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.. c:namespace:: usb_core 165.. kernel-doc:: drivers/usb/core/message.c 166 :export: 167 168.. kernel-doc:: drivers/usb/core/file.c 169 :export: 170 171.. kernel-doc:: drivers/usb/core/driver.c 172 :export: 173 174.. kernel-doc:: drivers/usb/core/usb.c 175 :export: 176 177.. kernel-doc:: drivers/usb/core/hub.c 178 :export: 179 180Host Controller APIs 181==================== 182 183These APIs are only for use by host controller drivers, most of which 184implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI 185was one of the first interfaces, designed by Intel and also used by VIA; 186it doesn't do much in hardware. OHCI was designed later, to have the 187hardware do more work (bigger transfers, tracking protocol state, and so 188on). EHCI was designed with USB 2.0; its design has features that 189resemble OHCI (hardware does much more work) as well as UHCI (some parts 190of ISO support, TD list processing). XHCI was designed with USB 3.0. It 191continues to shift support for functionality into hardware. 192 193There are host controllers other than the "big three", although most PCI 194based controllers (and a few non-PCI based ones) use one of those 195interfaces. Not all host controllers use DMA; some use PIO, and there is 196also a simulator and a virtual host controller to pipe USB over the network. 197 198The same basic APIs are available to drivers for all those controllers. 199For historical reasons they are in two layers: :c:type:`struct 200usb_bus <usb_bus>` is a rather thin layer that became available 201in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>` 202is a more featureful layer 203that lets HCDs share common code, to shrink driver size and 204significantly reduce hcd-specific behaviors. 205 206.. kernel-doc:: drivers/usb/core/hcd.c 207 :export: 208 209.. kernel-doc:: drivers/usb/core/hcd-pci.c 210 :export: 211 212.. kernel-doc:: drivers/usb/core/buffer.c 213 :internal: 214 215The USB character device nodes 216============================== 217 218This chapter presents the Linux character device nodes. You may prefer 219to avoid writing new kernel code for your USB driver. User mode device 220drivers are usually packaged as applications or libraries, and may use 221character devices through some programming library that wraps it. 222Such libraries include: 223 224 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and 225 - `jUSB <http://jUSB.sourceforge.net>`__ for Java. 226 227Some old information about it can be seen at the "USB Device Filesystem" 228section of the USB Guide. The latest copy of the USB Guide can be found 229at http://www.linux-usb.org/ 230 231.. note:: 232 233 - They were used to be implemented via *usbfs*, but this is not part of 234 the sysfs debug interface. 235 236 - This particular documentation is incomplete, especially with respect 237 to the asynchronous mode. As of kernel 2.5.66 the code and this 238 (new) documentation need to be cross-reviewed. 239 240What files are in "devtmpfs"? 241----------------------------- 242 243Conventionally mounted at ``/dev/bus/usb/``, usbfs features include: 244 245- ``/dev/bus/usb/BBB/DDD`` ... magic files exposing the each device's 246 configuration descriptors, and supporting a series of ioctls for 247 making device requests, including I/O to devices. (Purely for access 248 by programs.) 249 250Each bus is given a number (``BBB``) based on when it was enumerated; within 251each bus, each device is given a similar number (``DDD``). Those ``BBB/DDD`` 252paths are not "stable" identifiers; expect them to change even if you 253always leave the devices plugged in to the same hub port. *Don't even 254think of saving these in application configuration files.* Stable 255identifiers are available, for user mode applications that want to use 256them. HID and networking devices expose these stable IDs, so that for 257example you can be sure that you told the right UPS to power down its 258second server. Pleast note that it doesn't (yet) expose those IDs. 259 260/dev/bus/usb/BBB/DDD 261-------------------- 262 263Use these files in one of these basic ways: 264 265- *They can be read,* producing first the device descriptor (18 bytes) and 266 then the descriptors for the current configuration. See the USB 2.0 spec 267 for details about those binary data formats. You'll need to convert most 268 multibyte values from little endian format to your native host byte 269 order, although a few of the fields in the device descriptor (both of 270 the BCD-encoded fields, and the vendor and product IDs) will be 271 byteswapped for you. Note that configuration descriptors include 272 descriptors for interfaces, altsettings, endpoints, and maybe additional 273 class descriptors. 