the current configuration. See the USB 2.0 spec
266 for details about those binary data formats. You'll need to convert most
267 multibyte values from little endian format to your native host byte
268 order, although a few of the fields in the device descriptor (both of
269 the BCD-encoded fields, and the vendor and product IDs) will be
270 byteswapped for you. Note that configuration descriptors include
271 descriptors for interfaces, altsettings, endpoints, and maybe additional
272 class descriptors.
273
274 - *Perform USB operations* using *ioctl()* requests to make endpoint I/O
275 requests (synchronously or asynchronously) or manage the device. These
276 requests need the ``CAP_SYS_RAWIO`` capability, as well as filesystem
277 access permissions. Only one ioctl request can be made on one of these
278 device files at a time. This means that if you are synchronously reading
279 an endpoint from one thread, you won't be able to write to a different
280 endpoint from another thread until the read completes. This works for
281 *half duplex* protocols, but otherwise you'd use asynchronous i/o
282 requests.
283
284 Each connected USB device has one file. The ``BBB`` indicates the bus
285 number. The ``DDD`` indicates the device address on that bus. Both
286 of these numbers are assigned sequentially, and can be reused, so
287 you can't rely on them for stable access to devices. For example,
288 it's relatively common for devices to re-enumerate while they are
289 still connected (perhaps someone jostled their power supply, hub,
290 or USB cable), so a device might be ``002/027`` when you first connect
291 it and ``002/048`` sometime later.
292
293 These files can be read as binary data. The binary data consists
294 of first the device descriptor, then the descriptors for each
295 configuration of the device. Multi-byte fields in the device descriptor
296 are converted to host endianness by the kernel. The configuration
297 descriptors are in bus endian format! The configuration descriptor
298 are wTotalLength bytes apart. If a device returns less configuration
299 descriptor data than indicated by wTotalLength there will be a hole in
300 the file for the missing bytes. This information is also shown
301 in text form by the ``/sys/kernel/debug/usb/devices`` file, described later.
302
303 These files may also be used to write user-level drivers for the USB
304 devices. You would open the ``/dev/bus/usb/BBB/DDD`` file read/write,
305 read its descriptors to make sure it's the device you expect, and then
306 bind to an interface (or perhaps several) using an ioctl call. You
307 would issue more ioctls to the device to communicate to it using
308 control, bulk, or other kinds of USB transfers. The IOCTLs are
309 listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the
310 source code (``linux/drivers/usb/core/devio.c``) is the primary reference
311 for how to access devices through those files.
312
313 Note that since by default these ``BBB/DDD`` files are writable only by
314 root, only root can write such user mode drivers. You can selectively
315 grant read/write permissions to other users by using ``chmod``. Also,
316 usbfs mount options such as ``devmode=0666`` may be helpful.
317
318
319 Life Cycle of User Mode Drivers
320 -------------------------------
321
322 Such a driver first needs to find a device file for a device it knows
323 how to handle. Maybe it was told about it because a ``/sbin/hotplug``
324 event handling agent chose that driver to handle the new device. Or
325 maybe it's an application that scans all the ``/dev/bus/usb`` device files,
326 and ignores most devices. In either case, it should :c:func:`read()`
327 all the descriptors from the device file, and check them against what it
328 knows how to handle. It might just reject everything except a particular
329 vendor and product ID, or need a more complex policy.
330
331 Never assume there will only be one such device on the system at a time!
332 If your code can't handle more than one device at a time, at least
333 detect when there's more than one, and have your users choose which
334 device to use.
335
336 Once your user mode driver knows what device to use, it interacts with
337 it in either of two styles. The simple style is to make only control
338 requests; some devices don't need more complex interactions than those.
339 (An example might be software using vendor-specific control requests for
340 some initialization or configuration tasks, with a kernel driver for the
341 rest.)
342
343 More likely, you need a more complex style driver: one using non-control
344 endpoints, reading or writing data and claiming exclusive use of an
345 interface. *Bulk* transfers are easiest to use, but only their sibling
346 *interrupt* transfers work with low speed devices. Both interrupt and
347 *isochronous* transfers offer service guarantees because their bandwidth
348 is reserved. Such "periodic" transfers are awkward to use through usbfs,
349 unless you're using the asynchronous calls. However, interrupt transfers
350 can also be used in a synchronous "one shot" style.
