driver-api/usb/gadget.rst

11 This document presents a Linux-USB "Gadget" kernel mode API, for use
17 -  Supports USB 2.0, for high speed devices which can stream data at
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.
24 -  Flexible enough to expose more complex USB device capabilities such
28 -  USB "On-The-Go" (OTG) support, in conjunction with updates to the
29    Linux-USB host side.
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
36 -  Minimalist, so it's easier to support new device controller hardware.
50 The gadget API resembles the host side Linux-USB API in that both use
58 necessarily different (one side is a hardware-neutral master, the other
59 is a hardware-aware slave), the endpoint I/0 API used here should also
60 be usable for an overhead-reduced host side API.
80     devices only have one upstream port, they only have one of these
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
89     The lower boundary of this driver implements hardware-neutral USB
98     automatically for many bulk-oriented drivers.) Gadget driver
101     -  handling setup requests (ep0 protocol responses) possibly
102        including class-specific functionality
104     -  returning configuration and string descriptors
106     -  (re)setting configurations and interface altsettings, including
109     -  handling life cycle events, such as managing bindings to
113     -  managing IN and OUT transfers on all currently enabled endpoints
124     -  user mode code, using generic (gadgetfs) or application specific
127     -  networking subsystem (for network gadgets, like the CDC Ethernet
130     -  data capture drivers, perhaps video4Linux or a scanner driver; or
133     -  input subsystem (for HID gadgets)
135     -  sound subsystem (for audio gadgets)
137     -  file system (for PTP gadgets)
139     -  block i/o subsystem (for usb-storage gadgets)
141     -  ... and more
151 OTG-capable systems will also need to include a standard Linux-USB host
153 (HCDs), *USB Device Drivers* to support the OTG "Targeted Peripheral
154 List", and so forth. There will also be an *OTG Controller Driver*,
157 new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or
159 viewed as a more battery-friendly kind of device wakeup protocol.
167 USB-IF protocols for HID, networking, storage, or audio classes. Some
170 hardware-specific, any more than network protocols like X11, HTTP, or
171 NFS are. Such gadget-side interface drivers should eventually be
206 such as device-to-device DMA (without temporary storage in a memory
207 buffer) that would be added using hardware-specific APIs.
214 device configuration and management. The API supports limited run-time
223 Like the Linux-USB host side API, this API exposes the "chunky" nature
225 packet boundaries are visible to drivers. Compared to RS-232 serial
230 drivers won't buffer two single byte writes into a single two-byte USB
236 -----------------
262    configurations, unless the hardware prevents such operation. For OTG
263    devices, each configuration descriptor includes an OTG descriptor.
271    allowed by that configuration. For OTG devices, setting a
296 Note that the lifecycle above can be slightly different for OTG devices.
297 Other than providing an additional OTG descriptor in each configuration,
298 only the HNP-related differences are particularly visible to driver
304 -------------------------------------
312 ------------------------
317 .. kernel-doc:: include/linux/usb/gadget.h
321 ------------------
327 .. kernel-doc:: drivers/usb/gadget/usbstring.c
330 .. kernel-doc:: drivers/usb/gadget/config.c
334 --------------------------
338 multi-configuration devices (also more than one function, but not
349 .. kernel-doc:: include/linux/usb/composite.h
352 .. kernel-doc:: drivers/usb/gadget/composite.c
356 --------------------------
359 to this framework. Near-term plans include converting all of them,
373 "Goku-S" (``goku_udc``), Renesas SH7705/7727 (``sh_udc``), MediaQ 11xx
431 solution for interoperability with systems such as MS-Windows and MacOS.
440 MS-Windows. One interesting use of that driver is in boot firmware (like
447 USB On-The-GO (OTG)
450 USB OTG support on Linux 2.6 was initially developed by Texas
452 processors. Other OTG systems should work in similar ways, but the
455 Systems need specialized hardware support to implement OTG, notably
456 including a special *Mini-AB* jack and associated transceiver to support
457 *Dual-Role* operation: they can act either as a host, using the standard
458 Linux-USB host side driver stack, or as a peripheral, using this
461 component (here called an "OTG Controller") affecting which driver stack
462 connects to the OTG port. In each role, the system can re-use the
463 existing pool of hardware-neutral drivers, layered on top of the
466 support OTG can also benefit non-OTG products.
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
472 -  Gadget drivers may need changes to support the two new OTG protocols,
476    peripheral. SRP support can be user-initiated just like remote
479 -  On the host side, USB device drivers need to be taught to trigger HNP
481    conserves battery power, which is useful even for non-OTG
484 -  Also on the host side, a driver must support the OTG "Targeted
486    not supported with a given Linux OTG host. *This whitelist is
487    product-specific; each product must modify* ``otg_whitelist.h`` *to
490    Non-OTG Linux hosts, like PCs and workstations, normally have some
500 Additional changes are needed below those hardware-neutral :c:type:`usb_bus`
502 detail. Those affect the hardware-specific code for each USB Host or
503 Peripheral controller, and how the HCD initializes (since OTG can be
504 active 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
506 machine logic as well as much of the root hub behavior for the OTG port.
507 The OTG controller driver needs to activate and deactivate USB
509 were needed inside usbcore, so that it can identify OTG-capable devices