Copyright © 2003-2004 David Brownell
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Table of Contents
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.
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:
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.
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.
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
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.
Table of Contents
Gadget drivers declare themselves through a struct 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.
This documentation was prepared using the standard Linux
kernel docproc tool, which turns text
and in-code comments into SGML DocBook and then into usable
formats such as HTML or PDF.
Other than the "Chapter 9" data types, most of the significant
data types and functions are described here.
However, docproc does not understand all the C constructs that are used, so 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 kernel includes 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.
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:
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.
Register a gadget driver that implements some higher level device function. That will then bind() to a usb_gadget, which activates the data line pullup sometime after detecting VBUS.
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.
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.
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.
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).
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.
Gadget drivers
rely on common USB structures and constants
defined in the
<linux/usb/ch9.h>
header file, which is standard in Linux 2.6 kernels.
These are the same types and constants used by host
side drivers (and usbcore).
usb_speed_string — Returns human readable-name of the speed.
const char * fsfuncusb_speed_string ( | speed); |
enum usb_device_speed speed;These are declared in
<linux/usb/gadget.h>,
and are used by gadget drivers to interact with
USB peripheral controller drivers.
struct usb_request — describes one i/o request
struct usb_request {
void * buf;
unsigned length;
dma_addr_t dma;
struct scatterlist * sg;
unsigned num_sgs;
unsigned num_mapped_sgs;
unsigned stream_id:16;
unsigned no_interrupt:1;
unsigned zero:1;
unsigned short_not_ok:1;
void (* complete) (struct usb_ep *ep,struct usb_request *req);
void * context;
struct list_head list;
int status;
unsigned actual;
}; Buffer used for data. Always provide this; some controllers only use PIO, or don't use DMA for some endpoints.
Length of that data
DMA address corresponding to 'buf'. If you don't set this field, and the usb controller needs one, it is responsible for mapping and unmapping the buffer.
a scatterlist for SG-capable controllers.
number of SG entries
number of SG entries mapped to DMA (internal)
The stream id, when USB3.0 bulk streams are being used
If true, hints that no completion irq is needed. Helpful sometimes with deep request queues that are handled directly by DMA controllers.
If true, when writing data, makes the last packet be “short” by adding a zero length packet as needed;
When reading data, makes short packets be treated as errors (queue stops advancing till cleanup).
Function called when request completes, so this request and its buffer may be re-used. The function will always be called with interrupts disabled, and it must not sleep. Reads terminate with a short packet, or when the buffer fills, whichever comes first. When writes terminate, some data bytes will usually still be in flight (often in a hardware fifo). Errors (for reads or writes) stop the queue from advancing until the completion function returns, so that any transfers invalidated by the error may first be dequeued.
For use by the completion callback
For use by the gadget driver.
Reports completion code, zero or a negative errno. Normally, faults block the transfer queue from advancing until the completion callback returns. Code “-ESHUTDOWN” indicates completion caused by device disconnect, or when the driver disabled the endpoint.
Reports bytes transferred to/from the buffer. For reads (OUT transfers) this may be less than the requested length. If the short_not_ok flag is set, short reads are treated as errors even when status otherwise indicates successful completion. Note that for writes (IN transfers) some data bytes may still reside in a device-side FIFO when the request is reported as complete.
These are allocated/freed through the endpoint they're used with. The hardware's driver can add extra per-request data to the memory it returns, which often avoids separate memory allocations (potential failures), later when the request is queued.
Request flags affect request handling, such as whether a zero length packet is written (the “zero” flag), whether a short read should be treated as an error (blocking request queue advance, the “short_not_ok” flag), or hinting that an interrupt is not required (the “no_interrupt” flag, for use with deep request queues).
Bulk endpoints can use any size buffers, and can also be used for interrupt transfers. interrupt-only endpoints can be much less functional.
struct usb_ep — device side representation of USB endpoint
struct usb_ep {
void * driver_data;
const char * name;
const struct usb_ep_ops * ops;
struct list_head ep_list;
unsigned maxpacket:16;
unsigned max_streams:16;
unsigned mult:2;
unsigned maxburst:5;
u8 address;
const struct usb_endpoint_descriptor * desc;
const struct usb_ss_ep_comp_descriptor * comp_desc;
}; for use by the gadget driver.
identifier for the endpoint, such as “ep-a” or “ep9in-bulk”
Function pointers used to access hardware-specific operations.
the gadget's ep_list holds all of its endpoints
The maximum packet size used on this endpoint. The initial value can sometimes be reduced (hardware allowing), according to the endpoint descriptor used to configure the endpoint.
The maximum number of streams supported by this EP (0 - 16, actual number is 2^n)
multiplier, 'mult' value for SS Isoc EPs
the maximum number of bursts supported by this EP (for usb3)
used to identify the endpoint when finding descriptor that matches connection speed
endpoint descriptor. This pointer is set before the endpoint is enabled and remains valid until the endpoint is disabled.
In case of SuperSpeed support, this is the endpoint companion descriptor that is used to configure the endpoint
usb_ep_enable — configure endpoint, making it usable
int fsfuncusb_ep_enable ( | ep); |
struct usb_ep * ep;epthe endpoint being configured. may not be the endpoint named “ep0”. drivers discover endpoints through the ep_list of a usb_gadget.
