1======================== 2USB Gadget API for Linux 3======================== 4 5:Author: David Brownell 6:Date: 20 August 2004 7 8Introduction 9============ 10 11This document presents a Linux-USB "Gadget" kernel mode API, for use 12within peripherals and other USB devices that embed Linux. It provides 13an overview of the API structure, and shows how that fits into a system 14development project. This is the first such API released on Linux to 15address a number of important problems, including: 16 17- Supports USB 2.0, for high speed devices which can stream data at 18 several dozen megabytes per second. 19 20- Handles devices with dozens of endpoints just as well as ones with 21 just two fixed-function ones. Gadget drivers can be written so 22 they're easy to port to new hardware. 23 24- Flexible enough to expose more complex USB device capabilities such 25 as multiple configurations, multiple interfaces, composite devices, 26 and alternate interface settings. 27 28- USB "On-The-Go" (OTG) support, in conjunction with updates to the 29 Linux-USB host side. 30 31- Sharing data structures and API models with the Linux-USB host side 32 API. This helps the OTG support, and looks forward to more-symmetric 33 frameworks (where the same I/O model is used by both host and device 34 side drivers). 35 36- Minimalist, so it's easier to support new device controller hardware. 37 I/O processing doesn't imply large demands for memory or CPU 38 resources. 39 40Most Linux developers will not be able to use this API, since they have 41USB ``host`` hardware in a PC, workstation, or server. Linux users with 42embedded systems are more likely to have USB peripheral hardware. To 43distinguish drivers running inside such hardware from the more familiar 44Linux "USB device drivers", which are host side proxies for the real USB 45devices, a different term is used: the drivers inside the peripherals 46are "USB gadget drivers". In USB protocol interactions, the device 47driver is the master (or "client driver") and the gadget driver is the 48slave (or "function driver"). 49 50The gadget API resembles the host side Linux-USB API in that both use 51queues of request objects to package I/O buffers, and those requests may 52be submitted or canceled. They share common definitions for the standard 53USB *Chapter 9* messages, structures, and constants. Also, both APIs 54bind and unbind drivers to devices. The APIs differ in detail, since the 55host side's current URB framework exposes a number of implementation 56details and assumptions that are inappropriate for a gadget API. While 57the model for control transfers and configuration management is 58necessarily different (one side is a hardware-neutral master, the other 59is a hardware-aware slave), the endpoint I/0 API used here should also 60be usable for an overhead-reduced host side API. 61 62Structure of Gadget Drivers 63=========================== 64 65A system running inside a USB peripheral normally has at least three 66layers inside the kernel to handle USB protocol processing, and may have 67additional layers in user space code. The ``gadget`` API is used by the 68middle layer to interact with the lowest level (which directly handles 69hardware). 70 71In Linux, from the bottom up, these layers are: 72 73*USB Controller Driver* 74 This is the lowest software level. It is the only layer that talks 75 to hardware, through registers, fifos, dma, irqs, and the like. The 76 ``<linux/usb/gadget.h>`` API abstracts the peripheral controller 77 endpoint hardware. That hardware is exposed through endpoint 78 objects, which accept streams of IN/OUT buffers, and through 79 callbacks that interact with gadget drivers. Since normal USB 80 devices only have one upstream port, they only have one of these 81 drivers. The controller driver can support any number of different 82 gadget drivers, but only one of them can be used at a time. 83 84 Examples of such controller hardware include the PCI-based NetChip 85 2280 USB 2.0 high speed controller, the SA-11x0 or PXA-25x UDC 86 (found within many PDAs), and a variety of other products. 87 88*Gadget Driver* 89 The lower boundary of this driver implements hardware-neutral USB 90 functions, using calls to the controller driver. Because such 91 hardware varies widely in capabilities and restrictions, and is used 92 in embedded environments where space is at a premium, the gadget 93 driver is often configured at compile time to work with endpoints 94 supported by one particular controller. Gadget drivers may be 95 portable to several different controllers, using conditional 96 compilation. (Recent kernels substantially simplify the work 97 involved in supporting new hardware, by *autoconfiguring* endpoints 98 automatically for many bulk-oriented drivers.) Gadget driver 99 responsibilities include: 100 101 - handling setup requests (ep0 protocol responses) possibly 102 including class-specific functionality 103 104 - returning configuration and string descriptors 105 106 - (re)setting configurations and interface altsettings, including 107 enabling and configuring endpoints 108 109 - handling life cycle events, such as managing bindings to 110 hardware, USB suspend/resume, remote wakeup, and disconnection 111 from the USB host. 112 113 - managing IN and OUT transfers on all currently enabled endpoints 114 115 Such drivers may be modules of proprietary code, although that 116 approach is discouraged in the Linux community. 