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 ``<linux/usb/ch9.h>`` you will find the USB data types defined in 113chapter 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. 116 117.. kernel-doc:: include/linux/usb/ch9.h 118 :internal: 119 120.. _usb_header: 121 122Host-Side Data Types and Macros 123=============================== 124 125The host side API exposes several layers to drivers, some of which are 126more necessary than others. These support lifecycle models for host side 127drivers and devices, and support passing buffers through usbcore to some 128HCD that performs the I/O for the device driver. 129 130.. kernel-doc:: include/linux/usb.h 131 :internal: 132 133USB Core APIs 134============= 135 136There are two basic I/O models in the USB API. The most elemental one is 137asynchronous: drivers submit requests in the form of an URB, and the 138URB's completion callback handles the next step. All USB transfer types 139support that model, although there are special cases for control URBs 140(which always have setup and status stages, but may not have a data 141stage) and isochronous URBs (which allow large packets and include 142per-packet fault reports). Built on top of that is synchronous API 143support, where a driver calls a routine that allocates one or more URBs, 144submits them, and waits until they complete. There are synchronous 145wrappers for single-buffer control and bulk transfers (which are awkward 146to use in some driver disconnect scenarios), and for scatterlist based 147streaming i/o (bulk or interrupt). 148 149USB drivers need to provide buffers that can be used for DMA, although 150they don't necessarily need to provide the DMA mapping themselves. There 151are APIs to use used when allocating DMA buffers, which can prevent use 152of bounce buffers on some systems. In some cases, drivers may be able to 153rely on 64bit DMA to eliminate another kind of bounce buffer. 154 155.. kernel-doc:: drivers/usb/core/urb.c 156 :export: 157 158.. kernel-doc:: drivers/usb/core/message.c 159 :export: 160 161.. kernel-doc:: drivers/usb/core/file.c 162 :export: 163 164.. kernel-doc:: drivers/usb/core/driver.c 165 :export: 166 167.. kernel-doc:: drivers/usb/core/usb.c 168 :export: 169 170.. kernel-doc:: drivers/usb/core/hub.c 171 :export: 172 173Host Controller APIs 174==================== 175 176These APIs are only for use by host controller drivers, most of which 177implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI 178was one of the first interfaces, designed by Intel and also used by VIA; 179it doesn't do much in hardware. OHCI was designed later, to have the 180hardware do more work (bigger transfers, tracking protocol state, and so 181on). EHCI was designed with USB 2.0; its design has features that 182resemble OHCI (hardware does much more work) as well as UHCI (some parts 183of ISO support, TD list processing). XHCI was designed with USB 3.0. It 184continues to shift support for functionality into hardware. 185 186There are host controllers other than the "big three", although most PCI 187based controllers (and a few non-PCI based ones) use one of those 188interfaces. Not all host controllers use DMA; some use PIO, and there is 189also a simulator and a virtual host controller to pipe USB over the network. 190 191The same basic APIs are available to drivers for all those controllers. 192For historical reasons they are in two layers: :c:type:`struct 193usb_bus <usb_bus>` is a rather thin layer that became available 194in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>` 195is a more featureful layer 196that lets HCDs share common code, to shrink driver size and 197significantly reduce hcd-specific behaviors. 198 199.. kernel-doc:: drivers/usb/core/hcd.c 200 :export: 201 202.. kernel-doc:: drivers/usb/core/hcd-pci.c 203 :export: 204 205.. kernel-doc:: drivers/usb/core/buffer.c 206 :internal: 207 208The USB character device nodes 209============================== 210 211This chapter presents the Linux character device nodes. You may prefer 212to avoid writing new kernel code for your USB driver. User mode device 213drivers are usually packaged as applications or libraries, and may use 214character devices through some programming library that wraps it. 215Such libraries include: 216 217 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and 218 - `jUSB <http://jUSB.sourceforge.net>`__ for Java. 219 220Some old information about it can be seen at the "USB Device Filesystem" 221section of the USB Guide. The latest copy of the USB Guide can be found 222at http://www.linux-usb.org/ 223 224.. note:: 225 226 - They were used to be implemented via *usbfs*, but this is not part of 227 the sysfs debug interface. 