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_GETDRIVER 424 Returns the name of the kernel driver bound to a given interface (a 425 string). Parameter is a pointer to this structure, which is 426 modified:: 427 428 struct usbdevfs_getdriver { 429 unsigned int interface; 430 char driver[USBDEVFS_MAXDRIVERNAME + 1]; 431 }; 432 433 File modification time is not updated by this request. 434 435USBDEVFS_IOCTL 436 Passes a request from userspace through to a kernel driver that has 437 an ioctl entry in the *struct usb_driver* it registered:: 438 439 struct usbdevfs_ioctl { 440 int ifno; 441 int ioctl_code; 442 void *data; 443 }; 444 445 /* user mode call looks like this. 446 * 'request' becomes the driver->ioctl() 'code' parameter. 447 * the size of 'param' is encoded in 'request', and that data 448 * is copied to or from the driver->ioctl() 'buf' parameter. 449 */ 450 static int 451 usbdev_ioctl (int fd, int ifno, unsigned request, void *param) 452 { 453 struct usbdevfs_ioctl wrapper; 454 455 wrapper.ifno = ifno; 456 wrapper.ioctl_code = request; 457 wrapper.data = param; 458 459 return ioctl (fd, USBDEVFS_IOCTL, &wrapper); 460 } 461 462 File modification time is not updated by this request. 463 464 This request lets kernel drivers talk to user mode code through 465 filesystem operations even when they don't create a character or 466 block special device. It's also been used to do things like ask 467 devices what device special file should be used. Two pre-defined 468 ioctls are used to disconnect and reconnect kernel drivers, so that 469 user mode code can completely manage binding and configuration of 470 devices. 471 472USBDEVFS_RELEASEINTERFACE 473 This is used to release the claim usbfs made on interface, either 474 implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the 475 file descriptor is closed. The ioctl parameter is an integer holding 476 the number of the interface (bInterfaceNumber from descriptor); File 477 modification time is not updated by this request. 478 479 .. warning:: 480 481 *No security check is made to ensure that the task which made 482 the claim is the one which is releasing it. This means that user 483 mode driver may interfere other ones.* 484 485USBDEVFS_RESETEP 486 Resets the data toggle value for an endpoint (bulk or interrupt) to 487 DATA0. The ioctl parameter is an integer endpoint number (1 to 15, 488 as identified in the endpoint descriptor), with USB_DIR_IN added 489 if the device's endpoint sends data to the host. 490 491 .. Warning:: 492 493 *Avoid using this request. It should probably be removed.* Using 494 it typically means the device and driver will lose toggle 495 synchronization. If you really lost synchronization, you likely 496 need to completely handshake with the device, using a request 497 like CLEAR_HALT or SET_INTERFACE. 498 499USBDEVFS_DROP_PRIVILEGES 500 This is used to relinquish the ability to do certain operations 501 which are considered to be privileged on a usbfs file descriptor. 502 This includes claiming arbitrary interfaces, resetting a device on 503 which there are currently claimed interfaces from other users, and 504 issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask 505 of interfaces the user is allowed to claim on this file descriptor. 506 You may issue this ioctl more than one time to narrow said mask. 507 508Synchronous I/O Support 509~~~~~~~~~~~~~~~~~~~~~~~ 510 511Synchronous requests involve the kernel blocking until the user mode 512request completes, either by finishing successfully or by reporting an 513error. In most cases this is the simplest way to use usbfs, although as 514noted above it does prevent performing I/O to more than one endpoint at 515a time. 516 517USBDEVFS_BULK 518 Issues a bulk read or write request to the device. The ioctl 519 parameter is a pointer to this structure:: 520 521 struct usbdevfs_bulktransfer { 522 unsigned int ep; 523 unsigned int len; 524 unsigned int timeout; /* in milliseconds */ 525 void *data; 526 }; 527 528 The ``ep`` value identifies a bulk endpoint number (1 to 15, as 529 identified in an endpoint descriptor), masked with USB_DIR_IN when 530 referring to an endpoint which sends data to the host from the 531 device. The length of the data buffer is identified by ``len``; Recent 532 kernels support requests up to about 128KBytes. *FIXME say how read 533 length is returned, and how short reads are handled.*. 534 535USBDEVFS_CLEAR_HALT 536 Clears endpoint halt (stall) and resets the endpoint toggle. This is 537 only meaningful for bulk or interrupt endpoints. The ioctl parameter 538 is an integer endpoint number (1 to 15, as identified in an endpoint 539 descriptor), masked with USB_DIR_IN when referring to an endpoint 540 which sends data to the host from the device. 