274 275- *Perform USB operations* using *ioctl()* requests to make endpoint I/O 276 requests (synchronously or asynchronously) or manage the device. These 277 requests need the ``CAP_SYS_RAWIO`` capability, as well as filesystem 278 access permissions. Only one ioctl request can be made on one of these 279 device files at a time. This means that if you are synchronously reading 280 an endpoint from one thread, you won't be able to write to a different 281 endpoint from another thread until the read completes. This works for 282 *half duplex* protocols, but otherwise you'd use asynchronous i/o 283 requests. 284 285Each connected USB device has one file. The ``BBB`` indicates the bus 286number. The ``DDD`` indicates the device address on that bus. Both 287of these numbers are assigned sequentially, and can be reused, so 288you can't rely on them for stable access to devices. For example, 289it's relatively common for devices to re-enumerate while they are 290still connected (perhaps someone jostled their power supply, hub, 291or USB cable), so a device might be ``002/027`` when you first connect 292it and ``002/048`` sometime later. 293 294These files can be read as binary data. The binary data consists 295of first the device descriptor, then the descriptors for each 296configuration of the device. Multi-byte fields in the device descriptor 297are converted to host endianness by the kernel. The configuration 298descriptors are in bus endian format! The configuration descriptor 299are wTotalLength bytes apart. If a device returns less configuration 300descriptor data than indicated by wTotalLength there will be a hole in 301the file for the missing bytes. This information is also shown 302in text form by the ``/sys/kernel/debug/usb/devices`` file, described later. 303 304These files may also be used to write user-level drivers for the USB 305devices. You would open the ``/dev/bus/usb/BBB/DDD`` file read/write, 306read its descriptors to make sure it's the device you expect, and then 307bind to an interface (or perhaps several) using an ioctl call. You 308would issue more ioctls to the device to communicate to it using 309control, bulk, or other kinds of USB transfers. The IOCTLs are 310listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the 311source code (``linux/drivers/usb/core/devio.c``) is the primary reference 312for how to access devices through those files. 313 314Note that since by default these ``BBB/DDD`` files are writable only by 315root, only root can write such user mode drivers. You can selectively 316grant read/write permissions to other users by using ``chmod``. Also, 317usbfs mount options such as ``devmode=0666`` may be helpful. 318 319 320Life Cycle of User Mode Drivers 321------------------------------- 322 323Such a driver first needs to find a device file for a device it knows 324how to handle. Maybe it was told about it because a ``/sbin/hotplug`` 325event handling agent chose that driver to handle the new device. Or 326maybe it's an application that scans all the ``/dev/bus/usb`` device files, 327and ignores most devices. In either case, it should :c:func:`read()` 328all the descriptors from the device file, and check them against what it 329knows how to handle. It might just reject everything except a particular 330vendor and product ID, or need a more complex policy. 331 332Never assume there will only be one such device on the system at a time! 333If your code can't handle more than one device at a time, at least 334detect when there's more than one, and have your users choose which 335device to use. 336 337Once your user mode driver knows what device to use, it interacts with 338it in either of two styles. The simple style is to make only control 339requests; some devices don't need more complex interactions than those. 340(An example might be software using vendor-specific control requests for 341some initialization or configuration tasks, with a kernel driver for the 342rest.) 343 344More likely, you need a more complex style driver: one using non-control 345endpoints, reading or writing data and claiming exclusive use of an 346interface. *Bulk* transfers are easiest to use, but only their sibling 347*interrupt* transfers work with low speed devices. Both interrupt and 348*isochronous* transfers offer service guarantees because their bandwidth 349is reserved. Such "periodic" transfers are awkward to use through usbfs, 350unless you're using the asynchronous calls. However, interrupt transfers 351can also be used in a synchronous "one shot" style. 