228 229 - This particular documentation is incomplete, especially with respect 230 to the asynchronous mode. As of kernel 2.5.66 the code and this 231 (new) documentation need to be cross-reviewed. 232 233What files are in "devtmpfs"? 234----------------------------- 235 236Conventionally mounted at ``/dev/bus/usb/``, usbfs features include: 237 238- ``/dev/bus/usb/BBB/DDD`` ... magic files exposing the each device's 239 configuration descriptors, and supporting a series of ioctls for 240 making device requests, including I/O to devices. (Purely for access 241 by programs.) 242 243Each bus is given a number (``BBB``) based on when it was enumerated; within 244each bus, each device is given a similar number (``DDD``). Those ``BBB/DDD`` 245paths are not "stable" identifiers; expect them to change even if you 246always leave the devices plugged in to the same hub port. *Don't even 247think of saving these in application configuration files.* Stable 248identifiers are available, for user mode applications that want to use 249them. HID and networking devices expose these stable IDs, so that for 250example you can be sure that you told the right UPS to power down its 251second server. Pleast note that it doesn't (yet) expose those IDs. 252 253/dev/bus/usb/BBB/DDD 254-------------------- 255 256Use these files in one of these basic ways: 257 258- *They can be read,* producing first the device descriptor (18 bytes) and 259 then the descriptors for the current configuration. See the USB 2.0 spec 260 for details about those binary data formats. You'll need to convert most 261 multibyte values from little endian format to your native host byte 262 order, although a few of the fields in the device descriptor (both of 263 the BCD-encoded fields, and the vendor and product IDs) will be 264 byteswapped for you. Note that configuration descriptors include 265 descriptors for interfaces, altsettings, endpoints, and maybe additional 266 class descriptors. 267 268- *Perform USB operations* using *ioctl()* requests to make endpoint I/O 269 requests (synchronously or asynchronously) or manage the device. These 270 requests need the ``CAP_SYS_RAWIO`` capability, as well as filesystem 271 access permissions. Only one ioctl request can be made on one of these 272 device files at a time. This means that if you are synchronously reading 273 an endpoint from one thread, you won't be able to write to a different 274 endpoint from another thread until the read completes. This works for 275 *half duplex* protocols, but otherwise you'd use asynchronous i/o 276 requests. 277 278Each connected USB device has one file. The ``BBB`` indicates the bus 279number. The ``DDD`` indicates the device address on that bus. Both 280of these numbers are assigned sequentially, and can be reused, so 281you can't rely on them for stable access to devices. For example, 282it's relatively common for devices to re-enumerate while they are 283still connected (perhaps someone jostled their power supply, hub, 284or USB cable), so a device might be ``002/027`` when you first connect 285it and ``002/048`` sometime later. 286 287These files can be read as binary data. The binary data consists 288of first the device descriptor, then the descriptors for each 289configuration of the device. Multi-byte fields in the device descriptor 290are converted to host endianness by the kernel. The configuration 291descriptors are in bus endian format! The configuration descriptor 292are wTotalLength bytes apart. If a device returns less configuration 293descriptor data than indicated by wTotalLength there will be a hole in 294the file for the missing bytes. This information is also shown 295in text form by the ``/sys/kernel/debug/usb/devices`` file, described later. 296 297These files may also be used to write user-level drivers for the USB 298devices. You would open the ``/dev/bus/usb/BBB/DDD`` file read/write, 299read its descriptors to make sure it's the device you expect, and then 300bind to an interface (or perhaps several) using an ioctl call. You 301would issue more ioctls to the device to communicate to it using 302control, bulk, or other kinds of USB transfers. The IOCTLs are 303listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the 304source code (``linux/drivers/usb/core/devio.c``) is the primary reference 305for how to access devices through those files. 306 307Note that since by default these ``BBB/DDD`` files are writable only by 308root, only root can write such user mode drivers. You can selectively 309grant read/write permissions to other users by using ``chmod``. Also, 310usbfs mount options such as ``devmode=0666`` may be helpful. 