1.. _usb-hostside-api:
2
3===========================
4The Linux-USB Host Side API
5===========================
6
7Introduction to USB on Linux
8============================
9
10A Universal Serial Bus (USB) is used to connect a host, such as a PC or
11workstation, to a number of peripheral devices. USB uses a tree
12structure, with the host as the root (the system's master), hubs as
13interior nodes, and peripherals as leaves (and slaves). Modern PCs
14support several such trees of USB devices, usually
15a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy
16USB 2.0 (480 MBit/s) busses just in case.
17
18That master/slave asymmetry was designed-in for a number of reasons, one
19being ease of use. It is not physically possible to mistake upstream and
20downstream or it does not matter with a type C plug (or they are built into the
21peripheral). Also, the host software doesn't need to deal with
22distributed auto-configuration since the pre-designated master node
23manages all that.
24
25Kernel developers added USB support to Linux early in the 2.2 kernel
26series and have been developing it further since then. Besides support
27for each new generation of USB, various host controllers gained support,
28new drivers for peripherals have been added and advanced features for latency
29measurement and improved power management introduced.
30
31Linux can run inside USB devices as well as on the hosts that control
32the devices. But USB device drivers running inside those peripherals
33don't do the same things as the ones running inside hosts, so they've
34been given a different name: *gadget drivers*. This document does not
35cover gadget drivers.
36
37USB Host-Side API Model
38=======================
39
40Host-side drivers for USB devices talk to the "usbcore" APIs. There are
41two. One is intended for *general-purpose* drivers (exposed through
42driver frameworks), and the other is for drivers that are *part of the
43core*. Such core drivers include the *hub* driver (which manages trees
44of USB devices) and several different kinds of *host controller
45drivers*, which control individual busses.
46
47The 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
89Accordingly, the USB Core API exposed to device drivers covers quite a
90lot of territory. You'll probably need to consult the USB 3.0
91specification, available online from www.usb.org at no cost, as well as
92class or device specifications.
93
94The only host-side drivers that actually touch hardware (reading/writing
95registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs
96provide the same functionality through the same API. In practice, that's
97becoming more true, but there are still differences
98that crop up especially with fault handling on the less common controllers.
99Different controllers don't
100necessarily report the same aspects of failures, and recovery from
101faults (including software-induced ones like unlinking an URB) isn't yet
102fully consistent. Device driver authors should make a point of doing
103disconnect testing (while the device is active) with each different host
104controller driver, to make sure drivers don't have bugs of their own as
105well as to make sure they aren't relying on some HCD-specific behavior.
106
107.. _usb_chapter9:
108
109USB-Standard Types
110==================
111
112In ``include/uapi/linux/usb/ch9.h`` you will find the USB data types defined
113in chapter 9 of the USB specification. These data types are used throughout
114USB, and in APIs including this host side API, gadget APIs, usb character
115devices and debugfs interfaces. That file is itself included by
116``include/linux/usb/ch9.h``, which also contains declarations of a few
117utility routines for manipulating these data types; the implementations
118are in ``drivers/usb/common/common.c``.
119
120.. kernel-doc:: drivers/usb/common/common.c
121   :export:
122
123In addition, some functions useful for creating debugging output are
124defined in ``drivers/usb/common/debug.c``.
125
126.. _usb_header:
127
128Host-Side Data Types and Macros
129===============================
130
131The host side API exposes several layers to drivers, some of which are
132more necessary than others. These support lifecycle models for host side
133drivers and devices, and support passing buffers through usbcore to some
134HCD that performs the I/O for the device driver.
135
136.. kernel-doc:: include/linux/usb.h
137   :internal:
138
139USB Core APIs
140=============
141
142There are two basic I/O models in the USB API. The most elemental one is
143asynchronous: drivers submit requests in the form of an URB, and the
144URB's completion callback handles the next step. All USB transfer types
145support that model, although there are special cases for control URBs
146(which always have setup and status stages, but may not have a data
147stage) and isochronous URBs (which allow large packets and include
148per-packet fault reports). Built on top of that is synchronous API
149support, where a driver calls a routine that allocates one or more URBs,
150submits them, and waits until they complete. There are synchronous
151wrappers for single-buffer control and bulk transfers (which are awkward
152to use in some driver disconnect scenarios), and for scatterlist based
153streaming i/o (bulk or interrupt).