117 118*Upper Level* 119 Most gadget drivers have an upper boundary that connects to some 120 Linux driver or framework in Linux. Through that boundary flows the 121 data which the gadget driver produces and/or consumes through 122 protocol transfers over USB. Examples include: 123 124 - user mode code, using generic (gadgetfs) or application specific 125 files in ``/dev`` 126 127 - networking subsystem (for network gadgets, like the CDC Ethernet 128 Model gadget driver) 129 130 - data capture drivers, perhaps video4Linux or a scanner driver; or 131 test and measurement hardware. 132 133 - input subsystem (for HID gadgets) 134 135 - sound subsystem (for audio gadgets) 136 137 - file system (for PTP gadgets) 138 139 - block i/o subsystem (for usb-storage gadgets) 140 141 - ... and more 142 143*Additional Layers* 144 Other layers may exist. These could include kernel layers, such as 145 network protocol stacks, as well as user mode applications building 146 on standard POSIX system call APIs such as ``open()``, ``close()``, 147 ``read()`` and ``write()``. On newer systems, POSIX Async I/O calls may 148 be an option. Such user mode code will not necessarily be subject to 149 the GNU General Public License (GPL). 150 151OTG-capable systems will also need to include a standard Linux-USB host 152side stack, with ``usbcore``, one or more *Host Controller Drivers* 153(HCDs), *USB Device Drivers* to support the OTG "Targeted Peripheral 154List", and so forth. There will also be an *OTG Controller Driver*, 155which is visible to gadget and device driver developers only indirectly. 156That helps the host and device side USB controllers implement the two 157new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or 158vice versa) using HNP during USB suspend processing, and SRP can be 159viewed as a more battery-friendly kind of device wakeup protocol. 160 161Over time, reusable utilities are evolving to help make some gadget 162driver tasks simpler. For example, building configuration descriptors 163from vectors of descriptors for the configurations interfaces and 164endpoints is now automated, and many drivers now use autoconfiguration 165to choose hardware endpoints and initialize their descriptors. A 166potential example of particular interest is code implementing standard 167USB-IF protocols for HID, networking, storage, or audio classes. Some 168developers are interested in KDB or KGDB hooks, to let target hardware 169be remotely debugged. Most such USB protocol code doesn't need to be 170hardware-specific, any more than network protocols like X11, HTTP, or 171NFS are. Such gadget-side interface drivers should eventually be 172combined, to implement composite devices. 173 174Kernel Mode Gadget API 175====================== 176 177Gadget drivers declare themselves through a struct 178:c:type:`usb_gadget_driver`, which is responsible for most parts of enumeration 179for a struct usb_gadget. The response to a set_configuration usually 180involves enabling one or more of the struct usb_ep objects exposed by 181the gadget, and submitting one or more struct usb_request buffers to 182transfer data. Understand those four data types, and their operations, 183and you will understand how this API works. 184 185.. Note:: 186 187 Other than the "Chapter 9" data types, most of the significant data 188 types and functions are described here. 189 190 However, some relevant information is likely omitted from what you 191 are reading. One example of such information is endpoint 192 autoconfiguration. You'll have to read the header file, and use 193 example source code (such as that for "Gadget Zero"), to fully 194 understand the API. 195 196 The part of the API implementing some basic driver capabilities is 197 specific to the version of the Linux kernel that's in use. The 2.6 198 and upper kernel versions include a *driver model* framework that has 199 no analogue on earlier kernels; so those parts of the gadget API are 200 not fully portable. (They are implemented on 2.4 kernels, but in a 201 different way.) The driver model state is another part of this API that is 202 ignored by the kerneldoc tools. 203 204The core API does not expose every possible hardware feature, only the 205most widely available ones. There are significant hardware features, 206such as device-to-device DMA (without temporary storage in a memory 207buffer) that would be added using hardware-specific APIs. 208 209This API allows drivers to use conditional compilation to handle 210endpoint capabilities of different hardware, but doesn't require that. 211Hardware tends to have arbitrary restrictions, relating to transfer 212types, addressing, packet sizes, buffering, and availability. As a rule, 213such differences only matter for "endpoint zero" logic that handles 214device configuration and management. The API supports limited run-time 215detection of capabilities, through naming conventions for endpoints. 