541 542 Use this on bulk or interrupt endpoints which have stalled, 543 returning ``-EPIPE`` status to a data transfer request. Do not issue 544 the control request directly, since that could invalidate the host's 545 record of the data toggle. 546 547USBDEVFS_CONTROL 548 Issues a control request to the device. The ioctl parameter points 549 to a structure like this:: 550 551 struct usbdevfs_ctrltransfer { 552 __u8 bRequestType; 553 __u8 bRequest; 554 __u16 wValue; 555 __u16 wIndex; 556 __u16 wLength; 557 __u32 timeout; /* in milliseconds */ 558 void *data; 559 }; 560 561 The first eight bytes of this structure are the contents of the 562 SETUP packet to be sent to the device; see the USB 2.0 specification 563 for details. The bRequestType value is composed by combining a 564 ``USB_TYPE_*`` value, a ``USB_DIR_*`` value, and a ``USB_RECIP_*`` 565 value (from ``linux/usb.h``). If wLength is nonzero, it describes 566 the length of the data buffer, which is either written to the device 567 (USB_DIR_OUT) or read from the device (USB_DIR_IN). 568 569 At this writing, you can't transfer more than 4 KBytes of data to or 570 from a device; usbfs has a limit, and some host controller drivers 571 have a limit. (That's not usually a problem.) *Also* there's no way 572 to say it's not OK to get a short read back from the device. 573 574USBDEVFS_RESET 575 Does a USB level device reset. The ioctl parameter is ignored. After 576 the reset, this rebinds all device interfaces. File modification 577 time is not updated by this request. 578 579.. warning:: 580 581 *Avoid using this call* until some usbcore bugs get fixed, since 582 it does not fully synchronize device, interface, and driver (not 583 just usbfs) state. 584 585USBDEVFS_SETINTERFACE 586 Sets the alternate setting for an interface. The ioctl parameter is 587 a pointer to a structure like this:: 588 589 struct usbdevfs_setinterface { 590 unsigned int interface; 591 unsigned int altsetting; 592 }; 593 594 File modification time is not updated by this request. 595 596 Those struct members are from some interface descriptor applying to 597 the current configuration. The interface number is the 598 bInterfaceNumber value, and the altsetting number is the 599 bAlternateSetting value. (This resets each endpoint in the 600 interface.) 601 602USBDEVFS_SETCONFIGURATION 603 Issues the :c:func:`usb_set_configuration()` call for the 604 device. The parameter is an integer holding the number of a 605 configuration (bConfigurationValue from descriptor). File 606 modification time is not updated by this request. 607 608.. warning:: 609 610 *Avoid using this call* until some usbcore bugs get fixed, since 611 it does not fully synchronize device, interface, and driver (not 612 just usbfs) state. 613 614Asynchronous I/O Support 615~~~~~~~~~~~~~~~~~~~~~~~~ 616 617As mentioned above, there are situations where it may be important to 618initiate concurrent operations from user mode code. This is particularly 619important for periodic transfers (interrupt and isochronous), but it can 620be used for other kinds of USB requests too. In such cases, the 621asynchronous requests described here are essential. Rather than 622submitting one request and having the kernel block until it completes, 623the blocking is separate. 624 625These requests are packaged into a structure that resembles the URB used 626by kernel device drivers. (No POSIX Async I/O support here, sorry.) It 627identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint 628(number, masked with USB_DIR_IN as appropriate), buffer and length, 629and a user "context" value serving to uniquely identify each request. 630(It's usually a pointer to per-request data.) Flags can modify requests 631(not as many as supported for kernel drivers). 632 633Each request can specify a realtime signal number (between SIGRTMIN and 634SIGRTMAX, inclusive) to request a signal be sent when the request 635completes. 636 637When usbfs returns these urbs, the status value is updated, and the 638buffer may have been modified. Except for isochronous transfers, the 639actual_length is updated to say how many bytes were transferred; if the 640USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if 641fewer bytes were read than were requested then you get an error report:: 642 643 struct usbdevfs_iso_packet_desc { 644 unsigned int length; 645 unsigned int actual_length; 646 unsigned int status; 647 }; 648 649 struct usbdevfs_urb { 650 unsigned char type; 651 unsigned char endpoint; 652 int status; 653 unsigned int flags; 654 void *buffer; 655 int buffer_length; 656 int actual_length; 657 int start_frame; 658 int number_of_packets; 659 int error_count; 660 unsigned int signr; 661 void *usercontext; 662 struct usbdevfs_iso_packet_desc iso_frame_desc[]; 663 }; 664 665For these asynchronous requests, the file modification time reflects 666when the request was initiated. This contrasts with their use with the 667synchronous requests, where it reflects when requests complete. 