154
155USB drivers need to provide buffers that can be used for DMA, although
156they don't necessarily need to provide the DMA mapping themselves. There
157are APIs to use used when allocating DMA buffers, which can prevent use
158of bounce buffers on some systems. In some cases, drivers may be able to
159rely 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
179Host Controller APIs
180====================
181
182These APIs are only for use by host controller drivers, most of which
183implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI
184was one of the first interfaces, designed by Intel and also used by VIA;
185it doesn't do much in hardware. OHCI was designed later, to have the
186hardware do more work (bigger transfers, tracking protocol state, and so
187on). EHCI was designed with USB 2.0; its design has features that
188resemble OHCI (hardware does much more work) as well as UHCI (some parts
189of ISO support, TD list processing). XHCI was designed with USB 3.0. It
190continues to shift support for functionality into hardware.
191
192There are host controllers other than the "big three", although most PCI
193based controllers (and a few non-PCI based ones) use one of those
194interfaces. Not all host controllers use DMA; some use PIO, and there is
195also a simulator and a virtual host controller to pipe USB over the network.
196
197The same basic APIs are available to drivers for all those controllers.
198For historical reasons they are in two layers: :c:type:`struct
199usb_bus <usb_bus>` is a rather thin layer that became available
200in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>`
201is a more featureful layer
202that lets HCDs share common code, to shrink driver size and
203significantly 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
214The USB character device nodes
215==============================
216
217This chapter presents the Linux character device nodes. You may prefer
218to avoid writing new kernel code for your USB driver. User mode device
219drivers are usually packaged as applications or libraries, and may use
220character devices through some programming library that wraps it.
221Such libraries include:
222
223 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and
224 - `jUSB <http://jUSB.sourceforge.net>`__ for Java.
225
226Some old information about it can be seen at the "USB Device Filesystem"
227section of the USB Guide. The latest copy of the USB Guide can be found
228at 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
239What files are in "devtmpfs"?
240-----------------------------
241
242Conventionally 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
249Each bus is given a number (``BBB``) based on when it was enumerated; within
250each bus, each device is given a similar number (``DDD``). Those ``BBB/DDD``
251paths are not "stable" identifiers; expect them to change even if you
252always leave the devices plugged in to the same hub port. *Don't even
253think of saving these in application configuration files.* Stable
254identifiers are available, for user mode applications that want to use
255them. HID and networking devices expose these stable IDs, so that for
256example you can be sure that you told the right UPS to power down its
257second server. Pleast note that it doesn't (yet) expose those IDs.
258
259/dev/bus/usb/BBB/DDD
260--------------------
261
262Use 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
284Each connected USB device has one file.  The ``BBB`` indicates the bus
285number.  The ``DDD`` indicates the device address on that bus.  Both
286of these numbers are assigned sequentially, and can be reused, so
287you can't rely on them for stable access to devices.  For example,
288it's relatively common for devices to re-enumerate while they are
289still connected (perhaps someone jostled their power supply, hub,
290or USB cable), so a device might be ``002/027`` when you first connect
291it and ``002/048`` sometime later.
292
293These files can be read as binary data.  The binary data consists
294of first the device descriptor, then the descriptors for each
295configuration of the device.  Multi-byte fields in the device descriptor
296are converted to host endianness by the kernel.  The configuration
297descriptors are in bus endian format! The configuration descriptor
298are wTotalLength bytes apart. If a device returns less configuration
299descriptor data than indicated by wTotalLength there will be a hole in
300the file for the missing bytes.  This information is also shown
301in text form by the ``/sys/kernel/debug/usb/devices`` file, described later.
302
303These files may also be used to write user-level drivers for the USB
304devices.  You would open the ``/dev/bus/usb/BBB/DDD`` file read/write,
305read its descriptors to make sure it's the device you expect, and then
306bind to an interface (or perhaps several) using an ioctl call.  You
307would issue more ioctls to the device to communicate to it using
308control, bulk, or other kinds of USB transfers.  The IOCTLs are
309listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the
310source code (``linux/drivers/usb/core/devio.c``) is the primary reference
311for how to access devices through those files.
312
313Note that since by default these ``BBB/DDD`` files are writable only by
314root, only root can write such user mode drivers.  You can selectively
315grant read/write permissions to other users by using ``chmod``.  Also,
316usbfs mount options such as ``devmode=0666`` may be helpful.
317
318
319Life Cycle of User Mode Drivers
320-------------------------------
321
322Such a driver first needs to find a device file for a device it knows
323how to handle. Maybe it was told about it because a ``/sbin/hotplug``
324event handling agent chose that driver to handle the new device. Or
325maybe it's an application that scans all the ``/dev/bus/usb`` device files,
326and ignores most devices. In either case, it should :c:func:`read()`
327all the descriptors from the device file, and check them against what it
328knows how to handle. It might just reject everything except a particular
329vendor and product ID, or need a more complex policy.