216Many drivers will be able to at least partially autoconfigure 217themselves. In particular, driver init sections will often have endpoint 218autoconfiguration logic that scans the hardware's list of endpoints to 219find ones matching the driver requirements (relying on those 220conventions), to eliminate some of the most common reasons for 221conditional compilation. 222 223Like the Linux-USB host side API, this API exposes the "chunky" nature 224of USB messages: I/O requests are in terms of one or more "packets", and 225packet boundaries are visible to drivers. Compared to RS-232 serial 226protocols, USB resembles synchronous protocols like HDLC (N bytes per 227frame, multipoint addressing, host as the primary station and devices as 228secondary stations) more than asynchronous ones (tty style: 8 data bits 229per frame, no parity, one stop bit). So for example the controller 230drivers won't buffer two single byte writes into a single two-byte USB 231IN packet, although gadget drivers may do so when they implement 232protocols where packet boundaries (and "short packets") are not 233significant. 234 235Driver Life Cycle 236----------------- 237 238Gadget drivers make endpoint I/O requests to hardware without needing to 239know many details of the hardware, but driver setup/configuration code 240needs to handle some differences. Use the API like this: 241 2421. Register a driver for the particular device side usb controller 243 hardware, such as the net2280 on PCI (USB 2.0), sa11x0 or pxa25x as 244 found in Linux PDAs, and so on. At this point the device is logically 245 in the USB ch9 initial state (``attached``), drawing no power and not 246 usable (since it does not yet support enumeration). Any host should 247 not see the device, since it's not activated the data line pullup 248 used by the host to detect a device, even if VBUS power is available. 249 2502. Register a gadget driver that implements some higher level device 251 function. That will then bind() to a :c:type:`usb_gadget`, which activates 252 the data line pullup sometime after detecting VBUS. 253 2543. The hardware driver can now start enumerating. The steps it handles 255 are to accept USB ``power`` and ``set_address`` requests. Other steps are 256 handled by the gadget driver. If the gadget driver module is unloaded 257 before the host starts to enumerate, steps before step 7 are skipped. 258 2594. The gadget driver's ``setup()`` call returns usb descriptors, based both 260 on what the bus interface hardware provides and on the functionality 261 being implemented. That can involve alternate settings or 262 configurations, unless the hardware prevents such operation. For OTG 263 devices, each configuration descriptor includes an OTG descriptor. 264 2655. The gadget driver handles the last step of enumeration, when the USB 266 host issues a ``set_configuration`` call. It enables all endpoints used 267 in that configuration, with all interfaces in their default settings. 268 That involves using a list of the hardware's endpoints, enabling each 269 endpoint according to its descriptor. It may also involve using 270 ``usb_gadget_vbus_draw`` to let more power be drawn from VBUS, as 271 allowed by that configuration. For OTG devices, setting a 272 configuration may also involve reporting HNP capabilities through a 273 user interface. 274 2756. Do real work and perform data transfers, possibly involving changes 276 to interface settings or switching to new configurations, until the 277 device is disconnect()ed from the host. Queue any number of transfer 278 requests to each endpoint. It may be suspended and resumed several 279 times before being disconnected. On disconnect, the drivers go back 280 to step 3 (above). 281 2827. When the gadget driver module is being unloaded, the driver unbind() 283 callback is issued. That lets the controller driver be unloaded. 284 285Drivers will normally be arranged so that just loading the gadget driver 286module (or statically linking it into a Linux kernel) allows the 287peripheral device to be enumerated, but some drivers will defer 288enumeration until some higher level component (like a user mode daemon) 289enables it. Note that at this lowest level there are no policies about 290how ep0 configuration logic is implemented, except that it should obey 291USB specifications. Such issues are in the domain of gadget drivers, 292including knowing about implementation constraints imposed by some USB 293controllers or understanding that composite devices might happen to be 294built by integrating reusable components. 295 296Note that the lifecycle above can be slightly different for OTG devices. 297Other than providing an additional OTG descriptor in each configuration, 298only the HNP-related differences are particularly visible to driver 299code. They involve reporting requirements during the ``SET_CONFIGURATION`` 300request, and the option to invoke HNP during some suspend callbacks. 301Also, SRP changes the semantics of ``usb_gadget_wakeup`` slightly. 302 303USB 2.0 Chapter 9 Types and Constants 304------------------------------------- 305 306Gadget drivers rely on common USB structures and constants defined in 307the :ref:`linux/usb/ch9.h <usb_chapter9>` header file, which is standard in 308Linux 2.6+ kernels. These are the same types and constants used by host side 309drivers (and usbcore). 