352 353Your user-mode driver should never need to worry about cleaning up 354request state when the device is disconnected, although it should close 355its open file descriptors as soon as it starts seeing the ENODEV errors. 356 357The ioctl() Requests 358-------------------- 359 360To use these ioctls, you need to include the following headers in your 361userspace program:: 362 363 #include <linux/usb.h> 364 #include <linux/usbdevice_fs.h> 365 #include <asm/byteorder.h> 366 367The standard USB device model requests, from "Chapter 9" of the USB 2.0 368specification, are automatically included from the ``<linux/usb/ch9.h>`` 369header. 370 371Unless noted otherwise, the ioctl requests described here will update 372the modification time on the usbfs file to which they are applied 373(unless they fail). A return of zero indicates success; otherwise, a 374standard USB error code is returned (These are documented in 375:ref:`usb-error-codes`). 376 377Each of these files multiplexes access to several I/O streams, one per 378endpoint. Each device has one control endpoint (endpoint zero) which 379supports a limited RPC style RPC access. Devices are configured by 380hub_wq (in the kernel) setting a device-wide *configuration* that 381affects things like power consumption and basic functionality. The 382endpoints are part of USB *interfaces*, which may have *altsettings* 383affecting things like which endpoints are available. Many devices only 384have a single configuration and interface, so drivers for them will 385ignore configurations and altsettings. 386 387Management/Status Requests 388~~~~~~~~~~~~~~~~~~~~~~~~~~ 389 390A number of usbfs requests don't deal very directly with device I/O. 391They mostly relate to device management and status. These are all 392synchronous requests. 393 394USBDEVFS_CLAIMINTERFACE 395 This is used to force usbfs to claim a specific interface, which has 396 not previously been claimed by usbfs or any other kernel driver. The 397 ioctl parameter is an integer holding the number of the interface 398 (bInterfaceNumber from descriptor). 399 400 Note that if your driver doesn't claim an interface before trying to 401 use one of its endpoints, and no other driver has bound to it, then 402 the interface is automatically claimed by usbfs. 403 404 This claim will be released by a RELEASEINTERFACE ioctl, or by 405 closing the file descriptor. File modification time is not updated 406 by this request. 407 408USBDEVFS_CONNECTINFO 409 Says whether the device is lowspeed. The ioctl parameter points to a 410 structure like this:: 411 412 struct usbdevfs_connectinfo { 413 unsigned int devnum; 414 unsigned char slow; 415 }; 416 417 File modification time is not updated by this request. 418 419 *You can't tell whether a "not slow" device is connected at high 420 speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should 421 know the devnum value already, it's the DDD value of the device file 422 name. 423 424USBDEVFS_GET_SPEED 425 Returns the speed of the device. The speed is returned as a 426 numerical value in accordance with enum usb_device_speed 427 428 File modification time is not updated by this request. 429 430USBDEVFS_GETDRIVER 431 Returns the name of the kernel driver bound to a given interface (a 432 string). Parameter is a pointer to this structure, which is 433 modified:: 434 435 struct usbdevfs_getdriver { 436 unsigned int interface; 437 char driver[USBDEVFS_MAXDRIVERNAME + 1]; 438 }; 439 440 File modification time is not updated by this request. 441 442USBDEVFS_IOCTL 443 Passes a request from userspace through to a kernel driver that has 444 an ioctl entry in the *struct usb_driver* it registered:: 445 446 struct usbdevfs_ioctl { 447 int ifno; 448 int ioctl_code; 449 void *data; 450 }; 451 452 /* user mode call looks like this. 453 * 'request' becomes the driver->ioctl() 'code' parameter. 454 * the size of 'param' is encoded in 'request', and that data 455 * is copied to or from the driver->ioctl() 'buf' parameter. 456 */ 457 static int 458 usbdev_ioctl (int fd, int ifno, unsigned request, void *param) 459 { 460 struct usbdevfs_ioctl wrapper; 461 462 wrapper.ifno = ifno; 463 wrapper.ioctl_code = request; 464 wrapper.data = param; 465 466 return ioctl (fd, USBDEVFS_IOCTL, &wrapper); 467 } 468 469 File modification time is not updated by this request. 