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 ========================
 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 usb_gadget. The response to a set_configuration usually
 involves enabling one or more of the struct usb_ep objects exposed by
 the gadget, and submitting one or more struct 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 usb_composite_driver,
 which in turn provides one or more struct 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.

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