310 311Core Objects and Methods 312------------------------ 313 314These are declared in ``<linux/usb/gadget.h>``, and are used by gadget 315drivers to interact with USB peripheral controller drivers. 316 317.. kernel-doc:: include/linux/usb/gadget.h 318 :internal: 319 320Optional Utilities 321------------------ 322 323The core API is sufficient for writing a USB Gadget Driver, but some 324optional utilities are provided to simplify common tasks. These 325utilities include endpoint autoconfiguration. 326 327.. kernel-doc:: drivers/usb/gadget/usbstring.c 328 :export: 329 330.. kernel-doc:: drivers/usb/gadget/config.c 331 :export: 332 333Composite Device Framework 334-------------------------- 335 336The core API is sufficient for writing drivers for composite USB devices 337(with more than one function in a given configuration), and also 338multi-configuration devices (also more than one function, but not 339necessarily sharing a given configuration). There is however an optional 340framework which makes it easier to reuse and combine functions. 341 342Devices using this framework provide a struct usb_composite_driver, 343which in turn provides one or more struct usb_configuration 344instances. Each such configuration includes at least one struct 345:c:type:`usb_function`, which packages a user visible role such as "network 346link" or "mass storage device". Management functions may also exist, 347such as "Device Firmware Upgrade". 348 349.. kernel-doc:: include/linux/usb/composite.h 350 :internal: 351 352.. kernel-doc:: drivers/usb/gadget/composite.c 353 :export: 354 355Composite Device Functions 356-------------------------- 357 358At this writing, a few of the current gadget drivers have been converted 359to this framework. Near-term plans include converting all of them, 360except for ``gadgetfs``. 361 362Peripheral Controller Drivers 363============================= 364 365The first hardware supporting this API was the NetChip 2280 controller, 366which supports USB 2.0 high speed and is based on PCI. This is the 367``net2280`` driver module. The driver supports Linux kernel versions 2.4 368and 2.6; contact NetChip Technologies for development boards and product 369information. 370 371Other hardware working in the ``gadget`` framework includes: Intel's PXA 37225x and IXP42x series processors (``pxa2xx_udc``), Toshiba TC86c001 373"Goku-S" (``goku_udc``), Renesas SH7705/7727 (``sh_udc``), MediaQ 11xx 374(``mq11xx_udc``), Hynix HMS30C7202 (``h7202_udc``), National 9303/4 375(``n9604_udc``), Texas Instruments OMAP (``omap_udc``), Sharp LH7A40x 376(``lh7a40x_udc``), and more. Most of those are full speed controllers. 377 378At this writing, there are people at work on drivers in this framework 379for several other USB device controllers, with plans to make many of 380them be widely available. 381 382A partial USB simulator, the ``dummy_hcd`` driver, is available. It can 383act like a net2280, a pxa25x, or an sa11x0 in terms of available 384endpoints and device speeds; and it simulates control, bulk, and to some 385extent interrupt transfers. That lets you develop some parts of a gadget 386driver on a normal PC, without any special hardware, and perhaps with 387the assistance of tools such as GDB running with User Mode Linux. At 388least one person has expressed interest in adapting that approach, 389hooking it up to a simulator for a microcontroller. Such simulators can 390help debug subsystems where the runtime hardware is unfriendly to 391software development, or is not yet available. 392 393Support for other controllers is expected to be developed and 394contributed over time, as this driver framework evolves. 395 396Gadget Drivers 397============== 398 399In addition to *Gadget Zero* (used primarily for testing and development 400with drivers for usb controller hardware), other gadget drivers exist. 401 402There's an ``ethernet`` gadget driver, which implements one of the most 403useful *Communications Device Class* (CDC) models. One of the standards 404for cable modem interoperability even specifies the use of this ethernet 405model as one of two mandatory options. Gadgets using this code look to a 406USB host as if they're an Ethernet adapter. It provides access to a 407network where the gadget's CPU is one host, which could easily be 408bridging, routing, or firewalling access to other networks. Since some 409hardware can't fully implement the CDC Ethernet requirements, this 410driver also implements a "good parts only" subset of CDC Ethernet. (That 411subset doesn't advertise itself as CDC Ethernet, to avoid creating 412problems.) 413 414Support for Microsoft's ``RNDIS`` protocol has been contributed by 415Pengutronix and Auerswald GmbH. This is like CDC Ethernet, but it runs 416on more slightly USB hardware (but less than the CDC subset). However, 417its main claim to fame is being able to connect directly to recent 418versions of Windows, using drivers that Microsoft bundles and supports, 419making it much simpler to network with Windows. 420 421There is also support for user mode gadget drivers, using ``gadgetfs``. 