311 312 313Life Cycle of User Mode Drivers 314------------------------------- 315 316Such a driver first needs to find a device file for a device it knows 317how to handle. Maybe it was told about it because a ``/sbin/hotplug`` 318event handling agent chose that driver to handle the new device. Or 319maybe it's an application that scans all the ``/dev/bus/usb`` device files, 320and ignores most devices. In either case, it should :c:func:`read()` 321all the descriptors from the device file, and check them against what it 322knows how to handle. It might just reject everything except a particular 323vendor and product ID, or need a more complex policy. 324 325Never assume there will only be one such device on the system at a time! 326If your code can't handle more than one device at a time, at least 327detect when there's more than one, and have your users choose which 328device to use. 329 330Once your user mode driver knows what device to use, it interacts with 331it in either of two styles. The simple style is to make only control 332requests; some devices don't need more complex interactions than those. 333(An example might be software using vendor-specific control requests for 334some initialization or configuration tasks, with a kernel driver for the 335rest.) 336 337More likely, you need a more complex style driver: one using non-control 338endpoints, reading or writing data and claiming exclusive use of an 339interface. *Bulk* transfers are easiest to use, but only their sibling 340*interrupt* transfers work with low speed devices. Both interrupt and 341*isochronous* transfers offer service guarantees because their bandwidth 342is reserved. Such "periodic" transfers are awkward to use through usbfs, 343unless you're using the asynchronous calls. However, interrupt transfers 344can also be used in a synchronous "one shot" style. 345 346Your user-mode driver should never need to worry about cleaning up 347request state when the device is disconnected, although it should close 348its open file descriptors as soon as it starts seeing the ENODEV errors. 349 350The ioctl() Requests 351-------------------- 352 353To use these ioctls, you need to include the following headers in your 354userspace program:: 355 356 #include <linux/usb.h> 357 #include <linux/usbdevice_fs.h> 358 #include <asm/byteorder.h> 359 360The standard USB device model requests, from "Chapter 9" of the USB 2.0 361specification, are automatically included from the ``<linux/usb/ch9.h>`` 362header. 363 364Unless noted otherwise, the ioctl requests described here will update 365the modification time on the usbfs file to which they are applied 366(unless they fail). A return of zero indicates success; otherwise, a 367standard USB error code is returned (These are documented in 368:ref:`usb-error-codes`). 369 370Each of these files multiplexes access to several I/O streams, one per 371endpoint. Each device has one control endpoint (endpoint zero) which 372supports a limited RPC style RPC access. Devices are configured by 373hub_wq (in the kernel) setting a device-wide *configuration* that 374affects things like power consumption and basic functionality. The 375endpoints are part of USB *interfaces*, which may have *altsettings* 376affecting things like which endpoints are available. Many devices only 377have a single configuration and interface, so drivers for them will 378ignore configurations and altsettings. 379 380Management/Status Requests 381~~~~~~~~~~~~~~~~~~~~~~~~~~ 382 383A number of usbfs requests don't deal very directly with device I/O. 384They mostly relate to device management and status. These are all 385synchronous requests. 386 387USBDEVFS_CLAIMINTERFACE 388 This is used to force usbfs to claim a specific interface, which has 389 not previously been claimed by usbfs or any other kernel driver. The 390 ioctl parameter is an integer holding the number of the interface 391 (bInterfaceNumber from descriptor). 392 393 Note that if your driver doesn't claim an interface before trying to 394 use one of its endpoints, and no other driver has bound to it, then 395 the interface is automatically claimed by usbfs. 396 397 This claim will be released by a RELEASEINTERFACE ioctl, or by 398 closing the file descriptor. File modification time is not updated 399 by this request. 400 401USBDEVFS_CONNECTINFO 402 Says whether the device is lowspeed. The ioctl parameter points to a 403 structure like this:: 404 405 struct usbdevfs_connectinfo { 406 unsigned int devnum; 407 unsigned char slow; 408 }; 409 410 File modification time is not updated by this request. 