470 471 This request lets kernel drivers talk to user mode code through 472 filesystem operations even when they don't create a character or 473 block special device. It's also been used to do things like ask 474 devices what device special file should be used. Two pre-defined 475 ioctls are used to disconnect and reconnect kernel drivers, so that 476 user mode code can completely manage binding and configuration of 477 devices. 478 479USBDEVFS_RELEASEINTERFACE 480 This is used to release the claim usbfs made on interface, either 481 implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the 482 file descriptor is closed. The ioctl parameter is an integer holding 483 the number of the interface (bInterfaceNumber from descriptor); File 484 modification time is not updated by this request. 485 486 .. warning:: 487 488 *No security check is made to ensure that the task which made 489 the claim is the one which is releasing it. This means that user 490 mode driver may interfere other ones.* 491 492USBDEVFS_RESETEP 493 Resets the data toggle value for an endpoint (bulk or interrupt) to 494 DATA0. The ioctl parameter is an integer endpoint number (1 to 15, 495 as identified in the endpoint descriptor), with USB_DIR_IN added 496 if the device's endpoint sends data to the host. 497 498 .. Warning:: 499 500 *Avoid using this request. It should probably be removed.* Using 501 it typically means the device and driver will lose toggle 502 synchronization. If you really lost synchronization, you likely 503 need to completely handshake with the device, using a request 504 like CLEAR_HALT or SET_INTERFACE. 505 506USBDEVFS_DROP_PRIVILEGES 507 This is used to relinquish the ability to do certain operations 508 which are considered to be privileged on a usbfs file descriptor. 509 This includes claiming arbitrary interfaces, resetting a device on 510 which there are currently claimed interfaces from other users, and 511 issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask 512 of interfaces the user is allowed to claim on this file descriptor. 513 You may issue this ioctl more than one time to narrow said mask. 514 515Synchronous I/O Support 516~~~~~~~~~~~~~~~~~~~~~~~ 517 518Synchronous requests involve the kernel blocking until the user mode 519request completes, either by finishing successfully or by reporting an 520error. In most cases this is the simplest way to use usbfs, although as 521noted above it does prevent performing I/O to more than one endpoint at 522a time. 523 524USBDEVFS_BULK 525 Issues a bulk read or write request to the device. The ioctl 526 parameter is a pointer to this structure:: 527 528 struct usbdevfs_bulktransfer { 529 unsigned int ep; 530 unsigned int len; 531 unsigned int timeout; /* in milliseconds */ 532 void *data; 533 }; 534 535 The ``ep`` value identifies a bulk endpoint number (1 to 15, as 536 identified in an endpoint descriptor), masked with USB_DIR_IN when 537 referring to an endpoint which sends data to the host from the 538 device. The length of the data buffer is identified by ``len``; Recent 539 kernels support requests up to about 128KBytes. *FIXME say how read 540 length is returned, and how short reads are handled.*. 541 542USBDEVFS_CLEAR_HALT 543 Clears endpoint halt (stall) and resets the endpoint toggle. This is 544 only meaningful for bulk or interrupt endpoints. The ioctl parameter 545 is an integer endpoint number (1 to 15, as identified in an endpoint 546 descriptor), masked with USB_DIR_IN when referring to an endpoint 547 which sends data to the host from the device. 548 549 Use this on bulk or interrupt endpoints which have stalled, 550 returning ``-EPIPE`` status to a data transfer request. Do not issue 551 the control request directly, since that could invalidate the host's 552 record of the data toggle. 553 554USBDEVFS_CONTROL 555 Issues a control request to the device. The ioctl parameter points 556 to a structure like this:: 557 558 struct usbdevfs_ctrltransfer { 559 __u8 bRequestType; 560 __u8 bRequest; 561 __u16 wValue; 562 __u16 wIndex; 563 __u16 wLength; 564 __u32 timeout; /* in milliseconds */ 565 void *data; 566 }; 567 568 The first eight bytes of this structure are the contents of the 569 SETUP packet to be sent to the device; see the USB 2.0 specification 570 for details. The bRequestType value is composed by combining a 571 ``USB_TYPE_*`` value, a ``USB_DIR_*`` value, and a ``USB_RECIP_*`` 572 value (from ``linux/usb.h``). If wLength is nonzero, it describes 573 the length of the data buffer, which is either written to the device 574 (USB_DIR_OUT) or read from the device (USB_DIR_IN). 