330
331Never assume there will only be one such device on the system at a time!
332If your code can't handle more than one device at a time, at least
333detect when there's more than one, and have your users choose which
334device to use.
335
336Once your user mode driver knows what device to use, it interacts with
337it in either of two styles. The simple style is to make only control
338requests; some devices don't need more complex interactions than those.
339(An example might be software using vendor-specific control requests for
340some initialization or configuration tasks, with a kernel driver for the
341rest.)
342
343More likely, you need a more complex style driver: one using non-control
344endpoints, reading or writing data and claiming exclusive use of an
345interface. *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
348is reserved. Such "periodic" transfers are awkward to use through usbfs,
349unless you're using the asynchronous calls. However, interrupt transfers
350can also be used in a synchronous "one shot" style.
351
352Your user-mode driver should never need to worry about cleaning up
353request state when the device is disconnected, although it should close
354its open file descriptors as soon as it starts seeing the ENODEV errors.
355
356The ioctl() Requests
357--------------------
358
359To use these ioctls, you need to include the following headers in your
360userspace program::
361
362    #include <linux/usb.h>
363    #include <linux/usbdevice_fs.h>
364    #include <asm/byteorder.h>
365
366The standard USB device model requests, from "Chapter 9" of the USB 2.0
367specification, are automatically included from the ``<linux/usb/ch9.h>``
368header.
369
370Unless noted otherwise, the ioctl requests described here will update
371the modification time on the usbfs file to which they are applied
372(unless they fail). A return of zero indicates success; otherwise, a
373standard USB error code is returned (These are documented in
374:ref:`usb-error-codes`).
375
376Each of these files multiplexes access to several I/O streams, one per
377endpoint. Each device has one control endpoint (endpoint zero) which
378supports a limited RPC style RPC access. Devices are configured by
379hub_wq (in the kernel) setting a device-wide *configuration* that
380affects things like power consumption and basic functionality. The
381endpoints are part of USB *interfaces*, which may have *altsettings*
382affecting things like which endpoints are available. Many devices only
383have a single configuration and interface, so drivers for them will
384ignore configurations and altsettings.
385
386Management/Status Requests
387~~~~~~~~~~~~~~~~~~~~~~~~~~
388
389A number of usbfs requests don't deal very directly with device I/O.
390They mostly relate to device management and status. These are all
391synchronous requests.
392
393USBDEVFS_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
407USBDEVFS_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
423USBDEVFS_GET_SPEED
424    Returns the speed of the device. The speed is returned as a
425    nummerical value in accordance with enum usb_device_speed
426
427    File modification time is not updated by this request.
428
429USBDEVFS_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
441USBDEVFS_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
478USBDEVFS_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
491USBDEVFS_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
505USBDEVFS_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
514Synchronous I/O Support
515~~~~~~~~~~~~~~~~~~~~~~~
516
517Synchronous requests involve the kernel blocking until the user mode
518request completes, either by finishing successfully or by reporting an
519error. In most cases this is the simplest way to use usbfs, although as
520noted above it does prevent performing I/O to more than one endpoint at
521a time.
522
523USBDEVFS_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
541USBDEVFS_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
553USBDEVFS_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
580USBDEVFS_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
591USBDEVFS_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
608USBDEVFS_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
620Asynchronous I/O Support
621~~~~~~~~~~~~~~~~~~~~~~~~
622
623As mentioned above, there are situations where it may be important to
624initiate concurrent operations from user mode code. This is particularly
625important for periodic transfers (interrupt and isochronous), but it can
626be used for other kinds of USB requests too. In such cases, the
627asynchronous requests described here are essential. Rather than
628submitting one request and having the kernel block until it completes,
629the blocking is separate.
630
631These requests are packaged into a structure that resembles the URB used
632by kernel device drivers. (No POSIX Async I/O support here, sorry.) It
633identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint
634(number, masked with USB_DIR_IN as appropriate), buffer and length,
635and 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
639Each request can specify a realtime signal number (between SIGRTMIN and
640SIGRTMAX, inclusive) to request a signal be sent when the request
641completes.
642
643When usbfs returns these urbs, the status value is updated, and the
644buffer may have been modified. Except for isochronous transfers, the
645actual_length is updated to say how many bytes were transferred; if the
646USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if
647fewer 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
671For these asynchronous requests, the file modification time reflects
672when the request was initiated. This contrasts with their use with the
673synchronous requests, where it reflects when requests complete.
674
675USBDEVFS_DISCARDURB
676    *TBS* File modification time is not updated by this request.
677
678USBDEVFS_DISCSIGNAL
679    *TBS* File modification time is not updated by this request.