411 412 *You can't tell whether a "not slow" device is connected at high 413 speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should 414 know the devnum value already, it's the DDD value of the device file 415 name. 416 417USBDEVFS_GETDRIVER 418 Returns the name of the kernel driver bound to a given interface (a 419 string). Parameter is a pointer to this structure, which is 420 modified:: 421 422 struct usbdevfs_getdriver { 423 unsigned int interface; 424 char driver[USBDEVFS_MAXDRIVERNAME + 1]; 425 }; 426 427 File modification time is not updated by this request. 428 429USBDEVFS_IOCTL 430 Passes a request from userspace through to a kernel driver that has 431 an ioctl entry in the *struct usb_driver* it registered:: 432 433 struct usbdevfs_ioctl { 434 int ifno; 435 int ioctl_code; 436 void *data; 437 }; 438 439 /* user mode call looks like this. 440 * 'request' becomes the driver->ioctl() 'code' parameter. 441 * the size of 'param' is encoded in 'request', and that data 442 * is copied to or from the driver->ioctl() 'buf' parameter. 443 */ 444 static int 445 usbdev_ioctl (int fd, int ifno, unsigned request, void *param) 446 { 447 struct usbdevfs_ioctl wrapper; 448 449 wrapper.ifno = ifno; 450 wrapper.ioctl_code = request; 451 wrapper.data = param; 452 453 return ioctl (fd, USBDEVFS_IOCTL, &wrapper); 454 } 455 456 File modification time is not updated by this request. 457 458 This request lets kernel drivers talk to user mode code through 459 filesystem operations even when they don't create a character or 460 block special device. It's also been used to do things like ask 461 devices what device special file should be used. Two pre-defined 462 ioctls are used to disconnect and reconnect kernel drivers, so that 463 user mode code can completely manage binding and configuration of 464 devices. 465 466USBDEVFS_RELEASEINTERFACE 467 This is used to release the claim usbfs made on interface, either 468 implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the 469 file descriptor is closed. The ioctl parameter is an integer holding 470 the number of the interface (bInterfaceNumber from descriptor); File 471 modification time is not updated by this request. 472 473 .. warning:: 474 475 *No security check is made to ensure that the task which made 476 the claim is the one which is releasing it. This means that user 477 mode driver may interfere other ones.* 478 479USBDEVFS_RESETEP 480 Resets the data toggle value for an endpoint (bulk or interrupt) to 481 DATA0. The ioctl parameter is an integer endpoint number (1 to 15, 482 as identified in the endpoint descriptor), with USB_DIR_IN added 483 if the device's endpoint sends data to the host. 484 485 .. Warning:: 486 487 *Avoid using this request. It should probably be removed.* Using 488 it typically means the device and driver will lose toggle 489 synchronization. If you really lost synchronization, you likely 490 need to completely handshake with the device, using a request 491 like CLEAR_HALT or SET_INTERFACE. 492 493USBDEVFS_DROP_PRIVILEGES 494 This is used to relinquish the ability to do certain operations 495 which are considered to be privileged on a usbfs file descriptor. 496 This includes claiming arbitrary interfaces, resetting a device on 497 which there are currently claimed interfaces from other users, and 498 issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask 499 of interfaces the user is allowed to claim on this file descriptor. 500 You may issue this ioctl more than one time to narrow said mask. 501 502Synchronous I/O Support 503~~~~~~~~~~~~~~~~~~~~~~~ 504 505Synchronous requests involve the kernel blocking until the user mode 506request completes, either by finishing successfully or by reporting an 507error. In most cases this is the simplest way to use usbfs, although as 508noted above it does prevent performing I/O to more than one endpoint at 509a time. 510 511USBDEVFS_BULK 512 Issues a bulk read or write request to the device. The ioctl 513 parameter is a pointer to this structure:: 514 515 struct usbdevfs_bulktransfer { 516 unsigned int ep; 517 unsigned int len; 518 unsigned int timeout; /* in milliseconds */ 519 void *data; 520 }; 521 522 The ``ep`` value identifies a bulk endpoint number (1 to 15, as 523 identified in an endpoint descriptor), masked with USB_DIR_IN when 524 referring to an endpoint which sends data to the host from the 525 device. The length of the data buffer is identified by ``len``; Recent 526 kernels support requests up to about 128KBytes. *FIXME say how read 527 length is returned, and how short reads are handled.*. 528 529USBDEVFS_CLEAR_HALT 530 Clears endpoint halt (stall) and resets the endpoint toggle. This is 531 only meaningful for bulk or interrupt endpoints. The ioctl parameter 532 is an integer endpoint number (1 to 15, as identified in an endpoint 533 descriptor), masked with USB_DIR_IN when referring to an endpoint 534 which sends data to the host from the device. 