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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
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.
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
145 network protocol stacks, as well as user mode applications building
148 be an option. Such user mode code will not necessarily be subject to
151 OTG-capable systems will also need to include a standard Linux-USB host
155 which is visible to gadget and device driver developers only indirectly.
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
174 Kernel Mode Gadget API
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 -----------------
288 enumeration until some higher level component (like a user mode daemon)
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
345 :c:type:`usb_function`, which packages a user visible role such as "network
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
387 the assistance of tools such as GDB running with User Mode Linux. At
421 There is also support for user mode gadget drivers, using ``gadgetfs``.
422 This provides a *User Mode API* that presents each endpoint as a single
425 user mode drivers, so that once a robust controller driver is available
426 many applications for it won't require new kernel mode software. Linux
427 2.6 *Async I/O (AIO)* support is available, so that user mode software
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)
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
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
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
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
506 machine logic as well as much of the root hub behavior for the OTG port.
509 were needed inside usbcore, so that it can identify OTG-capable devices