When configurations are set, or when interface settings change, the driver
will enable or disable the relevant endpoints. while it is enabled, an
endpoint may be used for i/o until the driver receives a disconnect from
the host or until the endpoint is disabled.
the ep0 implementation (which calls this routine) must ensure that the hardware capabilities of each endpoint match the descriptor provided for it. for example, an endpoint named “ep2in-bulk” would be usable for interrupt transfers as well as bulk, but it likely couldn't be used for iso transfers or for endpoint 14. some endpoints are fully configurable, with more generic names like “ep-a”. (remember that for USB, “in” means “towards the USB master”.)
returns zero, or a negative error code.
usb_ep_disable — endpoint is no longer usable
int fsfuncusb_ep_disable ( | ep); |
struct usb_ep * ep;
no other task may be using this endpoint when this is called.
any pending and uncompleted requests will complete with status
indicating disconnect (-ESHUTDOWN) before this call returns.
gadget drivers must call usb_ep_enable again before queueing
requests to the endpoint.
returns zero, or a negative error code.
usb_ep_alloc_request — allocate a request object to use with this endpoint
struct usb_request * fsfuncusb_ep_alloc_request ( | ep, | |
gfp_flags); |
struct usb_ep * ep;gfp_t gfp_flags;
Request objects must be allocated with this call, since they normally
need controller-specific setup and may even need endpoint-specific
resources such as allocation of DMA descriptors.
Requests may be submitted with usb_ep_queue, and receive a single
completion callback. Free requests with usb_ep_free_request, when
they are no longer needed.
Returns the request, or null if one could not be allocated.
usb_ep_free_request — frees a request object
void fsfuncusb_ep_free_request ( | ep, | |
req); |
struct usb_ep * ep;struct usb_request * req;usb_ep_queue — queues (submits) an I/O request to an endpoint.
int fsfuncusb_ep_queue ( | ep, | |
| req, | ||
gfp_flags); |
struct usb_ep * ep;struct usb_request * req;gfp_t gfp_flags;epthe endpoint associated with the request
reqthe request being submitted
gfp_flagsGFP_* flags to use in case the lower level driver couldn't pre-allocate all necessary memory with the request.
This tells the device controller to perform the specified request through
that endpoint (reading or writing a buffer). When the request completes,
including being canceled by usb_ep_dequeue, the request's completion
routine is called to return the request to the driver. Any endpoint
(except control endpoints like ep0) may have more than one transfer
request queued; they complete in FIFO order. Once a gadget driver
submits a request, that request may not be examined or modified until it
is given back to that driver through the completion callback.
Each request is turned into one or more packets. The controller driver never merges adjacent requests into the same packet. OUT transfers will sometimes use data that's already buffered in the hardware. Drivers can rely on the fact that the first byte of the request's buffer always corresponds to the first byte of some USB packet, for both IN and OUT transfers.
Bulk endpoints can queue any amount of data; the transfer is packetized automatically. The last packet will be short if the request doesn't fill it out completely. Zero length packets (ZLPs) should be avoided in portable protocols since not all usb hardware can successfully handle zero length packets. (ZLPs may be explicitly written, and may be implicitly written if the request 'zero' flag is set.) Bulk endpoints may also be used for interrupt transfers; but the reverse is not true, and some endpoints won't support every interrupt transfer. (Such as 768 byte packets.)
Interrupt-only endpoints are less functional than bulk endpoints, for example by not supporting queueing or not handling buffers that are larger than the endpoint's maxpacket size. They may also treat data toggle differently.
Control endpoints ... after getting a setup callback, the driver queues
one response (even if it would be zero length). That enables the
status ack, after transferring data as specified in the response. Setup
functions may return negative error codes to generate protocol stalls.
(Note that some USB device controllers disallow protocol stall responses
in some cases.) When control responses are deferred (the response is
written after the setup callback returns), then usb_ep_set_halt may be
used on ep0 to trigger protocol stalls. Depending on the controller,
it may not be possible to trigger a status-stage protocol stall when the
data stage is over, that is, from within the response's completion
routine.
For periodic endpoints, like interrupt or isochronous ones, the usb host arranges to poll once per interval, and the gadget driver usually will have queued some data to transfer at that time.
Returns zero, or a negative error code. Endpoints that are not enabled report errors; errors will also be reported when the usb peripheral is disconnected.
usb_ep_dequeue — dequeues (cancels, unlinks) an I/O request from an endpoint
int fsfuncusb_ep_dequeue ( | ep, | |
req); |
struct usb_ep * ep;struct usb_request * req;if the request is still active on the endpoint, it is dequeued and its completion routine is called (with status -ECONNRESET); else a negative error code is returned.
note that some hardware can't clear out write fifos (to unlink the request at the head of the queue) except as part of disconnecting from usb. such restrictions prevent drivers from supporting configuration changes, even to configuration zero (a “chapter 9” requirement).
usb_ep_set_halt — sets the endpoint halt feature.
int fsfuncusb_ep_set_halt ( | ep); |
struct usb_ep * ep;Use this to stall an endpoint, perhaps as an error report. Except for control endpoints, the endpoint stays halted (will not stream any data) until the host clears this feature; drivers may need to empty the endpoint's request queue first, to make sure no inappropriate transfers happen.