575 576 At this writing, you can't transfer more than 4 KBytes of data to or 577 from a device; usbfs has a limit, and some host controller drivers 578 have a limit. (That's not usually a problem.) *Also* there's no way 579 to say it's not OK to get a short read back from the device. 580 581USBDEVFS_RESET 582 Does a USB level device reset. The ioctl parameter is ignored. After 583 the reset, this rebinds all device interfaces. File modification 584 time is not updated by this request. 585 586.. warning:: 587 588 *Avoid using this call* until some usbcore bugs get fixed, since 589 it does not fully synchronize device, interface, and driver (not 590 just usbfs) state. 591 592USBDEVFS_SETINTERFACE 593 Sets the alternate setting for an interface. The ioctl parameter is 594 a pointer to a structure like this:: 595 596 struct usbdevfs_setinterface { 597 unsigned int interface; 598 unsigned int altsetting; 599 }; 600 601 File modification time is not updated by this request. 602 603 Those struct members are from some interface descriptor applying to 604 the current configuration. The interface number is the 605 bInterfaceNumber value, and the altsetting number is the 606 bAlternateSetting value. (This resets each endpoint in the 607 interface.) 608 609USBDEVFS_SETCONFIGURATION 610 Issues the :c:func:`usb_set_configuration()` call for the 611 device. The parameter is an integer holding the number of a 612 configuration (bConfigurationValue from descriptor). File 613 modification time is not updated by this request. 614 615.. warning:: 616 617 *Avoid using this call* until some usbcore bugs get fixed, since 618 it does not fully synchronize device, interface, and driver (not 619 just usbfs) state. 620 621Asynchronous I/O Support 622~~~~~~~~~~~~~~~~~~~~~~~~ 623 624As mentioned above, there are situations where it may be important to 625initiate concurrent operations from user mode code. This is particularly 626important for periodic transfers (interrupt and isochronous), but it can 627be used for other kinds of USB requests too. In such cases, the 628asynchronous requests described here are essential. Rather than 629submitting one request and having the kernel block until it completes, 630the blocking is separate. 631 632These requests are packaged into a structure that resembles the URB used 633by kernel device drivers. (No POSIX Async I/O support here, sorry.) It 634identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint 635(number, masked with USB_DIR_IN as appropriate), buffer and length, 636and a user "context" value serving to uniquely identify each request. 637(It's usually a pointer to per-request data.) Flags can modify requests 638(not as many as supported for kernel drivers). 639 640Each request can specify a realtime signal number (between SIGRTMIN and 641SIGRTMAX, inclusive) to request a signal be sent when the request 642completes. 643 644When usbfs returns these urbs, the status value is updated, and the 645buffer may have been modified. Except for isochronous transfers, the 646actual_length is updated to say how many bytes were transferred; if the 647USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if 648fewer bytes were read than were requested then you get an error report:: 649 650 struct usbdevfs_iso_packet_desc { 651 unsigned int length; 652 unsigned int actual_length; 653 unsigned int status; 654 }; 655 656 struct usbdevfs_urb { 657 unsigned char type; 658 unsigned char endpoint; 659 int status; 660 unsigned int flags; 661 void *buffer; 662 int buffer_length; 663 int actual_length; 664 int start_frame; 665 int number_of_packets; 666 int error_count; 667 unsigned int signr; 668 void *usercontext; 669 struct usbdevfs_iso_packet_desc iso_frame_desc[]; 670 }; 671 672For these asynchronous requests, the file modification time reflects 673when the request was initiated. This contrasts with their use with the 674synchronous requests, where it reflects when requests complete. 675 676USBDEVFS_DISCARDURB 677 *TBS* File modification time is not updated by this request. 678 679USBDEVFS_DISCSIGNAL 680 *TBS* File modification time is not updated by this request. 681 682USBDEVFS_REAPURB 683 *TBS* File modification time is not updated by this request. 684 685USBDEVFS_REAPURBNDELAY 686 *TBS* File modification time is not updated by this request. 