535 536 Use this on bulk or interrupt endpoints which have stalled, 537 returning ``-EPIPE`` status to a data transfer request. Do not issue 538 the control request directly, since that could invalidate the host's 539 record of the data toggle. 540 541USBDEVFS_CONTROL 542 Issues a control request to the device. The ioctl parameter points 543 to a structure like this:: 544 545 struct usbdevfs_ctrltransfer { 546 __u8 bRequestType; 547 __u8 bRequest; 548 __u16 wValue; 549 __u16 wIndex; 550 __u16 wLength; 551 __u32 timeout; /* in milliseconds */ 552 void *data; 553 }; 554 555 The first eight bytes of this structure are the contents of the 556 SETUP packet to be sent to the device; see the USB 2.0 specification 557 for details. The bRequestType value is composed by combining a 558 ``USB_TYPE_*`` value, a ``USB_DIR_*`` value, and a ``USB_RECIP_*`` 559 value (from ``linux/usb.h``). If wLength is nonzero, it describes 560 the length of the data buffer, which is either written to the device 561 (USB_DIR_OUT) or read from the device (USB_DIR_IN). 562 563 At this writing, you can't transfer more than 4 KBytes of data to or 564 from a device; usbfs has a limit, and some host controller drivers 565 have a limit. (That's not usually a problem.) *Also* there's no way 566 to say it's not OK to get a short read back from the device. 567 568USBDEVFS_RESET 569 Does a USB level device reset. The ioctl parameter is ignored. After 570 the reset, this rebinds all device interfaces. File modification 571 time is not updated by this request. 572 573.. warning:: 574 575 *Avoid using this call* until some usbcore bugs get fixed, since 576 it does not fully synchronize device, interface, and driver (not 577 just usbfs) state. 578 579USBDEVFS_SETINTERFACE 580 Sets the alternate setting for an interface. The ioctl parameter is 581 a pointer to a structure like this:: 582 583 struct usbdevfs_setinterface { 584 unsigned int interface; 585 unsigned int altsetting; 586 }; 587 588 File modification time is not updated by this request. 589 590 Those struct members are from some interface descriptor applying to 591 the current configuration. The interface number is the 592 bInterfaceNumber value, and the altsetting number is the 593 bAlternateSetting value. (This resets each endpoint in the 594 interface.) 595 596USBDEVFS_SETCONFIGURATION 597 Issues the :c:func:`usb_set_configuration()` call for the 598 device. The parameter is an integer holding the number of a 599 configuration (bConfigurationValue from descriptor). File 600 modification time is not updated by this request. 601 602.. warning:: 603 604 *Avoid using this call* until some usbcore bugs get fixed, since 605 it does not fully synchronize device, interface, and driver (not 606 just usbfs) state. 607 608Asynchronous I/O Support 609~~~~~~~~~~~~~~~~~~~~~~~~ 610 611As mentioned above, there are situations where it may be important to 612initiate concurrent operations from user mode code. This is particularly 613important for periodic transfers (interrupt and isochronous), but it can 614be used for other kinds of USB requests too. In such cases, the 615asynchronous requests described here are essential. Rather than 616submitting one request and having the kernel block until it completes, 617the blocking is separate. 618 619These requests are packaged into a structure that resembles the URB used 620by kernel device drivers. (No POSIX Async I/O support here, sorry.) It 621identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint 622(number, masked with USB_DIR_IN as appropriate), buffer and length, 623and a user "context" value serving to uniquely identify each request. 624(It's usually a pointer to per-request data.) Flags can modify requests 625(not as many as supported for kernel drivers). 626 627Each request can specify a realtime signal number (between SIGRTMIN and 628SIGRTMAX, inclusive) to request a signal be sent when the request 629completes. 630 631When usbfs returns these urbs, the status value is updated, and the 632buffer may have been modified. Except for isochronous transfers, the 633actual_length is updated to say how many bytes were transferred; if the 634USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if 635fewer bytes were read than were requested then you get an error report:: 636 637 struct usbdevfs_iso_packet_desc { 638 unsigned int length; 639 unsigned int actual_length; 640 unsigned int status; 641 }; 642 643 struct usbdevfs_urb { 644 unsigned char type; 645 unsigned char endpoint; 646 int status; 647 unsigned int flags; 648 void *buffer; 649 int buffer_length; 650 int actual_length; 651 int start_frame; 652 int number_of_packets; 653 int error_count; 654 unsigned int signr; 655 void *usercontext; 656 struct usbdevfs_iso_packet_desc iso_frame_desc[]; 657 }; 658 659For these asynchronous requests, the file modification time reflects 660when the request was initiated. This contrasts with their use with the 661synchronous requests, where it reflects when requests complete. 