Note that while an endpoint CLEAR_FEATURE will be invisible to the
gadget driver, a SET_INTERFACE will not be. To reset endpoints for the
current altsetting, see usb_ep_clear_halt. When switching altsettings,
it's simplest to use usb_ep_enable or usb_ep_disable for the endpoints.
Returns zero, or a negative error code. On success, this call sets underlying hardware state that blocks data transfers. Attempts to halt IN endpoints will fail (returning -EAGAIN) if any transfer requests are still queued, or if the controller hardware (usually a FIFO) still holds bytes that the host hasn't collected.
usb_ep_clear_halt — clears endpoint halt, and resets toggle
int fsfuncusb_ep_clear_halt ( | ep); |
struct usb_ep * ep;Use this when responding to the standard usb “set interface” request, for endpoints that aren't reconfigured, after clearing any other state in the endpoint's i/o queue.
Returns zero, or a negative error code. On success, this call clears the underlying hardware state reflecting endpoint halt and data toggle. Note that some hardware can't support this request (like pxa2xx_udc), and accordingly can't correctly implement interface altsettings.
usb_ep_set_wedge — sets the halt feature and ignores clear requests
int fsfuncusb_ep_set_wedge ( | ep); |
struct usb_ep * ep;usb_ep_fifo_status — returns number of bytes in fifo, or error
int fsfuncusb_ep_fifo_status ( | ep); |
struct usb_ep * ep;FIFO endpoints may have “unclaimed data” in them in certain cases, such as after aborted transfers. Hosts may not have collected all the IN data written by the gadget driver (and reported by a request completion). The gadget driver may not have collected all the data written OUT to it by the host. Drivers that need precise handling for fault reporting or recovery may need to use this call.
This returns the number of such bytes in the fifo, or a negative errno if the endpoint doesn't use a FIFO or doesn't support such precise handling.
usb_ep_fifo_flush — flushes contents of a fifo
void fsfuncusb_ep_fifo_flush ( | ep); |
struct usb_ep * ep;struct usb_gadget — represents a usb slave device
struct usb_gadget {
struct work_struct work;
const struct usb_gadget_ops * ops;
struct usb_ep * ep0;
struct list_head ep_list;
enum usb_device_speed speed;
enum usb_device_speed max_speed;
enum usb_device_state state;
unsigned sg_supported:1;
unsigned is_otg:1;
unsigned is_a_peripheral:1;
unsigned b_hnp_enable:1;
unsigned a_hnp_support:1;
unsigned a_alt_hnp_support:1;
const char * name;
struct device dev;
unsigned out_epnum;
unsigned in_epnum;
};
(internal use) Workqueue to be used for sysfs_notify
Function pointers used to access hardware-specific operations.
Endpoint zero, used when reading or writing responses to
driver setup requests
List of other endpoints supported by the device.
Speed of current connection to USB host.
Maximal speed the UDC can handle. UDC must support this and all slower speeds.
the state we are now (attached, suspended, configured, etc)
true if we can handle scatter-gather
True if the USB device port uses a Mini-AB jack, so that the gadget driver must provide a USB OTG descriptor.
False unless is_otg, the “A” end of a USB cable is in the Mini-AB jack, and HNP has been used to switch roles so that the “A” device currently acts as A-Peripheral, not A-Host.
OTG device feature flag, indicating that the A-Host enabled HNP support.
OTG device feature flag, indicating that the A-Host supports HNP at this port.
OTG device feature flag, indicating that the A-Host only supports HNP on a different root port.
Identifies the controller hardware type. Used in diagnostics and sometimes configuration.
Driver model state for this abstract device.
last used out ep number
last used in ep number
Gadgets have a mostly-portable “gadget driver” implementing device functions, handling all usb configurations and interfaces. Gadget drivers talk to hardware-specific code indirectly, through ops vectors. That insulates the gadget driver from hardware details, and packages the hardware endpoints through generic i/o queues. The “usb_gadget” and “usb_ep” interfaces provide that insulation from the hardware.
Except for the driver data, all fields in this structure are read-only to the gadget driver. That driver data is part of the “driver model” infrastructure in 2.6 (and later) kernels, and for earlier systems is grouped in a similar structure that's not known to the rest of the kernel.