687 688USBDEVFS_SUBMITURB 689 *TBS* 690 691The USB devices 692=============== 693 694The USB devices are now exported via debugfs: 695 696- ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB 697 devices on known to the kernel, and their configuration descriptors. 698 You can also poll() this to learn about new devices. 699 700/sys/kernel/debug/usb/devices 701----------------------------- 702 703This file is handy for status viewing tools in user mode, which can scan 704the text format and ignore most of it. More detailed device status 705(including class and vendor status) is available from device-specific 706files. For information about the current format of this file, see below. 707 708This file, in combination with the poll() system call, can also be used 709to detect when devices are added or removed:: 710 711 int fd; 712 struct pollfd pfd; 713 714 fd = open("/sys/kernel/debug/usb/devices", O_RDONLY); 715 pfd = { fd, POLLIN, 0 }; 716 for (;;) { 717 /* The first time through, this call will return immediately. */ 718 poll(&pfd, 1, -1); 719 720 /* To see what's changed, compare the file's previous and current 721 contents or scan the filesystem. (Scanning is more precise.) */ 722 } 723 724Note that this behavior is intended to be used for informational and 725debug purposes. It would be more appropriate to use programs such as 726udev or HAL to initialize a device or start a user-mode helper program, 727for instance. 728 729In this file, each device's output has multiple lines of ASCII output. 730 731I made it ASCII instead of binary on purpose, so that someone 732can obtain some useful data from it without the use of an 733auxiliary program. However, with an auxiliary program, the numbers 734in the first 4 columns of each ``T:`` line (topology info: 735Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram. 736 737Each line is tagged with a one-character ID for that line:: 738 739 T = Topology (etc.) 740 B = Bandwidth (applies only to USB host controllers, which are 741 virtualized as root hubs) 742 D = Device descriptor info. 743 P = Product ID info. (from Device descriptor, but they won't fit 744 together on one line) 745 S = String descriptors. 746 C = Configuration descriptor info. (* = active configuration) 747 I = Interface descriptor info. 748 E = Endpoint descriptor info. 749 750/sys/kernel/debug/usb/devices output format 751~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 752 753Legend:: 754 d = decimal number (may have leading spaces or 0's) 755 x = hexadecimal number (may have leading spaces or 0's) 756 s = string 757 758 759 760Topology info 761^^^^^^^^^^^^^ 762 763:: 764 765 T: Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd 766 | | | | | | | | |__MaxChildren 767 | | | | | | | |__Device Speed in Mbps 768 | | | | | | |__DeviceNumber 769 | | | | | |__Count of devices at this level 770 | | | | |__Connector/Port on Parent for this device 771 | | | |__Parent DeviceNumber 772 | | |__Level in topology for this bus 773 | |__Bus number 774 |__Topology info tag 775 776Speed may be: 777 778 ======= ====================================================== 779 1.5 Mbit/s for low speed USB 780 12 Mbit/s for full speed USB 781 480 Mbit/s for high speed USB (added for USB 2.0) 782 5000 Mbit/s for SuperSpeed USB (added for USB 3.0) 783 ======= ====================================================== 784 785For reasons lost in the mists of time, the Port number is always 786too low by 1. For example, a device plugged into port 4 will 787show up with ``Port=03``. 788 789Bandwidth info 790^^^^^^^^^^^^^^ 791 792:: 793 794 B: Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd 795 | | | |__Number of isochronous requests 796 | | |__Number of interrupt requests 797 | |__Total Bandwidth allocated to this bus 798 |__Bandwidth info tag 799 800Bandwidth allocation is an approximation of how much of one frame 801(millisecond) is in use. It reflects only periodic transfers, which 802are the only transfers that reserve bandwidth. Control and bulk 803transfers use all other bandwidth, including reserved bandwidth that 804is not used for transfers (such as for short packets). 805 806The percentage is how much of the "reserved" bandwidth is scheduled by 807those transfers. For a low or full speed bus (loosely, "USB 1.1"), 80890% of the bus bandwidth is reserved. For a high speed bus (loosely, 809"USB 2.0") 80% is reserved. 