668 669USBDEVFS_DISCARDURB 670 *TBS* File modification time is not updated by this request. 671 672USBDEVFS_DISCSIGNAL 673 *TBS* File modification time is not updated by this request. 674 675USBDEVFS_REAPURB 676 *TBS* File modification time is not updated by this request. 677 678USBDEVFS_REAPURBNDELAY 679 *TBS* File modification time is not updated by this request. 680 681USBDEVFS_SUBMITURB 682 *TBS* 683 684The USB devices 685=============== 686 687The USB devices are now exported via debugfs: 688 689- ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB 690 devices on known to the kernel, and their configuration descriptors. 691 You can also poll() this to learn about new devices. 692 693/sys/kernel/debug/usb/devices 694----------------------------- 695 696This file is handy for status viewing tools in user mode, which can scan 697the text format and ignore most of it. More detailed device status 698(including class and vendor status) is available from device-specific 699files. For information about the current format of this file, see below. 700 701This file, in combination with the poll() system call, can also be used 702to detect when devices are added or removed:: 703 704 int fd; 705 struct pollfd pfd; 706 707 fd = open("/sys/kernel/debug/usb/devices", O_RDONLY); 708 pfd = { fd, POLLIN, 0 }; 709 for (;;) { 710 /* The first time through, this call will return immediately. */ 711 poll(&pfd, 1, -1); 712 713 /* To see what's changed, compare the file's previous and current 714 contents or scan the filesystem. (Scanning is more precise.) */ 715 } 716 717Note that this behavior is intended to be used for informational and 718debug purposes. It would be more appropriate to use programs such as 719udev or HAL to initialize a device or start a user-mode helper program, 720for instance. 721 722In this file, each device's output has multiple lines of ASCII output. 723 724I made it ASCII instead of binary on purpose, so that someone 725can obtain some useful data from it without the use of an 726auxiliary program. However, with an auxiliary program, the numbers 727in the first 4 columns of each ``T:`` line (topology info: 728Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram. 729 730Each line is tagged with a one-character ID for that line:: 731 732 T = Topology (etc.) 733 B = Bandwidth (applies only to USB host controllers, which are 734 virtualized as root hubs) 735 D = Device descriptor info. 736 P = Product ID info. (from Device descriptor, but they won't fit 737 together on one line) 738 S = String descriptors. 739 C = Configuration descriptor info. (* = active configuration) 740 I = Interface descriptor info. 741 E = Endpoint descriptor info. 742 743/sys/kernel/debug/usb/devices output format 744~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 745 746Legend:: 747 d = decimal number (may have leading spaces or 0's) 748 x = hexadecimal number (may have leading spaces or 0's) 749 s = string 750 751 752 753Topology info 754^^^^^^^^^^^^^ 755 756:: 757 758 T: Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd 759 | | | | | | | | |__MaxChildren 760 | | | | | | | |__Device Speed in Mbps 761 | | | | | | |__DeviceNumber 762 | | | | | |__Count of devices at this level 763 | | | | |__Connector/Port on Parent for this device 764 | | | |__Parent DeviceNumber 765 | | |__Level in topology for this bus 766 | |__Bus number 767 |__Topology info tag 768 769Speed may be: 770 771 ======= ====================================================== 772 1.5 Mbit/s for low speed USB 773 12 Mbit/s for full speed USB 774 480 Mbit/s for high speed USB (added for USB 2.0); 775 also used for Wireless USB, which has no fixed speed 776 5000 Mbit/s for SuperSpeed USB (added for USB 3.0) 777 ======= ====================================================== 778 779For reasons lost in the mists of time, the Port number is always 780too low by 1. For example, a device plugged into port 4 will 781show up with ``Port=03``. 782 783Bandwidth info 784^^^^^^^^^^^^^^ 785 786:: 787 788 B: Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd 789 | | | |__Number of isochronous requests 790 | | |__Number of interrupt requests 791 | |__Total Bandwidth allocated to this bus 792 |__Bandwidth info tag 793 794Bandwidth allocation is an approximation of how much of one frame 795(millisecond) is in use. It reflects only periodic transfers, which 796are the only transfers that reserve bandwidth. Control and bulk 797transfers use all other bandwidth, including reserved bandwidth that 798is not used for transfers (such as for short packets). 