Values of the three OTG device feature flags are updated before the
setup call corresponding to USB_REQ_SET_CONFIGURATION, and before
driver suspend calls. They are valid only when is_otg, and when the
device is acting as a B-Peripheral (so is_a_peripheral is false).
gadget_is_dualspeed — return true iff the hardware handles high speed
int fsfuncgadget_is_dualspeed ( | g); |
struct usb_gadget * g;gadget_is_superspeed — return true if the hardware handles superspeed
int fsfuncgadget_is_superspeed ( | g); |
struct usb_gadget * g;gadget_is_otg — return true iff the hardware is OTG-ready
int fsfuncgadget_is_otg ( | g); |
struct usb_gadget * g;usb_gadget_frame_number — returns the current frame number
int fsfuncusb_gadget_frame_number ( | gadget); |
struct usb_gadget * gadget;usb_gadget_wakeup — tries to wake up the host connected to this gadget
int fsfuncusb_gadget_wakeup ( | gadget); |
struct usb_gadget * gadget;Returns zero on success, else negative error code if the hardware doesn't support such attempts, or its support has not been enabled by the usb host. Drivers must return device descriptors that report their ability to support this, or hosts won't enable it.
This may also try to use SRP to wake the host and start enumeration, even if OTG isn't otherwise in use. OTG devices may also start remote wakeup even when hosts don't explicitly enable it.
usb_gadget_set_selfpowered — sets the device selfpowered feature.
int fsfuncusb_gadget_set_selfpowered ( | gadget); |
struct usb_gadget * gadget;usb_gadget_clear_selfpowered — clear the device selfpowered feature.
int fsfuncusb_gadget_clear_selfpowered ( | gadget); |
struct usb_gadget * gadget;usb_gadget_vbus_connect — Notify controller that VBUS is powered
int fsfuncusb_gadget_vbus_connect ( | gadget); |
struct usb_gadget * gadget;This call is used by a driver for an external transceiver (or GPIO) that detects a VBUS power session starting. Common responses include resuming the controller, activating the D+ (or D-) pullup to let the host detect that a USB device is attached, and starting to draw power (8mA or possibly more, especially after SET_CONFIGURATION).
Returns zero on success, else negative errno.
usb_gadget_vbus_draw — constrain controller's VBUS power usage
int fsfuncusb_gadget_vbus_draw ( | gadget, | |
mA); |
struct usb_gadget * gadget;unsigned mA;usb_gadget_vbus_disconnect — notify controller about VBUS session end
int fsfuncusb_gadget_vbus_disconnect ( | gadget); |
struct usb_gadget * gadget;usb_gadget_connect — software-controlled connect to USB host
int fsfuncusb_gadget_connect ( | gadget); |
struct usb_gadget * gadget;
Enables the D+ (or potentially D-) pullup. The host will start
enumerating this gadget when the pullup is active and a VBUS session
is active (the link is powered). This pullup is always enabled unless
usb_gadget_disconnect has been used to disable it.
Returns zero on success, else negative errno.
usb_gadget_disconnect — software-controlled disconnect from USB host
int fsfuncusb_gadget_disconnect ( | gadget); |
struct usb_gadget * gadget;Disables the D+ (or potentially D-) pullup, which the host may see as a disconnect (when a VBUS session is active). Not all systems support software pullup controls.
This routine may be used during the gadget driver bind call to prevent
the peripheral from ever being visible to the USB host, unless later
usb_gadget_connect is called. For example, user mode components may
need to be activated before the system can talk to hosts.
Returns zero on success, else negative errno.
struct usb_gadget_driver — driver for usb 'slave' devices
struct usb_gadget_driver {
char * function;
enum usb_device_speed max_speed;
int (* bind) (struct usb_gadget *gadget,struct usb_gadget_driver *driver);
void (* unbind) (struct usb_gadget *);
int (* setup) (struct usb_gadget *,const struct usb_ctrlrequest *);
void (* disconnect) (struct usb_gadget *);
void (* suspend) (struct usb_gadget *);
void (* resume) (struct usb_gadget *);
struct device_driver driver;
}; String describing the gadget's function
Highest speed the driver handles.
the driver's bind callback
Invoked when the driver is unbound from a gadget, usually from rmmod (after a disconnect is reported). Called in a context that permits sleeping.
Invoked for ep0 control requests that aren't handled by the hardware level driver. Most calls must be handled by the gadget driver, including descriptor and configuration management. The 16 bit members of the setup data are in USB byte order. Called in_interrupt; this may not sleep. Driver queues a response to ep0, or returns negative to stall.
Invoked after all transfers have been stopped, when the host is disconnected. May be called in_interrupt; this may not sleep. Some devices can't detect disconnect, so this might not be called except as part of controller shutdown.
Invoked on USB suspend. May be called in_interrupt.
Invoked on USB resume. May be called in_interrupt.
Driver model state for this driver.
Devices are disabled till a gadget driver successfully binds, which
means the driver will handle setup requests needed to enumerate (and
meet “chapter 9” requirements) then do some useful work.
If gadget->is_otg is true, the gadget driver must provide an OTG
descriptor during enumeration, or else fail the bind call. In such
cases, no USB traffic may flow until both bind returns without
having called usb_gadget_disconnect, and the USB host stack has
initialized.
Drivers use hardware-specific knowledge to configure the usb hardware.
endpoint addressing is only one of several hardware characteristics that
are in descriptors the ep0 implementation returns from setup calls.
Except for ep0 implementation, most driver code shouldn't need change to run on top of different usb controllers. It'll use endpoints set up by that ep0 implementation.
The usb controller driver handles a few standard usb requests. Those include set_address, and feature flags for devices, interfaces, and endpoints (the get_status, set_feature, and clear_feature requests).