351
352 Your user-mode driver should never need to worry about cleaning up
353 request state when the device is disconnected, although it should close
354 its open file descriptors as soon as it starts seeing the ENODEV errors.
355
356 The ioctl() Requests
357 --------------------
358
359 To use these ioctls, you need to include the following headers in your
360 userspace program::
361
362 #include <linux/usb.h>
363 #include <linux/usbdevice_fs.h>
364 #include <asm/byteorder.h>
365
366 The standard USB device model requests, from "Chapter 9" of the USB 2.0
367 specification, are automatically included from the ``<linux/usb/ch9.h>``
368 header.
369
370 Unless noted otherwise, the ioctl requests described here will update
371 the modification time on the usbfs file to which they are applied
372 (unless they fail). A return of zero indicates success; otherwise, a
373 standard USB error code is returned (These are documented in
374 :ref:`usb-error-codes`).
375
376 Each of these files multiplexes access to several I/O streams, one per
377 endpoint. Each device has one control endpoint (endpoint zero) which
378 supports a limited RPC style RPC access. Devices are configured by
379 hub_wq (in the kernel) setting a device-wide *configuration* that
380 affects things like power consumption and basic functionality. The
381 endpoints are part of USB *interfaces*, which may have *altsettings*
382 affecting things like which endpoints are available. Many devices only
383 have a single configuration and interface, so drivers for them will
384 ignore configurations and altsettings.
385
386 Management/Status Requests
387 ~~~~~~~~~~~~~~~~~~~~~~~~~~
388
389 A number of usbfs requests don't deal very directly with device I/O.
390 They mostly relate to device management and status. These are all
391 synchronous requests.
392
393 USBDEVFS_CLAIMINTERFACE
394 This is used to force usbfs to claim a specific interface, which has
395 not previously been claimed by usbfs or any other kernel driver. The
396 ioctl parameter is an integer holding the number of the interface
397 (bInterfaceNumber from descriptor).
398
399 Note that if your driver doesn't claim an interface before trying to
400 use one of its endpoints, and no other driver has bound to it, then
401 the interface is automatically claimed by usbfs.
402
403 This claim will be released by a RELEASEINTERFACE ioctl, or by
404 closing the file descriptor. File modification time is not updated
405 by this request.
406
407 USBDEVFS_CONNECTINFO
408 Says whether the device is lowspeed. The ioctl parameter points to a
409 structure like this::
410
411 struct usbdevfs_connectinfo {
412 unsigned int devnum;
413 unsigned char slow;
414 };
415
416 File modification time is not updated by this request.
417
418 *You can't tell whether a "not slow" device is connected at high
419 speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should
420 know the devnum value already, it's the DDD value of the device file
421 name.
422
423 USBDEVFS_GET_SPEED
424 Returns the speed of the device. The speed is returned as a
425 numerical value in accordance with enum usb_device_speed
426
427 File modification time is not updated by this request.
428
429 USBDEVFS_GETDRIVER
430 Returns the name of the kernel driver bound to a given interface (a
431 string). Parameter is a pointer to this structure, which is
432 modified::
433
434 struct usbdevfs_getdriver {
435 unsigned int interface;
436 char driver[USBDEVFS_MAXDRIVERNAME + 1];
437 };
438
439 File modification time is not updated by this request.
440
441 USBDEVFS_IOCTL
442 Passes a request from userspace through to a kernel driver that has
443 an ioctl entry in the *struct usb_driver* it registered::
444
445 struct usbdevfs_ioctl {
446 int ifno;
447 int ioctl_code;
448 void *data;
449 };
450
451 /* user mode call looks like this.
452 * 'request' becomes the driver->ioctl() 'code' parameter.
453 * the size of 'param' is encoded in 'request', and that data
454 * is copied to or from the driver->ioctl() 'buf' parameter.