662 663USBDEVFS_DISCARDURB 664 *TBS* File modification time is not updated by this request. 665 666USBDEVFS_DISCSIGNAL 667 *TBS* File modification time is not updated by this request. 668 669USBDEVFS_REAPURB 670 *TBS* File modification time is not updated by this request. 671 672USBDEVFS_REAPURBNDELAY 673 *TBS* File modification time is not updated by this request. 674 675USBDEVFS_SUBMITURB 676 *TBS* 677 678The USB devices 679=============== 680 681The USB devices are now exported via debugfs: 682 683- ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB 684 devices on known to the kernel, and their configuration descriptors. 685 You can also poll() this to learn about new devices. 686 687/sys/kernel/debug/usb/devices 688----------------------------- 689 690This file is handy for status viewing tools in user mode, which can scan 691the text format and ignore most of it. More detailed device status 692(including class and vendor status) is available from device-specific 693files. For information about the current format of this file, see below. 694 695This file, in combination with the poll() system call, can also be used 696to detect when devices are added or removed:: 697 698 int fd; 699 struct pollfd pfd; 700 701 fd = open("/sys/kernel/debug/usb/devices", O_RDONLY); 702 pfd = { fd, POLLIN, 0 }; 703 for (;;) { 704 /* The first time through, this call will return immediately. */ 705 poll(&pfd, 1, -1); 706 707 /* To see what's changed, compare the file's previous and current 708 contents or scan the filesystem. (Scanning is more precise.) */ 709 } 710 711Note that this behavior is intended to be used for informational and 712debug purposes. It would be more appropriate to use programs such as 713udev or HAL to initialize a device or start a user-mode helper program, 714for instance. 715 716In this file, each device's output has multiple lines of ASCII output. 717 718I made it ASCII instead of binary on purpose, so that someone 719can obtain some useful data from it without the use of an 720auxiliary program. However, with an auxiliary program, the numbers 721in the first 4 columns of each ``T:`` line (topology info: 722Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram. 723 724Each line is tagged with a one-character ID for that line:: 725 726 T = Topology (etc.) 727 B = Bandwidth (applies only to USB host controllers, which are 728 virtualized as root hubs) 729 D = Device descriptor info. 730 P = Product ID info. (from Device descriptor, but they won't fit 731 together on one line) 732 S = String descriptors. 733 C = Configuration descriptor info. (* = active configuration) 734 I = Interface descriptor info. 735 E = Endpoint descriptor info. 736 737/sys/kernel/debug/usb/devices output format 738~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 739 740Legend:: 741 d = decimal number (may have leading spaces or 0's) 742 x = hexadecimal number (may have leading spaces or 0's) 743 s = string 744 745 746 747Topology info 748^^^^^^^^^^^^^ 749 750:: 751 752 T: Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd 753 | | | | | | | | |__MaxChildren 754 | | | | | | | |__Device Speed in Mbps 755 | | | | | | |__DeviceNumber 756 | | | | | |__Count of devices at this level 757 | | | | |__Connector/Port on Parent for this device 758 | | | |__Parent DeviceNumber 759 | | |__Level in topology for this bus 760 | |__Bus number 761 |__Topology info tag 762 763Speed may be: 764 765 ======= ====================================================== 766 1.5 Mbit/s for low speed USB 767 12 Mbit/s for full speed USB 768 480 Mbit/s for high speed USB (added for USB 2.0); 769 also used for Wireless USB, which has no fixed speed 770 5000 Mbit/s for SuperSpeed USB (added for USB 3.0) 771 ======= ====================================================== 772 773For reasons lost in the mists of time, the Port number is always 774too low by 1. For example, a device plugged into port 4 will 775show up with ``Port=03``. 776 777Bandwidth info 778^^^^^^^^^^^^^^ 779 780:: 781 782 B: Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd 783 | | | |__Number of isochronous requests 784 | | |__Number of interrupt requests 785 | |__Total Bandwidth allocated to this bus 786 |__Bandwidth info tag 787 788Bandwidth allocation is an approximation of how much of one frame 789(millisecond) is in use. It reflects only periodic transfers, which 790are the only transfers that reserve bandwidth. Control and bulk 791transfers use all other bandwidth, including reserved bandwidth that 792is not used for transfers (such as for short packets). 