Accordingly, the driver's setup callback must always implement all
get_descriptor requests, returning at least a device descriptor and
a configuration descriptor. Drivers must make sure the endpoint
descriptors match any hardware constraints. Some hardware also constrains
other descriptors. (The pxa250 allows only configurations 1, 2, or 3).
The driver's setup callback must also implement set_configuration,
and should also implement set_interface, get_configuration, and
get_interface. Setting a configuration (or interface) is where
endpoints should be activated or (config 0) shut down.
(Note that only the default control endpoint is supported. Neither hosts nor devices generally support control traffic except to ep0.)
Most devices will ignore USB suspend/resume operations, and so will not provide those callbacks. However, some may need to change modes when the host is not longer directing those activities. For example, local controls (buttons, dials, etc) may need to be re-enabled since the (remote) host can't do that any longer; or an error state might be cleared, to make the device behave identically whether or not power is maintained.
usb_gadget_probe_driver — probe a gadget driver
int fsfuncusb_gadget_probe_driver ( | driver); |
struct usb_gadget_driver * driver;
Call this in your gadget driver's module initialization function,
to tell the underlying usb controller driver about your driver.
The bind() function will be called to bind it to a gadget before this
registration call returns. It's expected that the bind() function will
be in init sections.
usb_gadget_unregister_driver — unregister a gadget driver
int fsfuncusb_gadget_unregister_driver ( | driver); |
struct usb_gadget_driver * driver;
Call this in your gadget driver's module cleanup function,
to tell the underlying usb controller that your driver is
going away. If the controller is connected to a USB host,
it will first disconnect. The driver is also requested
to unbind and clean up any device state, before this procedure
finally returns. It's expected that the unbind functions
will in in exit sections, so may not be linked in some kernels.
struct usb_string — wraps a C string and its USB id
struct usb_string {
u8 id;
const char * s;
}; 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.
usb_gadget_get_string — fill out a string descriptor
int fsfuncusb_gadget_get_string ( | table, | |
| id, | ||
buf); |
struct usb_gadget_strings * table;int id;u8 * buf;tableof c strings encoded using UTF-8
idstring id, from low byte of wValue in get string descriptor
bufat least 256 bytes, must be 16-bit aligned
Finds the UTF-8 string matching the ID, and converts it into a string descriptor in utf16-le. Returns length of descriptor (always even) or negative errno
If your driver needs stings in multiple languages, you'll probably “switch (wIndex) { ... }” in your ep0 string descriptor logic, using this routine after choosing which set of UTF-8 strings to use. Note that US-ASCII is a strict subset of UTF-8; any string bytes with the eighth bit set will be multibyte UTF-8 characters, not ISO-8859/1 characters (which are also widely used in C strings).
usb_descriptor_fillbuf — fill buffer with descriptors
int fsfuncusb_descriptor_fillbuf ( | buf, | |
| buflen, | ||
src); |
void * buf;unsigned buflen;const struct usb_descriptor_header ** src;bufBuffer to be filled
buflenSize of buf
srcArray of descriptor pointers, terminated by null pointer.
Copies descriptors into the buffer, returning the length or a negative error code if they can't all be copied. Useful when assembling descriptors for an associated set of interfaces used as part of configuring a composite device; or in other cases where sets of descriptors need to be marshaled.
usb_gadget_config_buf — builts a complete configuration descriptor
int fsfuncusb_gadget_config_buf ( | config, | |
| buf, | ||
| length, | ||
desc); |
const struct usb_config_descriptor * config;void * buf;unsigned length;const struct usb_descriptor_header ** desc;configHeader for the descriptor, including characteristics such as power requirements and number of interfaces.
bufBuffer for the resulting configuration descriptor.
lengthLength of buffer. If this is not big enough to hold the entire configuration descriptor, an error code will be returned.
descNull-terminated vector of pointers to the descriptors (interface, endpoint, etc) defining all functions in this device configuration.
This copies descriptors into the response buffer, building a descriptor for that configuration. It returns the buffer length or a negative status code. The config.wTotalLength field is set to match the length of the result, but other descriptor fields (including power usage and interface count) must be set by the caller.
Gadget drivers could use this when constructing a config descriptor in response to USB_REQ_GET_DESCRIPTOR. They will need to patch the resulting bDescriptorType value if USB_DT_OTHER_SPEED_CONFIG is needed.
usb_copy_descriptors — copy a vector of USB descriptors
struct usb_descriptor_header ** fsfuncusb_copy_descriptors ( | src); |
struct usb_descriptor_header ** src;This makes a copy of a vector of USB descriptors. Its primary use is to support usb_function objects which can have multiple copies, each needing different descriptors. Functions may have static tables of descriptors, which are used as templates and customized with identifiers (for interfaces, strings, endpoints, and more) as needed by a given function instance.