455 */
456 static int
457 usbdev_ioctl (int fd, int ifno, unsigned request, void *param)
458 {
459 struct usbdevfs_ioctl wrapper;
460
461 wrapper.ifno = ifno;
462 wrapper.ioctl_code = request;
463 wrapper.data = param;
464
465 return ioctl (fd, USBDEVFS_IOCTL, &wrapper);
466 }
467
468 File modification time is not updated by this request.
469
470 This request lets kernel drivers talk to user mode code through
471 filesystem operations even when they don't create a character or
472 block special device. It's also been used to do things like ask
473 devices what device special file should be used. Two pre-defined
474 ioctls are used to disconnect and reconnect kernel drivers, so that
475 user mode code can completely manage binding and configuration of
476 devices.
477
478 USBDEVFS_RELEASEINTERFACE
479 This is used to release the claim usbfs made on interface, either
480 implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the
481 file descriptor is closed. The ioctl parameter is an integer holding
482 the number of the interface (bInterfaceNumber from descriptor); File
483 modification time is not updated by this request.
484
485 .. warning::
486
487 *No security check is made to ensure that the task which made
488 the claim is the one which is releasing it. This means that user
489 mode driver may interfere other ones.*
490
491 USBDEVFS_RESETEP
492 Resets the data toggle value for an endpoint (bulk or interrupt) to
493 DATA0. The ioctl parameter is an integer endpoint number (1 to 15,
494 as identified in the endpoint descriptor), with USB_DIR_IN added
495 if the device's endpoint sends data to the host.
496
497 .. Warning::
498
499 *Avoid using this request. It should probably be removed.* Using
500 it typically means the device and driver will lose toggle
501 synchronization. If you really lost synchronization, you likely
502 need to completely handshake with the device, using a request
503 like CLEAR_HALT or SET_INTERFACE.
504
505 USBDEVFS_DROP_PRIVILEGES
506 This is used to relinquish the ability to do certain operations
507 which are considered to be privileged on a usbfs file descriptor.
508 This includes claiming arbitrary interfaces, resetting a device on
509 which there are currently claimed interfaces from other users, and
510 issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask
511 of interfaces the user is allowed to claim on this file descriptor.
512 You may issue this ioctl more than one time to narrow said mask.
513
514 Synchronous I/O Support
515 ~~~~~~~~~~~~~~~~~~~~~~~
516
517 Synchronous requests involve the kernel blocking until the user mode
518 request completes, either by finishing successfully or by reporting an
519 error. In most cases this is the simplest way to use usbfs, although as
520 noted above it does prevent performing I/O to more than one endpoint at
521 a time.
522
523 USBDEVFS_BULK
524 Issues a bulk read or write request to the device. The ioctl
525 parameter is a pointer to this structure::
526
527 struct usbdevfs_bulktransfer {
528 unsigned int ep;
529 unsigned int len;
530 unsigned int timeout; /* in milliseconds */
531 void *data;
532 };
533
534 The ``ep`` value identifies a bulk endpoint number (1 to 15, as
535 identified in an endpoint descriptor), masked with USB_DIR_IN when
536 referring to an endpoint which sends data to the host from the
537 device. The length of the data buffer is identified by ``len``; Recent
538 kernels support requests up to about 128KBytes. *FIXME say how read
539 length is returned, and how short reads are handled.*.
540
541 USBDEVFS_CLEAR_HALT
542 Clears endpoint halt (stall) and resets the endpoint toggle. This is
543 only meaningful for bulk or interrupt endpoints. The ioctl parameter
544 is an integer endpoint number (1 to 15, as identified in an endpoint
545 descriptor), masked with USB_DIR_IN when referring to an endpoint
546 which sends data to the host from the device.
547
548 Use this on bulk or interrupt endpoints which have stalled,
549 returning ``-EPIPE`` status to a data transfer request. Do not issue
550 the control request directly, since that could invalidate the host's
551 record of the data toggle.
552
553 USBDEVFS_CONTROL
554 Issues a control request to the device. The ioctl parameter points
555 to a structure like this::
556
557 struct usbdevfs_ctrltransfer {
558 __u8 bRequestType;
559 __u8 bRequest;
560 __u16 wValue;
561 __u16 wIndex;
562 __u16 wLength;
563 __u32 timeout; /* in milliseconds */
564 void *data;
565 };
566
567 The first eight bytes of this structure are the contents of the
568 SETUP packet to be sent to the device; see the USB 2.0 specification
569 for details. The bRequestType value is composed by combining a
570 ``USB_TYPE_*`` value, a ``USB_DIR_*`` value, and a ``USB_RECIP_*``
571 value (from ``linux/usb.h``). If wLength is nonzero, it describes
572 the length of the data buffer, which is either written to the device
573 (USB_DIR_OUT) or read from the device (USB_DIR_IN).