810 811 812Device descriptor info & Product ID info 813^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 814 815:: 816 817 D: Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 818 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 819 820where:: 821 822 D: Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 823 | | | | | | |__NumberConfigurations 824 | | | | | |__MaxPacketSize of Default Endpoint 825 | | | | |__DeviceProtocol 826 | | | |__DeviceSubClass 827 | | |__DeviceClass 828 | |__Device USB version 829 |__Device info tag #1 830 831where:: 832 833 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 834 | | | |__Product revision number 835 | | |__Product ID code 836 | |__Vendor ID code 837 |__Device info tag #2 838 839 840String descriptor info 841^^^^^^^^^^^^^^^^^^^^^^ 842:: 843 844 S: Manufacturer=ssss 845 | |__Manufacturer of this device as read from the device. 846 | For USB host controller drivers (virtual root hubs) this may 847 | be omitted, or (for newer drivers) will identify the kernel 848 | version and the driver which provides this hub emulation. 849 |__String info tag 850 851 S: Product=ssss 852 | |__Product description of this device as read from the device. 853 | For older USB host controller drivers (virtual root hubs) this 854 | indicates the driver; for newer ones, it's a product (and vendor) 855 | description that often comes from the kernel's PCI ID database. 856 |__String info tag 857 858 S: SerialNumber=ssss 859 | |__Serial Number of this device as read from the device. 860 | For USB host controller drivers (virtual root hubs) this is 861 | some unique ID, normally a bus ID (address or slot name) that 862 | can't be shared with any other device. 863 |__String info tag 864 865 866 867Configuration descriptor info 868^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 869:: 870 871 C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA 872 | | | | | |__MaxPower in mA 873 | | | | |__Attributes 874 | | | |__ConfiguratioNumber 875 | | |__NumberOfInterfaces 876 | |__ "*" indicates the active configuration (others are " ") 877 |__Config info tag 878 879USB devices may have multiple configurations, each of which act 880rather differently. For example, a bus-powered configuration 881might be much less capable than one that is self-powered. Only 882one device configuration can be active at a time; most devices 883have only one configuration. 884 885Each configuration consists of one or more interfaces. Each 886interface serves a distinct "function", which is typically bound 887to a different USB device driver. One common example is a USB 888speaker with an audio interface for playback, and a HID interface 889for use with software volume control. 890 891Interface descriptor info (can be multiple per Config) 892^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 893:: 894 895 I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss 896 | | | | | | | | |__Driver name 897 | | | | | | | | or "(none)" 898 | | | | | | | |__InterfaceProtocol 899 | | | | | | |__InterfaceSubClass 900 | | | | | |__InterfaceClass 901 | | | | |__NumberOfEndpoints 902 | | | |__AlternateSettingNumber 903 | | |__InterfaceNumber 904 | |__ "*" indicates the active altsetting (others are " ") 905 |__Interface info tag 906 907A given interface may have one or more "alternate" settings. 908For example, default settings may not use more than a small 909amount of periodic bandwidth. To use significant fractions 910of bus bandwidth, drivers must select a non-default altsetting. 911 912Only one setting for an interface may be active at a time, and 913only one driver may bind to an interface at a time. Most devices 914have only one alternate setting per interface. 915 916 917Endpoint descriptor info (can be multiple per Interface) 918^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 919 920:: 921 922 E: Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss 923 | | | | |__Interval (max) between transfers 924 | | | |__EndpointMaxPacketSize 925 | | |__Attributes(EndpointType) 926 | |__EndpointAddress(I=In,O=Out) 927 |__Endpoint info tag 928 929The interval is nonzero for all periodic (interrupt or isochronous) 930endpoints. For high speed endpoints the transfer interval may be 931measured in microseconds rather than milliseconds. 932 933For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects 934the per-microframe data transfer size. For "high bandwidth" 935endpoints, that can reflect two or three packets (for up to 9363KBytes every 125 usec) per endpoint. 937 938With the Linux-USB stack, periodic bandwidth reservations use the 939transfer intervals and sizes provided by URBs, which can be less 940than those found in endpoint descriptor. 