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 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".
struct usb_function — describes one function of a configuration
struct usb_function {
const char * name;
struct usb_gadget_strings ** strings;
struct usb_descriptor_header ** fs_descriptors;
struct usb_descriptor_header ** hs_descriptors;
struct usb_descriptor_header ** ss_descriptors;
struct usb_configuration * config;
int (* bind) (struct usb_configuration *,struct usb_function *);
void (* unbind) (struct usb_configuration *,struct usb_function *);
void (* free_func) (struct usb_function *f);
struct module * mod;
int (* set_alt) (struct usb_function *,unsigned interface, unsigned alt);
int (* get_alt) (struct usb_function *,unsigned interface);
void (* disable) (struct usb_function *);
int (* setup) (struct usb_function *,const struct usb_ctrlrequest *);
void (* suspend) (struct usb_function *);
void (* resume) (struct usb_function *);
int (* get_status) (struct usb_function *);
int (* func_suspend) (struct usb_function *,u8 suspend_opt);
}; For diagnostics, identifies the function.
tables of strings, keyed by identifiers assigned during bind
and by language IDs provided in control requests
Table of full (or low) speed descriptors, using interface and
string identifiers assigned during bind(). If this pointer is null,
the function will not be available at full speed (or at low speed).
Table of high speed descriptors, using interface and
string identifiers assigned during bind(). If this pointer is null,
the function will not be available at high speed.
Table of super speed descriptors, using interface and
string identifiers assigned during bind(). If this
pointer is null after initiation, the function will not
be available at super speed.
assigned when usb_add_function() is called; this is the
configuration with which this function is associated.
Before the gadget can register, all of its functions bind to the
available resources including string and interface identifiers used
in interface or class descriptors; endpoints; I/O buffers; and so on.
Reverses bind; called as a side effect of unregistering the
driver which added this function.
free the struct usb_function.
(internal) points to the module that created this structure.
(REQUIRED) Reconfigures altsettings; function drivers may initialize usb_ep.driver data at this time (when it is used). Note that setting an interface to its current altsetting resets interface state, and that all interfaces have a disabled state.
Returns the active altsetting. If this is not provided, then only altsetting zero is supported.
(REQUIRED) Indicates the function should be disabled. Reasons include host resetting or reconfiguring the gadget, and disconnection.
Used for interface-specific control requests.
Notifies functions when the host stops sending USB traffic.
Notifies functions when the host restarts USB traffic.
Returns function status as a reply to
GetStatus request when the recepient is Interface.
callback to be called when SetFeature(FUNCTION_SUSPEND) is reseived
A single USB function uses one or more interfaces, and should in most
cases support operation at both full and high speeds. Each function is
associated by usb_add_function() with a one configuration; that function
causes bind() to be called so resources can be allocated as part of
setting up a gadget driver. Those resources include endpoints, which
should be allocated using usb_ep_autoconfig().
To support dual speed operation, a function driver provides descriptors for both high and full speed operation. Except in rare cases that don't involve bulk endpoints, each speed needs different endpoint descriptors.
Function drivers choose their own strategies for managing instance data. The simplest strategy just declares it "static', which means the function can only be activated once. If the function needs to be exposed in more than one configuration at a given speed, it needs to support multiple usb_function structures (one for each configuration).
A more complex strategy might encapsulate a usb_function structure inside
a driver-specific instance structure to allows multiple activations. An
example of multiple activations might be a CDC ACM function that supports
two or more distinct instances within the same configuration, providing
several independent logical data links to a USB host.
struct usb_configuration — represents one gadget configuration
struct usb_configuration {
const char * label;
struct usb_gadget_strings ** strings;
const struct usb_descriptor_header ** descriptors;
void (* unbind) (struct usb_configuration *);
int (* setup) (struct usb_configuration *,const struct usb_ctrlrequest *);
u8 bConfigurationValue;
u8 iConfiguration;
u8 bmAttributes;
u16 MaxPower;
struct usb_composite_dev * cdev;
}; For diagnostics, describes the configuration.
Tables of strings, keyed by identifiers assigned during bind()
and by language IDs provided in control requests.
Table of descriptors preceding all function descriptors. Examples include OTG and vendor-specific descriptors.
Reverses bind; called as a side effect of unregistering the
driver which added this configuration.
Used to delegate control requests that aren't handled by standard device infrastructure or directed at a specific interface.
Copied into configuration descriptor.
Copied into configuration descriptor.
Copied into configuration descriptor.
Power consumtion in mA. Used to compute bMaxPower in the configuration descriptor after considering the bus speed.
assigned by usb_add_config() before calling bind(); this is
the device associated with this configuration.
Configurations are building blocks for gadget drivers structured around function drivers. Simple USB gadgets require only one function and one configuration, and handle dual-speed hardware by always providing the same functionality. Slightly more complex gadgets may have more than one single-function configuration at a given speed; or have configurations that only work at one speed.
Composite devices are, by definition, ones with configurations which include more than one function.
The lifecycle of a usb_configuration includes allocation, initialization
of the fields described above, and calling usb_add_config() to set up
internal data and bind it to a specific device. The configuration's
bind() method is then used to initialize all the functions and then
call usb_add_function() for them.
Those functions would normally be independent of each other, but that's
not mandatory. CDC WMC devices are an example where functions often
depend on other functions, with some functions subsidiary to others.