574
575 At this writing, you can't transfer more than 4 KBytes of data to or
576 from a device; usbfs has a limit, and some host controller drivers
577 have a limit. (That's not usually a problem.) *Also* there's no way
578 to say it's not OK to get a short read back from the device.
579
580 USBDEVFS_RESET
581 Does a USB level device reset. The ioctl parameter is ignored. After
582 the reset, this rebinds all device interfaces. File modification
583 time is not updated by this request.
584
585 .. warning::
586
587 *Avoid using this call* until some usbcore bugs get fixed, since
588 it does not fully synchronize device, interface, and driver (not
589 just usbfs) state.
590
591 USBDEVFS_SETINTERFACE
592 Sets the alternate setting for an interface. The ioctl parameter is
593 a pointer to a structure like this::
594
595 struct usbdevfs_setinterface {
596 unsigned int interface;
597 unsigned int altsetting;
598 };
599
600 File modification time is not updated by this request.
601
602 Those struct members are from some interface descriptor applying to
603 the current configuration. The interface number is the
604 bInterfaceNumber value, and the altsetting number is the
605 bAlternateSetting value. (This resets each endpoint in the
606 interface.)
607
608 USBDEVFS_SETCONFIGURATION
609 Issues the :c:func:`usb_set_configuration()` call for the
610 device. The parameter is an integer holding the number of a
611 configuration (bConfigurationValue from descriptor). File
612 modification time is not updated by this request.
613
614 .. warning::
615
616 *Avoid using this call* until some usbcore bugs get fixed, since
617 it does not fully synchronize device, interface, and driver (not
618 just usbfs) state.
619
620 Asynchronous I/O Support
621 ~~~~~~~~~~~~~~~~~~~~~~~~
622
623 As mentioned above, there are situations where it may be important to
624 initiate concurrent operations from user mode code. This is particularly
625 important for periodic transfers (interrupt and isochronous), but it can
626 be used for other kinds of USB requests too. In such cases, the
627 asynchronous requests described here are essential. Rather than
628 submitting one request and having the kernel block until it completes,
629 the blocking is separate.
630
631 These requests are packaged into a structure that resembles the URB used
632 by kernel device drivers. (No POSIX Async I/O support here, sorry.) It
633 identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint
634 (number, masked with USB_DIR_IN as appropriate), buffer and length,
635 and a user "context" value serving to uniquely identify each request.
636 (It's usually a pointer to per-request data.) Flags can modify requests
637 (not as many as supported for kernel drivers).
638
639 Each request can specify a realtime signal number (between SIGRTMIN and
640 SIGRTMAX, inclusive) to request a signal be sent when the request
641 completes.
642
643 When usbfs returns these urbs, the status value is updated, and the
644 buffer may have been modified. Except for isochronous transfers, the
645 actual_length is updated to say how many bytes were transferred; if the
646 USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if
647 fewer bytes were read than were requested then you get an error report::
648
649 struct usbdevfs_iso_packet_desc {
650 unsigned int length;
651 unsigned int actual_length;
652 unsigned int status;
653 };
654
655 struct usbdevfs_urb {
656 unsigned char type;
657 unsigned char endpoint;
658 int status;
659 unsigned int flags;
660 void *buffer;
661 int buffer_length;
662 int actual_length;
663 int start_frame;
664 int number_of_packets;
665 int error_count;
666 unsigned int signr;
667 void *usercontext;
668 struct usbdevfs_iso_packet_desc iso_frame_desc[];
669 };
670
671 For these asynchronous requests, the file modification time reflects
672 when the request was initiated. This contrasts with their use with the
673 synchronous requests, where it reflects when requests complete.
674
675 USBDEVFS_DISCARDURB
676 *TBS* File modification time is not updated by this request.
677
678 USBDEVFS_DISCSIGNAL
679 *TBS* File modification time is not updated by this request.
680
681 USBDEVFS_REAPURB
682 *TBS* File modification time is not updated by this request.
683
684 USBDEVFS_REAPURBNDELAY
685 *TBS* File modification time is not updated by this request.
