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