793 794The percentage is how much of the "reserved" bandwidth is scheduled by 795those transfers. For a low or full speed bus (loosely, "USB 1.1"), 79690% of the bus bandwidth is reserved. For a high speed bus (loosely, 797"USB 2.0") 80% is reserved. 798 799 800Device descriptor info & Product ID info 801^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 802 803:: 804 805 D: Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 806 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 807 808where:: 809 810 D: Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 811 | | | | | | |__NumberConfigurations 812 | | | | | |__MaxPacketSize of Default Endpoint 813 | | | | |__DeviceProtocol 814 | | | |__DeviceSubClass 815 | | |__DeviceClass 816 | |__Device USB version 817 |__Device info tag #1 818 819where:: 820 821 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 822 | | | |__Product revision number 823 | | |__Product ID code 824 | |__Vendor ID code 825 |__Device info tag #2 826 827 828String descriptor info 829^^^^^^^^^^^^^^^^^^^^^^ 830:: 831 832 S: Manufacturer=ssss 833 | |__Manufacturer of this device as read from the device. 834 | For USB host controller drivers (virtual root hubs) this may 835 | be omitted, or (for newer drivers) will identify the kernel 836 | version and the driver which provides this hub emulation. 837 |__String info tag 838 839 S: Product=ssss 840 | |__Product description of this device as read from the device. 841 | For older USB host controller drivers (virtual root hubs) this 842 | indicates the driver; for newer ones, it's a product (and vendor) 843 | description that often comes from the kernel's PCI ID database. 844 |__String info tag 845 846 S: SerialNumber=ssss 847 | |__Serial Number of this device as read from the device. 848 | For USB host controller drivers (virtual root hubs) this is 849 | some unique ID, normally a bus ID (address or slot name) that 850 | can't be shared with any other device. 851 |__String info tag 852 853 854 855Configuration descriptor info 856^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 857:: 858 859 C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA 860 | | | | | |__MaxPower in mA 861 | | | | |__Attributes 862 | | | |__ConfiguratioNumber 863 | | |__NumberOfInterfaces 864 | |__ "*" indicates the active configuration (others are " ") 865 |__Config info tag 866 867USB devices may have multiple configurations, each of which act 868rather differently. For example, a bus-powered configuration 869might be much less capable than one that is self-powered. Only 870one device configuration can be active at a time; most devices 871have only one configuration. 872 873Each configuration consists of one or more interfaces. Each 874interface serves a distinct "function", which is typically bound 875to a different USB device driver. One common example is a USB 876speaker with an audio interface for playback, and a HID interface 877for use with software volume control. 878 879Interface descriptor info (can be multiple per Config) 880^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 881:: 882 883 I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss 884 | | | | | | | | |__Driver name 885 | | | | | | | | or "(none)" 886 | | | | | | | |__InterfaceProtocol 887 | | | | | | |__InterfaceSubClass 888 | | | | | |__InterfaceClass 889 | | | | |__NumberOfEndpoints 890 | | | |__AlternateSettingNumber 891 | | |__InterfaceNumber 892 | |__ "*" indicates the active altsetting (others are " ") 893 |__Interface info tag 894 895A given interface may have one or more "alternate" settings. 896For example, default settings may not use more than a small 897amount of periodic bandwidth. To use significant fractions 898of bus bandwidth, drivers must select a non-default altsetting. 899 900Only one setting for an interface may be active at a time, and 901only one driver may bind to an interface at a time. Most devices 902have only one alternate setting per interface. 903 904 905Endpoint descriptor info (can be multiple per Interface) 906^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 907 908:: 909 910 E: Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss 911 | | | | |__Interval (max) between transfers 912 | | | |__EndpointMaxPacketSize 913 | | |__Attributes(EndpointType) 914 | |__EndpointAddress(I=In,O=Out) 915 |__Endpoint info tag 916 917The interval is nonzero for all periodic (interrupt or isochronous) 918endpoints. For high speed endpoints the transfer interval may be 919measured in microseconds rather than milliseconds. 920 921For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects 922the per-microframe data transfer size. For "high bandwidth" 923endpoints, that can reflect two or three packets (for up to 9243KBytes every 125 usec) per endpoint. 