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diff --git a/Documentation/driver-api/usb/gadget.rst b/Documentation/driver-api/usb/gadget.rst new file mode 100644 index 000000000..3e8a3809c --- /dev/null +++ b/Documentation/driver-api/usb/gadget.rst @@ -0,0 +1,510 @@ +======================== +USB Gadget API for Linux +======================== + +:Author: David Brownell +:Date: 20 August 2004 + +Introduction +============ + +This document presents a Linux-USB "Gadget" kernel mode API, for use +within peripherals and other USB devices that embed Linux. It provides +an overview of the API structure, and shows how that fits into a system +development project. This is the first such API released on Linux to +address a number of important problems, including: + +- Supports USB 2.0, for high speed devices which can stream data at + several dozen megabytes per second. + +- Handles devices with dozens of endpoints just as well as ones with + just two fixed-function ones. Gadget drivers can be written so + they're easy to port to new hardware. + +- Flexible enough to expose more complex USB device capabilities such + as multiple configurations, multiple interfaces, composite devices, + and alternate interface settings. + +- USB "On-The-Go" (OTG) support, in conjunction with updates to the + Linux-USB host side. + +- Sharing data structures and API models with the Linux-USB host side + API. This helps the OTG support, and looks forward to more-symmetric + frameworks (where the same I/O model is used by both host and device + side drivers). + +- Minimalist, so it's easier to support new device controller hardware. + I/O processing doesn't imply large demands for memory or CPU + resources. + +Most Linux developers will not be able to use this API, since they have +USB ``host`` hardware in a PC, workstation, or server. Linux users with +embedded systems are more likely to have USB peripheral hardware. To +distinguish drivers running inside such hardware from the more familiar +Linux "USB device drivers", which are host side proxies for the real USB +devices, a different term is used: the drivers inside the peripherals +are "USB gadget drivers". In USB protocol interactions, the device +driver is the master (or "client driver") and the gadget driver is the +slave (or "function driver"). + +The gadget API resembles the host side Linux-USB API in that both use +queues of request objects to package I/O buffers, and those requests may +be submitted or canceled. They share common definitions for the standard +USB *Chapter 9* messages, structures, and constants. Also, both APIs +bind and unbind drivers to devices. The APIs differ in detail, since the +host side's current URB framework exposes a number of implementation +details and assumptions that are inappropriate for a gadget API. While +the model for control transfers and configuration management is +necessarily different (one side is a hardware-neutral master, the other +is a hardware-aware slave), the endpoint I/0 API used here should also +be usable for an overhead-reduced host side API. + +Structure of Gadget Drivers +=========================== + +A system running inside a USB peripheral normally has at least three +layers inside the kernel to handle USB protocol processing, and may have +additional layers in user space code. The ``gadget`` API is used by the +middle layer to interact with the lowest level (which directly handles +hardware). + +In Linux, from the bottom up, these layers are: + +*USB Controller Driver* + This is the lowest software level. It is the only layer that talks + to hardware, through registers, fifos, dma, irqs, and the like. The + ``<linux/usb/gadget.h>`` API abstracts the peripheral controller + endpoint hardware. That hardware is exposed through endpoint + objects, which accept streams of IN/OUT buffers, and through + callbacks that interact with gadget drivers. Since normal USB + devices only have one upstream port, they only have one of these + drivers. The controller driver can support any number of different + gadget drivers, but only one of them can be used at a time. + + Examples of such controller hardware include the PCI-based NetChip + 2280 USB 2.0 high speed controller, the SA-11x0 or PXA-25x UDC + (found within many PDAs), and a variety of other products. + +*Gadget Driver* + The lower boundary of this driver implements hardware-neutral USB + functions, using calls to the controller driver. Because such + hardware varies widely in capabilities and restrictions, and is used + in embedded environments where space is at a premium, the gadget + driver is often configured at compile time to work with endpoints + supported by one particular controller. Gadget drivers may be + portable to several different controllers, using conditional + compilation. (Recent kernels substantially simplify the work + involved in supporting new hardware, by *autoconfiguring* endpoints + automatically for many bulk-oriented drivers.) Gadget driver + responsibilities include: + + - handling setup requests (ep0 protocol responses) possibly + including