686
687 USBDEVFS_SUBMITURB
688 *TBS*
689
690 The USB devices
691 ===============
692
693 The USB devices are now exported via debugfs:
694
695 - ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB
696 devices on known to the kernel, and their configuration descriptors.
697 You can also poll() this to learn about new devices.
698
699 /sys/kernel/debug/usb/devices
700 -----------------------------
701
702 This file is handy for status viewing tools in user mode, which can scan
703 the text format and ignore most of it. More detailed device status
704 (including class and vendor status) is available from device-specific
705 files. For information about the current format of this file, see below.
706
707 This file, in combination with the poll() system call, can also be used
708 to detect when devices are added or removed::
709
710 int fd;
711 struct pollfd pfd;
712
713 fd = open("/sys/kernel/debug/usb/devices", O_RDONLY);
714 pfd = { fd, POLLIN, 0 };
715 for (;;) {
716 /* The first time through, this call will return immediately. */
717 poll(&pfd, 1, -1);
718
719 /* To see what's changed, compare the file's previous and current
720 contents or scan the filesystem. (Scanning is more precise.) */
721 }
722
723 Note that this behavior is intended to be used for informational and
724 debug purposes. It would be more appropriate to use programs such as
725 udev or HAL to initialize a device or start a user-mode helper program,
726 for instance.
727
728 In this file, each device's output has multiple lines of ASCII output.
729
730 I made it ASCII instead of binary on purpose, so that someone
731 can obtain some useful data from it without the use of an
732 auxiliary program. However, with an auxiliary program, the numbers
733 in the first 4 columns of each ``T:`` line (topology info:
734 Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram.
735
736 Each line is tagged with a one-character ID for that line::
737
738 T = Topology (etc.)
739 B = Bandwidth (applies only to USB host controllers, which are
740 virtualized as root hubs)
741 D = Device descriptor info.
742 P = Product ID info. (from Device descriptor, but they won't fit
743 together on one line)
744 S = String descriptors.
745 C = Configuration descriptor info. (* = active configuration)
746 I = Interface descriptor info.
747 E = Endpoint descriptor info.
748
749 /sys/kernel/debug/usb/devices output format
750 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
751
752 Legend::
753 d = decimal number (may have leading spaces or 0's)
754 x = hexadecimal number (may have leading spaces or 0's)
755 s = string
756
757
758
759 Topology info
760 ^^^^^^^^^^^^^
761
762 ::
763
764 T: Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd
765 | | | | | | | | |__MaxChildren
766 | | | | | | | |__Device Speed in Mbps
767 | | | | | | |__DeviceNumber
768 | | | | | |__Count of devices at this level
769 | | | | |__Connector/Port on Parent for this device
770 | | | |__Parent DeviceNumber
771 | | |__Level in topology for this bus
772 | |__Bus number
773 |__Topology info tag
774
775 Speed may be:
776
777 ======= ======================================================
778 1.5 Mbit/s for low speed USB
779 12 Mbit/s for full speed USB
780 480 Mbit/s for high speed USB (added for USB 2.0)
781 5000 Mbit/s for SuperSpeed USB (added for USB 3.0)
782 ======= ======================================================
783
784 For reasons lost in the mists of time, the Port number is always
785 too low by 1. For example, a device plugged into port 4 will
786 show up with ``Port=03``.
787
788 Bandwidth info
789 ^^^^^^^^^^^^^^
790
791 ::
792
793 B: Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd
794 | | | |__Number of isochronous requests
795 | | |__Number of interrupt requests
796 | |__Total Bandwidth allocated to this bus
797 |__Bandwidth info tag
798
799 Bandwidth allocation is an approximation of how much of one frame
800 (millisecond) is in use. It reflects only periodic transfers, which
801 are the only transfers that reserve bandwidth. Control and bulk
802 transfers use all other bandwidth, including reserved bandwidth that
803 is not used for transfers (such as for short packets).
804
805 The percentage is how much of the "reserved" bandwidth is scheduled by
806 those transfers. For a low or full speed bus (loosely, "USB 1.1"),
807 90% of the bus bandwidth is reserved. For a high speed bus (loosely,
808 "USB 2.0") 80% is reserved.