799 800The percentage is how much of the "reserved" bandwidth is scheduled by 801those transfers. For a low or full speed bus (loosely, "USB 1.1"), 80290% of the bus bandwidth is reserved. For a high speed bus (loosely, 803"USB 2.0") 80% is reserved. 804 805 806Device descriptor info & Product ID info 807^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 808 809:: 810 811 D: Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 812 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 813 814where:: 815 816 D: Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 817 | | | | | | |__NumberConfigurations 818 | | | | | |__MaxPacketSize of Default Endpoint 819 | | | | |__DeviceProtocol 820 | | | |__DeviceSubClass 821 | | |__DeviceClass 822 | |__Device USB version 823 |__Device info tag #1 824 825where:: 826 827 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 828 | | | |__Product revision number 829 | | |__Product ID code 830 | |__Vendor ID code 831 |__Device info tag #2 832 833 834String descriptor info 835^^^^^^^^^^^^^^^^^^^^^^ 836:: 837 838 S: Manufacturer=ssss 839 | |__Manufacturer of this device as read from the device. 840 | For USB host controller drivers (virtual root hubs) this may 841 | be omitted, or (for newer drivers) will identify the kernel 842 | version and the driver which provides this hub emulation. 843 |__String info tag 844 845 S: Product=ssss 846 | |__Product description of this device as read from the device. 847 | For older USB host controller drivers (virtual root hubs) this 848 | indicates the driver; for newer ones, it's a product (and vendor) 849 | description that often comes from the kernel's PCI ID database. 850 |__String info tag 851 852 S: SerialNumber=ssss 853 | |__Serial Number of this device as read from the device. 854 | For USB host controller drivers (virtual root hubs) this is 855 | some unique ID, normally a bus ID (address or slot name) that 856 | can't be shared with any other device. 857 |__String info tag 858 859 860 861Configuration descriptor info 862^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 863:: 864 865 C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA 866 | | | | | |__MaxPower in mA 867 | | | | |__Attributes 868 | | | |__ConfiguratioNumber 869 | | |__NumberOfInterfaces 870 | |__ "*" indicates the active configuration (others are " ") 871 |__Config info tag 872 873USB devices may have multiple configurations, each of which act 874rather differently. For example, a bus-powered configuration 875might be much less capable than one that is self-powered. Only 876one device configuration can be active at a time; most devices 877have only one configuration. 878 879Each configuration consists of one or more interfaces. Each 880interface serves a distinct "function", which is typically bound 881to a different USB device driver. One common example is a USB 882speaker with an audio interface for playback, and a HID interface 883for use with software volume control. 884 885Interface descriptor info (can be multiple per Config) 886^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 887:: 888 889 I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss 890 | | | | | | | | |__Driver name 891 | | | | | | | | or "(none)" 892 | | | | | | | |__InterfaceProtocol 893 | | | | | | |__InterfaceSubClass 894 | | | | | |__InterfaceClass 895 | | | | |__NumberOfEndpoints 896 | | | |__AlternateSettingNumber 897 | | |__InterfaceNumber 898 | |__ "*" indicates the active altsetting (others are " ") 899 |__Interface info tag 900 901A given interface may have one or more "alternate" settings. 902For example, default settings may not use more than a small 903amount of periodic bandwidth. To use significant fractions 904of bus bandwidth, drivers must select a non-default altsetting. 905 906Only one setting for an interface may be active at a time, and 907only one driver may bind to an interface at a time. Most devices 908have only one alternate setting per interface. 909 910 911Endpoint descriptor info (can be multiple per Interface) 912^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 913 914:: 915 916 E: Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss 917 | | | | |__Interval (max) between transfers 918 | | | |__EndpointMaxPacketSize 919 | | |__Attributes(EndpointType) 920 | |__EndpointAddress(I=In,O=Out) 921 |__Endpoint info tag 922 923The interval is nonzero for all periodic (interrupt or isochronous) 924endpoints. For high speed endpoints the transfer interval may be 925measured in microseconds rather than milliseconds. 926 927For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects 928the per-microframe data transfer size. For "high bandwidth" 929endpoints, that can reflect two or three packets (for up to 9303KBytes every 125 usec) per endpoint. 