class-specific functionality + + - returning configuration and string descriptors + + - (re)setting configurations and interface altsettings, including + enabling and configuring endpoints + + - handling life cycle events, such as managing bindings to + hardware, USB suspend/resume, remote wakeup, and disconnection + from the USB host. + + - managing IN and OUT transfers on all currently enabled endpoints + + Such drivers may be modules of proprietary code, although that + approach is discouraged in the Linux community. + +*Upper Level* + Most gadget drivers have an upper boundary that connects to some + Linux driver or framework in Linux. Through that boundary flows the + data which the gadget driver produces and/or consumes through + protocol transfers over USB. Examples include: + + - user mode code, using generic (gadgetfs) or application specific + files in ``/dev`` + + - networking subsystem (for network gadgets, like the CDC Ethernet + Model gadget driver) + + - data capture drivers, perhaps video4Linux or a scanner driver; or + test and measurement hardware. + + - input subsystem (for HID gadgets) + + - sound subsystem (for audio gadgets) + + - file system (for PTP gadgets) + + - block i/o subsystem (for usb-storage gadgets) + + - ... and more + +*Additional Layers* + Other layers may exist. These could include kernel layers, such as + network protocol stacks, as well as user mode applications building + on standard POSIX system call APIs such as ``open()``, ``close()``, + ``read()`` and ``write()``. On newer systems, POSIX Async I/O calls may + be an option. Such user mode code will not necessarily be subject to + the GNU General Public License (GPL). + +OTG-capable systems will also need to include a standard Linux-USB host +side stack, with ``usbcore``, one or more *Host Controller Drivers* +(HCDs), *USB Device Drivers* to support the OTG "Targeted Peripheral +List", and so forth. There will also be an *OTG Controller Driver*, +which is visible to gadget and device driver developers only indirectly. +That helps the host and device side USB controllers implement the two +new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or +vice versa) using HNP during USB suspend processing, and SRP can be +viewed as a more battery-friendly kind of device wakeup protocol. + +Over time, reusable utilities are evolving to help make some gadget +driver tasks simpler. For example, building configuration descriptors +from vectors of descriptors for the configurations interfaces and +endpoints is now automated, and many drivers now use autoconfiguration +to choose hardware endpoints and initialize their descriptors. A +potential example of particular interest is code implementing standard +USB-IF protocols for HID, networking, storage, or audio classes. Some +developers are interested in KDB or KGDB hooks, to let target hardware +be remotely debugged. Most such USB protocol code doesn't need to be +hardware-specific, any more than network protocols like X11, HTTP, or +NFS are. Such gadget-side interface drivers should eventually be +combined, to implement composite devices. + +Kernel Mode Gadget API +====================== + +Gadget drivers declare themselves through a struct +:c:type:`usb_gadget_driver`, which is responsible for most parts of enumeration +for a struct :c:type:`usb_gadget`. The response to a set_configuration usually +involves enabling one or more of the struct :c:type:`usb_ep` objects exposed by +the gadget, and submitting one or more struct :c:type:`usb_request` buffers to +transfer data. Understand those four data types, and their operations, +and you will understand how this API works. + +.. Note:: + + Other than the "Chapter 9" data types, most of the significant data + types and functions are described here. + + However, some relevant information is likely omitted from what you + are reading. One example of such information is endpoint + autoconfiguration. You'll have to read the header file, and use + example source code (such as that for "Gadget Zero"), to fully + understand the API. + + The part of the API implementing some basic driver capabilities is + specific to the version of the Linux kernel that's in use. The 2.6 + and upper kernel versions include a *driver model* framework that has + no analogue on earlier kernels; so those parts of the gadget API are + not fully portable. (They are implemented on 2.4 kernels, but in a + different way.) The driver model state is another part of this API that is + ignored by the kerneldoc tools. + +The core API does not expose every possible hardware feature, only the +most widely available ones. There are significant hardware features, +such as device-to-device DMA (without temporary storage in a memory +buffer) that would be added using hardware-specific APIs. + +This API allows drivers to use conditional compilation to handle +endpoint capabilities of different hardware, but doesn't require that. +Hardware tends to have arbitrary restrictions, relating to transfer +types, addressing, packet sizes, buffering, and availability. As a rule, +such differences