809
810
811 Device descriptor info & Product ID info
812 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
813
814 ::
815
816 D: Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
817 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx
818
819 where::
820
821 D: Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
822 | | | | | | |__NumberConfigurations
823 | | | | | |__MaxPacketSize of Default Endpoint
824 | | | | |__DeviceProtocol
825 | | | |__DeviceSubClass
826 | | |__DeviceClass
827 | |__Device USB version
828 |__Device info tag #1
829
830 where::
831
832 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx
833 | | | |__Product revision number
834 | | |__Product ID code
835 | |__Vendor ID code
836 |__Device info tag #2
837
838
839 String descriptor info
840 ^^^^^^^^^^^^^^^^^^^^^^
841 ::
842
843 S: Manufacturer=ssss
844 | |__Manufacturer of this device as read from the device.
845 | For USB host controller drivers (virtual root hubs) this may
846 | be omitted, or (for newer drivers) will identify the kernel
847 | version and the driver which provides this hub emulation.
848 |__String info tag
849
850 S: Product=ssss
851 | |__Product description of this device as read from the device.
852 | For older USB host controller drivers (virtual root hubs) this
853 | indicates the driver; for newer ones, it's a product (and vendor)
854 | description that often comes from the kernel's PCI ID database.
855 |__String info tag
856
857 S: SerialNumber=ssss
858 | |__Serial Number of this device as read from the device.
859 | For USB host controller drivers (virtual root hubs) this is
860 | some unique ID, normally a bus ID (address or slot name) that
861 | can't be shared with any other device.
862 |__String info tag
863
864
865
866 Configuration descriptor info
867 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
868 ::
869
870 C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA
871 | | | | | |__MaxPower in mA
872 | | | | |__Attributes
873 | | | |__ConfiguratioNumber
874 | | |__NumberOfInterfaces
875 | |__ "*" indicates the active configuration (others are " ")
876 |__Config info tag
877
878 USB devices may have multiple configurations, each of which act
879 rather differently. For example, a bus-powered configuration
880 might be much less capable than one that is self-powered. Only
881 one device configuration can be active at a time; most devices
882 have only one configuration.
883
884 Each configuration consists of one or more interfaces. Each
885 interface serves a distinct "function", which is typically bound
886 to a different USB device driver. One common example is a USB
887 speaker with an audio interface for playback, and a HID interface
888 for use with software volume control.
889
890 Interface descriptor info (can be multiple per Config)
891 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
892 ::
893
894 I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss
895 | | | | | | | | |__Driver name
896 | | | | | | | | or "(none)"
897 | | | | | | | |__InterfaceProtocol
898 | | | | | | |__InterfaceSubClass
899 | | | | | |__InterfaceClass
900 | | | | |__NumberOfEndpoints
901 | | | |__AlternateSettingNumber
902 | | |__InterfaceNumber
903 | |__ "*" indicates the active altsetting (others are " ")
904 |__Interface info tag
905
906 A given interface may have one or more "alternate" settings.
907 For example, default settings may not use more than a small
908 amount of periodic bandwidth. To use significant fractions
909 of bus bandwidth, drivers must select a non-default altsetting.
910
911 Only one setting for an interface may be active at a time, and
912 only one driver may bind to an interface at a time. Most devices
913 have only one alternate setting per interface.
914
915
916 Endpoint descriptor info (can be multiple per Interface)
917 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
918
919 ::
920
921 E: Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss
922 | | | | |__Interval (max) between transfers
923 | | | |__EndpointMaxPacketSize
924 | | |__Attributes(EndpointType)
925 | |__EndpointAddress(I=In,O=Out)
926 |__Endpoint info tag
927
928 The interval is nonzero for all periodic (interrupt or isochronous)
929 endpoints. For high speed endpoints the transfer interval may be
930 measured in microseconds rather than milliseconds.
931
932 For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects
933 the per-microframe data transfer size. For "high bandwidth"
934 endpoints, that can reflect two or three packets (for up to
935 3KBytes every 125 usec) per endpoint.
936
937 With the Linux-USB stack, periodic bandwidth reservations use the
938 transfer intervals and sizes provided by URBs, which can be less
939 than those found in endpoint descriptor.