925 926With the Linux-USB stack, periodic bandwidth reservations use the 927transfer intervals and sizes provided by URBs, which can be less 928than those found in endpoint descriptor. 929 930Usage examples 931~~~~~~~~~~~~~~ 932 933If a user or script is interested only in Topology info, for 934example, use something like ``grep ^T: /sys/kernel/debug/usb/devices`` 935for only the Topology lines. A command like 936``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list 937only the lines that begin with the characters in square brackets, 938where the valid characters are TDPCIE. With a slightly more able 939script, it can display any selected lines (for example, only T, D, 940and P lines) and change their output format. (The ``procusb`` 941Perl script is the beginning of this idea. It will list only 942selected lines [selected from TBDPSCIE] or "All" lines from 943``/sys/kernel/debug/usb/devices``.) 944 945The Topology lines can be used to generate a graphic/pictorial 946of the USB devices on a system's root hub. (See more below 947on how to do this.) 948 949The Interface lines can be used to determine what driver is 950being used for each device, and which altsetting it activated. 951 952The Configuration lines could be used to list maximum power 953(in milliamps) that a system's USB devices are using. 954For example, ``grep ^C: /sys/kernel/debug/usb/devices``. 955 956 957Here's an example, from a system which has a UHCI root hub, 958an external hub connected to the root hub, and a mouse and 959a serial converter connected to the external hub. 960 961:: 962 963 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 964 B: Alloc= 28/900 us ( 3%), #Int= 2, #Iso= 0 965 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 966 P: Vendor=0000 ProdID=0000 Rev= 0.00 967 S: Product=USB UHCI Root Hub 968 S: SerialNumber=dce0 969 C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr= 0mA 970 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 971 E: Ad=81(I) Atr=03(Int.) MxPS= 8 Ivl=255ms 972 973 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 974 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 975 P: Vendor=0451 ProdID=1446 Rev= 1.00 976 C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA 977 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 978 E: Ad=81(I) Atr=03(Int.) MxPS= 1 Ivl=255ms 979 980 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 981 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 982 P: Vendor=04b4 ProdID=0001 Rev= 0.00 983 C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA 984 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 985 E: Ad=81(I) Atr=03(Int.) MxPS= 3 Ivl= 10ms 986 987 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 988 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 989 P: Vendor=0565 ProdID=0001 Rev= 1.08 990 S: Manufacturer=Peracom Networks, Inc. 991 S: Product=Peracom USB to Serial Converter 992 C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA 993 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 994 E: Ad=81(I) Atr=02(Bulk) MxPS= 64 Ivl= 16ms 995 E: Ad=01(O) Atr=02(Bulk) MxPS= 16 Ivl= 16ms 996 E: Ad=82(I) Atr=03(Int.) MxPS= 8 Ivl= 8ms 997 998 999Selecting only the ``T:`` and ``I:`` lines from this (for example, by using 1000``procusb ti``), we have 1001 1002:: 1003 1004 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 1005 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 1006 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 1007 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 1008 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 1009 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 1010 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 1011 1012 1013Physically this looks like (or could be converted to):: 1014 1015 +------------------+ 1016 | PC/root_hub (12)| Dev# = 1 1017 +------------------+ (nn) is Mbps. 1018 Level 0 | CN.0 | CN.1 | [CN = connector/port #] 1019 +------------------+ 1020 / 1021 / 1022 +-----------------------+ 1023 Level 1 | Dev#2: 4-port hub (12)| 1024 +-----------------------+ 1025 |CN.0 |CN.1 |CN.2 |CN.3 | 1026 +-----------------------+ 1027 \ \____________________ 1028 \_____ \ 1029 \ \ 1030 +--------------------+ +--------------------+ 1031 Level 2 | Dev# 3: mouse (1.5)| | Dev# 4: serial (12)| 1032 +--------------------+ +--------------------+ 1033 1034 1035 1036Or, in a more tree-like structure (ports [Connectors] without 1037connections could be omitted):: 1038 1039 PC: Dev# 1, root hub, 2 ports, 12 Mbps 1040 |_ CN.0: Dev# 2, hub, 4 ports, 12 Mbps 1041 |_ CN.0: Dev #3, mouse, 1.5 Mbps 1042 |_ CN.1: 1043 |_ CN.2: Dev #4, serial, 12 Mbps 1044 |_ CN.3: 1045 |_ CN.1: 1046