only matter for "endpoint zero" logic that handles +device configuration and management. The API supports limited run-time +detection of capabilities, through naming conventions for endpoints. +Many drivers will be able to at least partially autoconfigure +themselves. In particular, driver init sections will often have endpoint +autoconfiguration logic that scans the hardware's list of endpoints to +find ones matching the driver requirements (relying on those +conventions), to eliminate some of the most common reasons for +conditional compilation. + +Like the Linux-USB host side API, this API exposes the "chunky" nature +of USB messages: I/O requests are in terms of one or more "packets", and +packet boundaries are visible to drivers. Compared to RS-232 serial +protocols, USB resembles synchronous protocols like HDLC (N bytes per +frame, multipoint addressing, host as the primary station and devices as +secondary stations) more than asynchronous ones (tty style: 8 data bits +per frame, no parity, one stop bit). So for example the controller +drivers won't buffer two single byte writes into a single two-byte USB +IN packet, although gadget drivers may do so when they implement +protocols where packet boundaries (and "short packets") are not +significant. + +Driver Life Cycle +----------------- + +Gadget drivers make endpoint I/O requests to hardware without needing to +know many details of the hardware, but driver setup/configuration code +needs to handle some differences. Use the API like this: + +1. Register a driver for the particular device side usb controller + hardware, such as the net2280 on PCI (USB 2.0), sa11x0 or pxa25x as + found in Linux PDAs, and so on. At this point the device is logically + in the USB ch9 initial state (``attached``), drawing no power and not + usable (since it does not yet support enumeration). Any host should + not see the device, since it's not activated the data line pullup + used by the host to detect a device, even if VBUS power is available. + +2. Register a gadget driver that implements some higher level device + function. That will then bind() to a :c:type:`usb_gadget`, which activates + the data line pullup sometime after detecting VBUS. + +3. The hardware driver can now start enumerating. The steps it handles + are to accept USB ``power`` and ``set_address`` requests. Other steps are + handled by the gadget driver. If the gadget driver module is unloaded + before the host starts to enumerate, steps before step 7 are skipped. + +4. The gadget driver's ``setup()`` call returns usb descriptors, based both + on what the bus interface hardware provides and on the functionality + being implemented. That can involve alternate settings or + configurations, unless the hardware prevents such operation. For OTG + devices, each configuration descriptor includes an OTG descriptor. + +5. The gadget driver handles the last step of enumeration, when the USB + host issues a ``set_configuration`` call. It enables all endpoints used + in that configuration, with all interfaces in their default settings. + That involves using a list of the hardware's endpoints, enabling each + endpoint according to its descriptor. It may also involve using + ``usb_gadget_vbus_draw`` to let more power be drawn from VBUS, as + allowed by that configuration. For OTG devices, setting a + configuration may also involve reporting HNP capabilities through a + user interface. + +6. Do real work and perform data transfers, possibly involving changes + to interface settings or switching to new configurations, until the + device is disconnect()ed from the host. Queue any number of transfer + requests to each endpoint. It may be suspended and resumed several + times before being disconnected. On disconnect, the drivers go back + to step 3 (above). + +7. When the gadget driver module is being unloaded, the driver unbind() + callback is issued. That lets the controller driver be unloaded. + +Drivers will normally be arranged so that just loading the gadget driver +module (or statically linking it into a Linux kernel) allows the +peripheral device to be enumerated, but some drivers will defer +enumeration until some higher level component (like a user mode daemon) +enables it. Note that at this lowest level there are no policies about +how ep0 configuration logic is implemented, except that it should obey +USB specifications. Such issues are in the domain of gadget drivers, +including knowing about implementation constraints imposed by some USB +controllers or understanding that composite devices might happen to be +built by integrating reusable components. + +Note that the lifecycle above can be slightly different for OTG devices. +Other than providing an additional OTG descriptor in each configuration, +only the HNP-related differences are particularly visible to driver +code. They involve reporting requirements during the ``SET_CONFIGURATION`` +request, and the option to invoke HNP during some suspend callbacks. +Also, SRP changes the semantics of ``usb_gadget_wakeup`` slightly. + +USB 2.0 Chapter 9 Types and Constants +------------------------------------- + +Gadget drivers rely on common USB structures and constants defined in +the :ref:`linux/usb/ch9.h <usb_chapter9>` header file, which is standard in +Linux 2.6+ kernels. These are the same types and constants used by host side +drivers (and usbcore). + +Core Objects and Methods +------------------------ + +These are declared in ``<linux/usb/gadget.h>``, and are used by gadget +drivers to interact with USB peripheral controller drivers. + +.. kernel-doc:: include/linux/usb/gadget.h + :internal: + +Optional Utilities +------------------ + +The core API is sufficient for writing a USB Gadget Driver, but some +optional utilities are provided to simplify common tasks. These +utilities include endpoint autoconfiguration. + +.. kernel-doc:: drivers/usb/gadget/usbstring.c + :export: + +.. kernel-doc:: drivers/usb/gadget/config.c + :export: + +Composite Device Framework +-------------------------- + +The core API is sufficient for writing drivers for composite USB devices +(with more than one function in a given configuration), and also +multi-configuration devices (also more than one function, but not +necessarily sharing a given configuration). There is however an optional +framework which makes it easier to reuse and combine functions. + +Devices using this framework provide a struct :c:type:`usb_composite_driver`, +which in turn provides one or more struct :c:type:`usb_configuration` +instances. Each such configuration includes at least one struct +:c:type:`usb_function`, which packages a user visible role such as "network +link" or "mass storage device". Management functions may also exist, +such as "Device Firmware Upgrade". + +.. kernel-doc:: include/linux/usb/composite.h + :internal: + +.. kernel-doc:: drivers/usb/gadget/composite.c + :export: + +Composite Device Functions +-------------------------- + +At this writing, a few of the current gadget drivers have been converted +to this framework. Near-term plans include converting all of them, +except for ``gadgetfs``. + +Peripheral Controller Drivers +============================= + +The first hardware supporting this API was the NetChip 2280 controller, +which supports USB 2.0 high speed and is based on PCI. This is the +``net2280`` driver module. The driver supports Linux kernel versions 2.4 +and 2.6; contact NetChip Technologies for development boards and product +information. + +Other hardware working in the ``gadget`` framework includes: Intel's PXA +25x and IXP42x series processors (``pxa2xx_udc``), Toshiba TC86c001 +"Goku-S" (``goku_udc``), Renesas SH7705/7727 (``sh_udc``), MediaQ 11xx +(``mq11xx_udc``), Hynix HMS30C7202 (``h7202_udc``), National 9303/4 +(``n9604_udc``), Texas Instruments OMAP (``omap_udc``), Sharp LH7A40x +(``lh7a40x_udc``), and more. Most of those are full speed controllers. + +At this writing, there are people at work on drivers in this framework +for several other USB device controllers, with plans to make many of +them be widely available. + +A partial USB simulator, the ``dummy_hcd`` driver, is available. It can +act like a net2280, a pxa25x, or an sa11x0 in terms of available +endpoints and device speeds; and it simulates control, bulk, and to some +extent interrupt transfers. That lets you develop some parts of a gadget +driver on a normal PC, without any special hardware, and perhaps with +the assistance of tools such as GDB running with User Mode Linux. At +least one person has expressed interest in adapting that approach, +hooking it up to a simulator for a microcontroller. Such simulators can +help debug subsystems where the runtime hardware is unfriendly to +software development, or is not yet available. + +Support for other controllers is expected to be developed and +contributed over time, as this driver framework evolves. + +Gadget Drivers +============== + +In addition to *Gadget Zero* (used primarily for testing and development +with drivers for usb controller hardware), other gadget drivers exist. + +There's an ``ethernet`` gadget driver, which implements one of the most +useful *Communications Device Class* (CDC) models. One of the standards +for cable modem interoperability even specifies the use of this ethernet +model as one of two mandatory options. Gadgets using this code look to a +USB host as if they're an Ethernet adapter. It provides access to a +network where the gadget's CPU is one host, which could easily be +bridging, routing, or firewalling access to other networks. Since some +hardware can't fully implement the CDC Ethernet requirements, this +driver also implements a "good parts only" subset of CDC Ethernet. (That +subset doesn't advertise itself as CDC Ethernet, to avoid creating +problems.) + +Support for Microsoft's ``RNDIS`` protocol has been contributed by +Pengutronix and Auerswald GmbH. This is like CDC Ethernet, but it runs +on more slightly USB hardware (but less than the CDC subset). However, +its main claim to fame is being able to connect directly to recent +versions of Windows, using drivers that Microsoft bundles and supports, +making it much simpler to