941 942Usage examples 943~~~~~~~~~~~~~~ 944 945If a user or script is interested only in Topology info, for 946example, use something like ``grep ^T: /sys/kernel/debug/usb/devices`` 947for only the Topology lines. A command like 948``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list 949only the lines that begin with the characters in square brackets, 950where the valid characters are TDPCIE. With a slightly more able 951script, it can display any selected lines (for example, only T, D, 952and P lines) and change their output format. (The ``procusb`` 953Perl script is the beginning of this idea. It will list only 954selected lines [selected from TBDPSCIE] or "All" lines from 955``/sys/kernel/debug/usb/devices``.) 956 957The Topology lines can be used to generate a graphic/pictorial 958of the USB devices on a system's root hub. (See more below 959on how to do this.) 960 961The Interface lines can be used to determine what driver is 962being used for each device, and which altsetting it activated. 963 964The Configuration lines could be used to list maximum power 965(in milliamps) that a system's USB devices are using. 966For example, ``grep ^C: /sys/kernel/debug/usb/devices``. 967 968 969Here's an example, from a system which has a UHCI root hub, 970an external hub connected to the root hub, and a mouse and 971a serial converter connected to the external hub. 972 973:: 974 975 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 976 B: Alloc= 28/900 us ( 3%), #Int= 2, #Iso= 0 977 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 978 P: Vendor=0000 ProdID=0000 Rev= 0.00 979 S: Product=USB UHCI Root Hub 980 S: SerialNumber=dce0 981 C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr= 0mA 982 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 983 E: Ad=81(I) Atr=03(Int.) MxPS= 8 Ivl=255ms 984 985 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 986 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 987 P: Vendor=0451 ProdID=1446 Rev= 1.00 988 C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA 989 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 990 E: Ad=81(I) Atr=03(Int.) MxPS= 1 Ivl=255ms 991 992 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 993 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 994 P: Vendor=04b4 ProdID=0001 Rev= 0.00 995 C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA 996 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 997 E: Ad=81(I) Atr=03(Int.) MxPS= 3 Ivl= 10ms 998 999 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 1000 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 1001 P: Vendor=0565 ProdID=0001 Rev= 1.08 1002 S: Manufacturer=Peracom Networks, Inc. 1003 S: Product=Peracom USB to Serial Converter 1004 C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA 1005 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 1006 E: Ad=81(I) Atr=02(Bulk) MxPS= 64 Ivl= 16ms 1007 E: Ad=01(O) Atr=02(Bulk) MxPS= 16 Ivl= 16ms 1008 E: Ad=82(I) Atr=03(Int.) MxPS= 8 Ivl= 8ms 1009 1010 1011Selecting only the ``T:`` and ``I:`` lines from this (for example, by using 1012``procusb ti``), we have 1013 1014:: 1015 1016 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 1017 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 1018 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 1019 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 1020 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 1021 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 1022 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 1023 1024 1025Physically this looks like (or could be converted to):: 1026 1027 +------------------+ 1028 | PC/root_hub (12)| Dev# = 1 1029 +------------------+ (nn) is Mbps. 1030 Level 0 | CN.0 | CN.1 | [CN = connector/port #] 1031 +------------------+ 1032 / 1033 / 1034 +-----------------------+ 1035 Level 1 | Dev#2: 4-port hub (12)| 1036 +-----------------------+ 1037 |CN.0 |CN.1 |CN.2 |CN.3 | 1038 +-----------------------+ 1039 \ \____________________ 1040 \_____ \ 1041 \ \ 1042 +--------------------+ +--------------------+ 1043 Level 2 | Dev# 3: mouse (1.5)| | Dev# 4: serial (12)| 1044 +--------------------+ +--------------------+ 1045 1046 1047 1048Or, in a more tree-like structure (ports [Connectors] without 1049connections could be omitted):: 1050 1051 PC: Dev# 1, root hub, 2 ports, 12 Mbps 1052 |_ CN.0: Dev# 2, hub, 4 ports, 12 Mbps 1053 |_ CN.0: Dev #3, mouse, 1.5 Mbps 1054 |_ CN.1: 1055 |_ CN.2: Dev #4, serial, 12 Mbps 1056 |_ CN.3: 1057 |_ CN.1: 1058