Such interdependency may be managed in any way, so long as all of the
descriptors complete by the time the composite driver returns from
its bind routine.
struct usb_composite_driver — groups configurations into a gadget
struct usb_composite_driver {
const char * name;
const struct usb_device_descriptor * dev;
struct usb_gadget_strings ** strings;
enum usb_device_speed max_speed;
unsigned needs_serial:1;
int (* bind) (struct usb_composite_dev *cdev);
int (* unbind) (struct usb_composite_dev *);
void (* disconnect) (struct usb_composite_dev *);
void (* suspend) (struct usb_composite_dev *);
void (* resume) (struct usb_composite_dev *);
struct usb_gadget_driver gadget_driver;
}; For diagnostics, identifies the driver.
Template descriptor for the device, including default device identifiers.
tables of strings, keyed by identifiers assigned during bind
and language IDs provided in control requests. Note: The first entries
are predefined. The first entry that may be used is
USB_GADGET_FIRST_AVAIL_IDX
Highest speed the driver supports.
set to 1 if the gadget needs userspace to provide a serial number. If one is not provided, warning will be printed.
(REQUIRED) Used to allocate resources that are shared across the
whole device, such as string IDs, and add its configurations using
usb_add_config(). This may fail by returning a negative errno
value; it should return zero on successful initialization.
Reverses bind; called as a side effect of unregistering
this driver.
optional driver disconnect method
Notifies when the host stops sending USB traffic, after function notifications
Notifies configuration when the host restarts USB traffic, before function notifications
Gadget driver controlling this driver
Devices default to reporting self powered operation. Devices which rely
on bus powered operation should report this in their bind method.
Before returning from bind, various fields in the template descriptor
may be overridden. These include the idVendor/idProduct/bcdDevice values
normally to bind the appropriate host side driver, and the three strings
(iManufacturer, iProduct, iSerialNumber) normally used to provide user
meaningful device identifiers. (The strings will not be defined unless
they are defined in dev and strings.) The correct ep0 maxpacket size
is also reported, as defined by the underlying controller driver.
struct usb_composite_dev — represents one composite usb gadget
struct usb_composite_dev {
struct usb_gadget * gadget;
struct usb_request * req;
struct usb_configuration * config;
}; read-only, abstracts the gadget's usb peripheral controller
used for control responses; buffer is pre-allocated
the currently active configuration
One of these devices is allocated and initialized before the
associated device driver's bind is called.
it appears that some WUSB devices will need to be built by combining a normal (wired) gadget with a wireless one. This revision of the gadget framework should probably try to make sure doing that won't hurt too much.
(a) a second gadget here, discovery mechanism TBD, but likely needing separate “register/unregister WUSB gadget” calls; (b) updates to usb_gadget to include flags “is it wireless”, “is it wired”, plus (presumably in a wrapper structure) bandgroup and PHY info; (c) presumably a wireless_ep wrapping a usb_ep, and reporting wireless-specific parameters like maxburst and maxsequence; (d) configurations that are specific to wireless links; (e) function drivers that understand wireless configs and will support wireless for (additional) function instances; (f) a function to support association setup (like CBAF), not necessarily requiring a wireless adapter; (g) composite device setup that can create one or more wireless configs, including appropriate association setup support; (h) more, TBD.
config_ep_by_speed — configures the given endpoint according to gadget speed.
int fsfuncconfig_ep_by_speed ( | g, | |
| f, | ||
_ep); |
struct usb_gadget * g;struct usb_function * f;struct usb_ep * _ep;error code, 0 on success
This function chooses the right descriptors for a given endpoint according to gadget speed and saves it in the endpoint desc field. If the endpoint already has a descriptor assigned to it - overwrites it with currently corresponding descriptor. The endpoint maxpacket field is updated according to the chosen descriptor.
usb_add_function — add a function to a configuration
int fsfuncusb_add_function ( | config, | |
function); |
struct usb_configuration * config;struct usb_function * function;
After initialization, each configuration must have one or more
functions added to it. Adding a function involves calling its bind()
method to allocate resources such as interface and string identifiers
and endpoints.
This function returns the value of the function's bind, which is
zero for success else a negative errno value.
usb_function_deactivate — prevent function and gadget enumeration
int fsfuncusb_function_deactivate ( | function); |
struct usb_function * function;
Blocks response of the gadget driver to host enumeration by
preventing the data line pullup from being activated. This is
normally called during bind() processing to change from the
initial “ready to respond” state, or when a required resource
becomes available.
For example, drivers that serve as a passthrough to a userspace daemon can block enumeration unless that daemon (such as an OBEX, MTP, or print server) is ready to handle host requests.
Not all systems support software control of their USB peripheral data pullups.
Returns zero on success, else negative errno.
usb_function_activate — allow function and gadget enumeration
int fsfuncusb_function_activate ( | function); |
struct usb_function * function;usb_interface_id — allocate an unused interface ID
int fsfuncusb_interface_id ( | config, | |
function); |
struct usb_configuration * config;struct usb_function * function;configconfiguration associated with the interface
functionfunction handling the interface
usb_interface_id is called from usb_function.bind callbacks to
allocate new interface IDs. The function driver will then store that
ID in interface, association, CDC union, and other descriptors. It
will also handle any control requests targeted at that interface,
particularly changing its altsetting via set_alt. There may
also be class-specific or vendor-specific requests to handle.