network with Windows. + +There is also support for user mode gadget drivers, using ``gadgetfs``. +This provides a *User Mode API* that presents each endpoint as a single +file descriptor. I/O is done using normal ``read()`` and ``read()`` calls. +Familiar tools like GDB and pthreads can be used to develop and debug +user mode drivers, so that once a robust controller driver is available +many applications for it won't require new kernel mode software. Linux +2.6 *Async I/O (AIO)* support is available, so that user mode software +can stream data with only slightly more overhead than a kernel driver. + +There's a USB Mass Storage class driver, which provides a different +solution for interoperability with systems such as MS-Windows and MacOS. +That *Mass Storage* driver uses a file or block device as backing store +for a drive, like the ``loop`` driver. The USB host uses the BBB, CB, or +CBI versions of the mass storage class specification, using transparent +SCSI commands to access the data from the backing store. + +There's a "serial line" driver, useful for TTY style operation over USB. +The latest version of that driver supports CDC ACM style operation, like +a USB modem, and so on most hardware it can interoperate easily with +MS-Windows. One interesting use of that driver is in boot firmware (like +a BIOS), which can sometimes use that model with very small systems +without real serial lines. + +Support for other kinds of gadget is expected to be developed and +contributed over time, as this driver framework evolves. + +USB On-The-GO (OTG) +=================== + +USB OTG support on Linux 2.6 was initially developed by Texas +Instruments for `OMAP <http://www.omap.com>`__ 16xx and 17xx series +processors. Other OTG systems should work in similar ways, but the +hardware level details could be very different. + +Systems need specialized hardware support to implement OTG, notably +including a special *Mini-AB* jack and associated transceiver to support +*Dual-Role* operation: they can act either as a host, using the standard +Linux-USB host side driver stack, or as a peripheral, using this +``gadget`` framework. To do that, the system software relies on small +additions to those programming interfaces, and on a new internal +component (here called an "OTG Controller") affecting which driver stack +connects to the OTG port. In each role, the system can re-use the +existing pool of hardware-neutral drivers, layered on top of the +controller driver interfaces (:c:type:`usb_bus` or :c:type:`usb_gadget`). +Such drivers need at most minor changes, and most of the calls added to +support OTG can also benefit non-OTG products. + +- Gadget drivers test the ``is_otg`` flag, and use it to determine + whether or not to include an OTG descriptor in each of their + configurations. + +- Gadget drivers may need changes to support the two new OTG protocols, + exposed in new gadget attributes such as ``b_hnp_enable`` flag. HNP + support should be reported through a user interface (two LEDs could + suffice), and is triggered in some cases when the host suspends the + peripheral. SRP support can be user-initiated just like remote + wakeup, probably by pressing the same button. + +- On the host side, USB device drivers need to be taught to trigger HNP + at appropriate moments, using ``usb_suspend_device()``. That also + conserves battery power, which is useful even for non-OTG + configurations. + +- Also on the host side, a driver must support the OTG "Targeted + Peripheral List". That's just a whitelist, used to reject peripherals + not supported with a given Linux OTG host. *This whitelist is + product-specific; each product must modify* ``otg_whitelist.h`` *to + match its interoperability specification.* + + Non-OTG Linux hosts, like PCs and workstations, normally have some + solution for adding drivers, so that peripherals that aren't + recognized can eventually be supported. That approach is unreasonable + for consumer products that may never have their firmware upgraded, + and where it's usually unrealistic to expect traditional + PC/workstation/server kinds of support model to work. For example, + it's often impractical to change device firmware once the product has + been distributed, so driver bugs can't normally be fixed if they're + found after shipment. + +Additional changes are needed below those hardware-neutral :c:type:`usb_bus` +and :c:type:`usb_gadget` driver interfaces; those aren't discussed here in any +detail. Those affect the hardware-specific code for each USB Host or +Peripheral controller, and how the HCD initializes (since OTG can be +active only on a single port). They also involve what may be called an +*OTG Controller Driver*, managing the OTG transceiver and the OTG state +machine logic as well as much of the root hub behavior for the OTG port. +The OTG controller driver needs to activate and deactivate USB +controllers depending on the relevant device role. Some related changes +were needed inside usbcore, so that it can identify OTG-capable devices +and respond appropriately to HNP or SRP protocols. |