422This provides a *User Mode API* that presents each endpoint as a single 423file descriptor. I/O is done using normal ``read()`` and ``read()`` calls. 424Familiar tools like GDB and pthreads can be used to develop and debug 425user mode drivers, so that once a robust controller driver is available 426many applications for it won't require new kernel mode software. Linux 4272.6 *Async I/O (AIO)* support is available, so that user mode software 428can stream data with only slightly more overhead than a kernel driver. 429 430There's a USB Mass Storage class driver, which provides a different 431solution for interoperability with systems such as MS-Windows and MacOS. 432That *Mass Storage* driver uses a file or block device as backing store 433for a drive, like the ``loop`` driver. The USB host uses the BBB, CB, or 434CBI versions of the mass storage class specification, using transparent 435SCSI commands to access the data from the backing store. 436 437There's a "serial line" driver, useful for TTY style operation over USB. 438The latest version of that driver supports CDC ACM style operation, like 439a USB modem, and so on most hardware it can interoperate easily with 440MS-Windows. One interesting use of that driver is in boot firmware (like 441a BIOS), which can sometimes use that model with very small systems 442without real serial lines. 443 444Support for other kinds of gadget is expected to be developed and 445contributed over time, as this driver framework evolves. 446 447USB On-The-GO (OTG) 448=================== 449 450USB OTG support on Linux 2.6 was initially developed by Texas 451Instruments for `OMAP <http://www.omap.com>`__ 16xx and 17xx series 452processors. Other OTG systems should work in similar ways, but the 453hardware level details could be very different. 454 455Systems need specialized hardware support to implement OTG, notably 456including a special *Mini-AB* jack and associated transceiver to support 457*Dual-Role* operation: they can act either as a host, using the standard 458Linux-USB host side driver stack, or as a peripheral, using this 459``gadget`` framework. To do that, the system software relies on small 460additions to those programming interfaces, and on a new internal 461component (here called an "OTG Controller") affecting which driver stack 462connects to the OTG port. In each role, the system can re-use the 463existing pool of hardware-neutral drivers, layered on top of the 464controller driver interfaces (:c:type:`usb_bus` or :c:type:`usb_gadget`). 465Such drivers need at most minor changes, and most of the calls added to 466support OTG can also benefit non-OTG products. 467 468- Gadget drivers test the ``is_otg`` flag, and use it to determine 469 whether or not to include an OTG descriptor in each of their 470 configurations. 471 472- Gadget drivers may need changes to support the two new OTG protocols, 473 exposed in new gadget attributes such as ``b_hnp_enable`` flag. HNP 474 support should be reported through a user interface (two LEDs could 475 suffice), and is triggered in some cases when the host suspends the 476 peripheral. SRP support can be user-initiated just like remote 477 wakeup, probably by pressing the same button. 478 479- On the host side, USB device drivers need to be taught to trigger HNP 480 at appropriate moments, using ``usb_suspend_device()``. That also 481 conserves battery power, which is useful even for non-OTG 482 configurations. 483 484- Also on the host side, a driver must support the OTG "Targeted 485 Peripheral List". That's just a whitelist, used to reject peripherals 486 not supported with a given Linux OTG host. *This whitelist is 487 product-specific; each product must modify* ``otg_whitelist.h`` *to 488 match its interoperability specification.* 489 490 Non-OTG Linux hosts, like PCs and workstations, normally have some 491 solution for adding drivers, so that peripherals that aren't 492 recognized can eventually be supported. That approach is unreasonable 493 for consumer products that may never have their firmware upgraded, 494 and where it's usually unrealistic to expect traditional 495 PC/workstation/server kinds of support model to work. For example, 496 it's often impractical to change device firmware once the product has 497 been distributed, so driver bugs can't normally be fixed if they're 498 found after shipment. 499 500Additional changes are needed below those hardware-neutral :c:type:`usb_bus` 501and :c:type:`usb_gadget` driver interfaces; those aren't discussed here in any 502detail. Those affect the hardware-specific code for each USB Host or 503Peripheral controller, and how the HCD initializes (since OTG can be 504active only on a single port). They also involve what may be called an 505*OTG Controller Driver*, managing the OTG transceiver and the OTG state 506machine logic as well as much of the root hub behavior for the OTG port. 507The OTG controller driver needs to activate and deactivate USB 508controllers depending on the relevant device role. Some related changes 509were needed inside usbcore, so that it can identify OTG-capable devices 510and respond appropriately to HNP or SRP protocols. 511