All interface identifier should be allocated using this routine, to ensure that for example different functions don't wrongly assign different meanings to the same identifier. Note that since interface identifiers are configuration-specific, functions used in more than one configuration (or more than once in a given configuration) need multiple versions of the relevant descriptors.
Returns the interface ID which was allocated; or -ENODEV if no more interface IDs can be allocated.
usb_add_config — add a configuration to a device.
int fsfuncusb_add_config ( | cdev, | |
| config, | ||
bind); |
struct usb_composite_dev * cdev;struct usb_configuration * config;int (*bind)
(struct usb_configuration *);cdevwraps the USB gadget
configthe configuration, with bConfigurationValue assigned
bindthe configuration's bind function
One of the main tasks of a composite bind() routine is to
add each of the configurations it supports, using this routine.
This function returns the value of the configuration's bind(), which
is zero for success else a negative errno value. Binding configurations
assigns global resources including string IDs, and per-configuration
resources such as interface IDs and endpoints.
usb_string_id — allocate an unused string ID
int fsfuncusb_string_id ( | cdev); |
struct usb_composite_dev * cdev;
usb_string_id() is called from bind callbacks to allocate
string IDs. Drivers for functions, configurations, or gadgets will
then store that ID in the appropriate descriptors and string table.
All string identifier should be allocated using this,
usb_string_ids_tab() or usb_string_ids_n() routine, to ensure
that for example different functions don't wrongly assign different
meanings to the same identifier.
usb_string_ids_tab — allocate unused string IDs in batch
int fsfuncusb_string_ids_tab ( | cdev, | |
str); |
struct usb_composite_dev * cdev;struct usb_string * str;cdevthe device whose string descriptor IDs are being allocated
stran array of usb_string objects to assign numbers to
usb_string_ids() is called from bind callbacks to allocate
string IDs. Drivers for functions, configurations, or gadgets will
then copy IDs from the string table to the appropriate descriptors
and string table for other languages.
All string identifier should be allocated using this,
usb_string_id() or usb_string_ids_n() routine, to ensure that for
example different functions don't wrongly assign different meanings
to the same identifier.
usb_gstrings_attach — attach gadget strings to a cdev and assign ids
struct usb_string * fsfuncusb_gstrings_attach ( | cdev, | |
| sp, | ||
n_strings); |
struct usb_composite_dev * cdev;struct usb_gadget_strings ** sp;unsigned n_strings;cdevthe device whose string descriptor IDs are being allocated and attached.
span array of usb_gadget_strings to attach.
n_stringsnumber of entries in each usb_strings array (sp[]->strings)
This function will create a deep copy of usb_gadget_strings and usb_string and attach it to the cdev. The actual string (usb_string.s) will not be copied but only a referenced will be made. The struct usb_gadget_strings array may contain multiple languges and should be NULL terminated. The ->language pointer of each struct usb_gadget_strings has to contain the same amount of entries.
usb_string_ids_n — allocate unused string IDs in batch
int fsfuncusb_string_ids_n ( | c, | |
n); |
struct usb_composite_dev * c;unsigned n;cthe device whose string descriptor IDs are being allocated
nnumber of string IDs to allocate
Returns the first requested ID. This ID and next n-1 IDs are now
valid IDs. At least provided that n is non-zero because if it
is, returns last requested ID which is now very useful information.
usb_string_ids_n() is called from bind callbacks to allocate
string IDs. Drivers for functions, configurations, or gadgets will
then store that ID in the appropriate descriptors and string table.
All string identifier should be allocated using this,
usb_string_id() or usb_string_ids_n() routine, to ensure that for
example different functions don't wrongly assign different meanings
to the same identifier.
usb_composite_probe — register a composite driver
int fsfuncusb_composite_probe ( | driver); |
struct usb_composite_driver * driver;
This function is used to register drivers using the composite driver
framework. The return value is zero, or a negative errno value.
Those values normally come from the driver's bind method, which does
all the work of setting up the driver to match the hardware.
On successful return, the gadget is ready to respond to requests from
the host, unless one of its components invokes usb_gadget_disconnect
while it was binding. That would usually be done in order to wait for
some userspace participation.
usb_composite_unregister — unregister a composite driver
void fsfuncusb_composite_unregister ( | driver); |
struct usb_composite_driver * driver;usb_composite_setup_continue — Continue with the control transfer
void fsfuncusb_composite_setup_continue ( | cdev); |
struct usb_composite_dev * cdev;
This function must be called by the USB function driver to continue
with the control transfer's data/status stage in case it had requested to
delay the data/status stages. A USB function's setup handler (e.g. set_alt)
can request the composite framework to delay the setup request's data/status
stages by returning USB_GADGET_DELAYED_STATUS.
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".
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.
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 OTG support on Linux 2.6 was initially developed by Texas Instruments for OMAP 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 transciever 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 (usb_bus or 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 usb_bus and 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.