680
681USBDEVFS_REAPURB
682    *TBS* File modification time is not updated by this request.
683
684USBDEVFS_REAPURBNDELAY
685    *TBS* File modification time is not updated by this request.
686
687USBDEVFS_SUBMITURB
688    *TBS*
689
690The USB devices
691===============
692
693The 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
702This file is handy for status viewing tools in user mode, which can scan
703the text format and ignore most of it. More detailed device status
704(including class and vendor status) is available from device-specific
705files. For information about the current format of this file, see below.
706
707This file, in combination with the poll() system call, can also be used
708to 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
723Note that this behavior is intended to be used for informational and
724debug purposes. It would be more appropriate to use programs such as
725udev or HAL to initialize a device or start a user-mode helper program,
726for instance.
727
728In this file, each device's output has multiple lines of ASCII output.
729
730I made it ASCII instead of binary on purpose, so that someone
731can obtain some useful data from it without the use of an
732auxiliary program.  However, with an auxiliary program, the numbers
733in the first 4 columns of each ``T:`` line (topology info:
734Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram.
735
736Each 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
752Legend::
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
759Topology 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
775Speed 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
784For reasons lost in the mists of time, the Port number is always
785too low by 1.  For example, a device plugged into port 4 will
786show up with ``Port=03``.
787
788Bandwidth 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
799Bandwidth allocation is an approximation of how much of one frame
800(millisecond) is in use.  It reflects only periodic transfers, which
801are the only transfers that reserve bandwidth.  Control and bulk
802transfers use all other bandwidth, including reserved bandwidth that
803is not used for transfers (such as for short packets).
804
805The percentage is how much of the "reserved" bandwidth is scheduled by
806those transfers.  For a low or full speed bus (loosely, "USB 1.1"),
80790% of the bus bandwidth is reserved.  For a high speed bus (loosely,
808"USB 2.0") 80% is reserved.
809
810
811Device 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
819where::
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
830where::
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
839String 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
866Configuration 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
878USB devices may have multiple configurations, each of which act
879rather differently.  For example, a bus-powered configuration
880might be much less capable than one that is self-powered.  Only
881one device configuration can be active at a time; most devices
882have only one configuration.
883
884Each configuration consists of one or more interfaces.  Each
885interface serves a distinct "function", which is typically bound
886to a different USB device driver.  One common example is a USB
887speaker with an audio interface for playback, and a HID interface
888for use with software volume control.
889
890Interface 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
906A given interface may have one or more "alternate" settings.
907For example, default settings may not use more than a small
908amount of periodic bandwidth.  To use significant fractions
909of bus bandwidth, drivers must select a non-default altsetting.
910
911Only one setting for an interface may be active at a time, and
912only one driver may bind to an interface at a time.  Most devices
913have only one alternate setting per interface.
914
915
916Endpoint 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
928The interval is nonzero for all periodic (interrupt or isochronous)
929endpoints.  For high speed endpoints the transfer interval may be
930measured in microseconds rather than milliseconds.
931
932For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects
933the per-microframe data transfer size.  For "high bandwidth"
934endpoints, that can reflect two or three packets (for up to
9353KBytes every 125 usec) per endpoint.
936
937With the Linux-USB stack, periodic bandwidth reservations use the
938transfer intervals and sizes provided by URBs, which can be less
939than those found in endpoint descriptor.
940
941Usage examples
942~~~~~~~~~~~~~~
943
944If a user or script is interested only in Topology info, for
945example, use something like ``grep ^T: /sys/kernel/debug/usb/devices``
946for only the Topology lines.  A command like
947``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list
948only the lines that begin with the characters in square brackets,
949where the valid characters are TDPCIE.  With a slightly more able
950script, it can display any selected lines (for example, only T, D,
951and P lines) and change their output format.  (The ``procusb``
952Perl script is the beginning of this idea.  It will list only
953selected lines [selected from TBDPSCIE] or "All" lines from
954``/sys/kernel/debug/usb/devices``.)
955
956The Topology lines can be used to generate a graphic/pictorial
957of the USB devices on a system's root hub.  (See more below
958on how to do this.)
959
960The Interface lines can be used to determine what driver is
961being used for each device, and which altsetting it activated.
962
963The Configuration lines could be used to list maximum power
964(in milliamps) that a system's USB devices are using.
965For example, ``grep ^C: /sys/kernel/debug/usb/devices``.
966
967
968Here's an example, from a system which has a UHCI root hub,
969an external hub connected to the root hub, and a mouse and
970a 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
1010Selecting 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
1024Physically 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
1047Or, in a more tree-like structure (ports [Connectors] without
1048connections 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:
1057