931 932With the Linux-USB stack, periodic bandwidth reservations use the 933transfer intervals and sizes provided by URBs, which can be less 934than those found in endpoint descriptor. 935 936Usage examples 937~~~~~~~~~~~~~~ 938 939If a user or script is interested only in Topology info, for 940example, use something like ``grep ^T: /sys/kernel/debug/usb/devices`` 941for only the Topology lines. A command like 942``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list 943only the lines that begin with the characters in square brackets, 944where the valid characters are TDPCIE. With a slightly more able 945script, it can display any selected lines (for example, only T, D, 946and P lines) and change their output format. (The ``procusb`` 947Perl script is the beginning of this idea. It will list only 948selected lines [selected from TBDPSCIE] or "All" lines from 949``/sys/kernel/debug/usb/devices``.) 950 951The Topology lines can be used to generate a graphic/pictorial 952of the USB devices on a system's root hub. (See more below 953on how to do this.) 954 955The Interface lines can be used to determine what driver is 956being used for each device, and which altsetting it activated. 957 958The Configuration lines could be used to list maximum power 959(in milliamps) that a system's USB devices are using. 960For example, ``grep ^C: /sys/kernel/debug/usb/devices``. 961 962 963Here's an example, from a system which has a UHCI root hub, 964an external hub connected to the root hub, and a mouse and 965a serial converter connected to the external hub. 966 967:: 968 969 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 970 B: Alloc= 28/900 us ( 3%), #Int= 2, #Iso= 0 971 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 972 P: Vendor=0000 ProdID=0000 Rev= 0.00 973 S: Product=USB UHCI Root Hub 974 S: SerialNumber=dce0 975 C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr= 0mA 976 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 977 E: Ad=81(I) Atr=03(Int.) MxPS= 8 Ivl=255ms 978 979 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 980 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 981 P: Vendor=0451 ProdID=1446 Rev= 1.00 982 C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA 983 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 984 E: Ad=81(I) Atr=03(Int.) MxPS= 1 Ivl=255ms 985 986 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 987 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 988 P: Vendor=04b4 ProdID=0001 Rev= 0.00 989 C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA 990 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 991 E: Ad=81(I) Atr=03(Int.) MxPS= 3 Ivl= 10ms 992 993 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 994 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 995 P: Vendor=0565 ProdID=0001 Rev= 1.08 996 S: Manufacturer=Peracom Networks, Inc. 997 S: Product=Peracom USB to Serial Converter 998 C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA 999 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 1000 E: Ad=81(I) Atr=02(Bulk) MxPS= 64 Ivl= 16ms 1001 E: Ad=01(O) Atr=02(Bulk) MxPS= 16 Ivl= 16ms 1002 E: Ad=82(I) Atr=03(Int.) MxPS= 8 Ivl= 8ms 1003 1004 1005Selecting only the ``T:`` and ``I:`` lines from this (for example, by using 1006``procusb ti``), we have 1007 1008:: 1009 1010 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 1011 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 1012 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 1013 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 1014 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 1015 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 1016 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 1017 1018 1019Physically this looks like (or could be converted to):: 1020 1021 +------------------+ 1022 | PC/root_hub (12)| Dev# = 1 1023 +------------------+ (nn) is Mbps. 1024 Level 0 | CN.0 | CN.1 | [CN = connector/port #] 1025 +------------------+ 1026 / 1027 / 1028 +-----------------------+ 1029 Level 1 | Dev#2: 4-port hub (12)| 1030 +-----------------------+ 1031 |CN.0 |CN.1 |CN.2 |CN.3 | 1032 +-----------------------+ 1033 \ \____________________ 1034 \_____ \ 1035 \ \ 1036 +--------------------+ +--------------------+ 1037 Level 2 | Dev# 3: mouse (1.5)| | Dev# 4: serial (12)| 1038 +--------------------+ +--------------------+ 1039 1040 1041 1042Or, in a more tree-like structure (ports [Connectors] without 1043connections could be omitted):: 1044 1045 PC: Dev# 1, root hub, 2 ports, 12 Mbps 1046 |_ CN.0: Dev# 2, hub, 4 ports, 12 Mbps 1047 |_ CN.0: Dev #3, mouse, 1.5 Mbps 1048 |_ CN.1: 1049 |_ CN.2: Dev #4, serial, 12 Mbps 1050 |_ CN.3: 1051 |_ CN.1: 1052