940
941 Usage examples
942 ~~~~~~~~~~~~~~
943
944 If a user or script is interested only in Topology info, for
945 example, use something like ``grep ^T: /sys/kernel/debug/usb/devices``
946 for only the Topology lines. A command like
947 ``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list
948 only the lines that begin with the characters in square brackets,
949 where the valid characters are TDPCIE. With a slightly more able
950 script, it can display any selected lines (for example, only T, D,
951 and P lines) and change their output format. (The ``procusb``
952 Perl script is the beginning of this idea. It will list only
953 selected lines [selected from TBDPSCIE] or "All" lines from
954 ``/sys/kernel/debug/usb/devices``.)
955
956 The Topology lines can be used to generate a graphic/pictorial
957 of the USB devices on a system's root hub. (See more below
958 on how to do this.)
959
960 The Interface lines can be used to determine what driver is
961 being used for each device, and which altsetting it activated.
962
963 The Configuration lines could be used to list maximum power
964 (in milliamps) that a system's USB devices are using.
965 For example, ``grep ^C: /sys/kernel/debug/usb/devices``.
966
967
968 Here's an example, from a system which has a UHCI root hub,
969 an external hub connected to the root hub, and a mouse and
970 a serial converter connected to the external hub.
971
972 ::
973
974 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2
975 B: Alloc= 28/900 us ( 3%), #Int= 2, #Iso= 0
976 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
977 P: Vendor=0000 ProdID=0000 Rev= 0.00
978 S: Product=USB UHCI Root Hub
979 S: SerialNumber=dce0
980 C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr= 0mA
981 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub
982 E: Ad=81(I) Atr=03(Int.) MxPS= 8 Ivl=255ms
983
984 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4
985 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
986 P: Vendor=0451 ProdID=1446 Rev= 1.00
987 C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA
988 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub
989 E: Ad=81(I) Atr=03(Int.) MxPS= 1 Ivl=255ms
990
991 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0
992 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
993 P: Vendor=04b4 ProdID=0001 Rev= 0.00
994 C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA
995 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse
996 E: Ad=81(I) Atr=03(Int.) MxPS= 3 Ivl= 10ms
997
998 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0
999 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
1000 P: Vendor=0565 ProdID=0001 Rev= 1.08
1001 S: Manufacturer=Peracom Networks, Inc.
1002 S: Product=Peracom USB to Serial Converter
1003 C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA
1004 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial
1005 E: Ad=81(I) Atr=02(Bulk) MxPS= 64 Ivl= 16ms
1006 E: Ad=01(O) Atr=02(Bulk) MxPS= 16 Ivl= 16ms
1007 E: Ad=82(I) Atr=03(Int.) MxPS= 8 Ivl= 8ms
1008
1009
1010 Selecting only the ``T:`` and ``I:`` lines from this (for example, by using
1011 ``procusb ti``), we have
1012
1013 ::
1014
1015 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2
1016 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4
1017 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub
1018 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0
1019 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse
1020 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0
1021 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial
1022
1023
1024 Physically this looks like (or could be converted to)::
1025
1026 +------------------+
1027 | PC/root_hub (12)| Dev# = 1
1028 +------------------+ (nn) is Mbps.
1029 Level 0 | CN.0 | CN.1 | [CN = connector/port #]
1030 +------------------+
1031 /
1032 /
1033 +-----------------------+
1034 Level 1 | Dev#2: 4-port hub (12)|
1035 +-----------------------+
1036 |CN.0 |CN.1 |CN.2 |CN.3 |
1037 +-----------------------+
1038 \ \____________________
1039 \_____ \
1040 \ \
1041 +--------------------+ +--------------------+
1042 Level 2 | Dev# 3: mouse (1.5)| | Dev# 4: serial (12)|
1043 +--------------------+ +--------------------+
1044
1045
1046
1047 Or, in a more tree-like structure (ports [Connectors] without
1048 connections could be omitted)::
1049
1050 PC: Dev# 1, root hub, 2 ports, 12 Mbps
1051 |_ CN.0: Dev# 2, hub, 4 ports, 12 Mbps
1052 |_ CN.0: Dev #3, mouse, 1.5 Mbps
1053 |_ CN.1:
1054 |_ CN.2: Dev #4, serial, 12 Mbps
1055 |_ CN.3:
1056 |_ CN.1:
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