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| -rw-r--r-- | Documentation/hid/hid-transport.txt | 316 |
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diff --git a/Documentation/hid/hid-transport.txt b/Documentation/hid/hid-transport.txt new file mode 100644 index 000000000000..9dbbceaef4f3 --- /dev/null +++ b/Documentation/hid/hid-transport.txt @@ -0,0 +1,316 @@ + HID I/O Transport Drivers + =========================== + +The HID subsystem is independent of the underlying transport driver. Initially, +only USB was supported, but other specifications adopted the HID design and +provided new transport drivers. The kernel includes at least support for USB, +Bluetooth, I2C and user-space I/O drivers. + +1) HID Bus +========== + +The HID subsystem is designed as a bus. Any I/O subsystem may provide HID +devices and register them with the HID bus. HID core then loads generic device +drivers on top of it. The transport drivers are responsible of raw data +transport and device setup/management. HID core is responsible of +report-parsing, report interpretation and the user-space API. Device specifics +and quirks are handled by all layers depending on the quirk. + + +-----------+ +-----------+ +-----------+ +-----------+ + | Device #1 | | Device #i | | Device #j | | Device #k | + +-----------+ +-----------+ +-----------+ +-----------+ + \\ // \\ // + +------------+ +------------+ + | I/O Driver | | I/O Driver | + +------------+ +------------+ + || || + +------------------+ +------------------+ + | Transport Driver | | Transport Driver | + +------------------+ +------------------+ + \___ ___/ + \ / + +----------------+ + | HID Core | + +----------------+ + / | | \ + / | | \ + ____________/ | | \_________________ + / | | \ + / | | \ + +----------------+ +-----------+ +------------------+ +------------------+ + | Generic Driver | | MT Driver | | Custom Driver #1 | | Custom Driver #2 | + +----------------+ +-----------+ +------------------+ +------------------+ + +Example Drivers: + I/O: USB, I2C, Bluetooth-l2cap + Transport: USB-HID, I2C-HID, BT-HIDP + +Everything below "HID Core" is simplified in this graph as it is only of +interest to HID device drivers. Transport drivers do not need to know the +specifics. + +1.1) Device Setup +----------------- + +I/O drivers normally provide hotplug detection or device enumeration APIs to the +transport drivers. Transport drivers use this to find any suitable HID device. +They allocate HID device objects and register them with HID core. Transport +drivers are not required to register themselves with HID core. HID core is never +aware of which transport drivers are available and is not interested in it. It +is only interested in devices. + +Transport drivers attach a constant "struct hid_ll_driver" object with each +device. Once a device is registered with HID core, the callbacks provided via +this struct are used by HID core to communicate with the device. + +Transport drivers are responsible of detecting device failures and unplugging. +HID core will operate a device as long as it is registered regardless of any +device failures. Once transport drivers detect unplug or failure events, they +must unregister the device from HID core and HID core will stop using the +provided callbacks. + +1.2) Transport Driver Requirements +---------------------------------- + +The terms "asynchronous" and "synchronous" in this document describe the +transmission behavior regarding acknowledgements. An asynchronous channel must +not perform any synchronous operations like waiting for acknowledgements or +verifications. Generally, HID calls operating on asynchronous channels must be +running in atomic-context just fine. +On the other hand, synchronous channels can be implemented by the transport +driver in whatever way they like. They might just be the same as asynchronous +channels, but they can also provide acknowledgement reports, automatic +retransmission on failure, etc. in a blocking manner. If such functionality is +required on asynchronous channels, a transport-driver must implement that via +its own worker threads. + +HID core requires transport drivers to follow a given design. A Transport +driver must provide two bi-directional I/O channels to each HID device. These +channels must not necessarily be bi-directional in the hardware itself. A +transport driver might just provide 4 uni-directional channels. Or it might +multiplex all four on a single physical channel. However, in this document we +will describe them as two bi-directional channels as they have several +properties in common. + + - Interrupt Channel (intr): The intr channel is used for asynchronous data + reports. No management commands or data acknowledgements are sent on this + channel. Any unrequested incoming or outgoing data report must be sent on + this channel and is never acknowledged by the remote side. Devices usually + send their input events on this channel. Outgoing events are normally + not send via intr, except if high throughput is required. + - Control Channel (ctrl): The ctrl channel is used for synchronous requests and + device management. Unrequested data input events must not be sent on this + channel and are normally ignored. Instead, devices only send management + events or answers to host requests on this channel. + The control-channel is used for direct blocking queries to the device + independent of any events on the intr-channel. + Outgoing reports are usually sent on the ctrl channel via synchronous + SET_REPORT requests. + +Communication between devices and HID core is mostly done via HID reports. A +report can be of one of three types: + + - INPUT Report: Input reports provide data from device to host. This + data may include button events, axis events, battery status or more. This + data is generated by the device and sent to the host with or without + requiring explicit requests. Devices can choose to send data continuously or + only on change. + - OUTPUT Report: Output reports change device states. They are sent from host + to device and may include LED requests, rumble requests or more. Output + reports are never sent from device to host, but a host can retrieve their + current state. + Hosts may choose to send output reports either continuously or only on + change. + - FEATURE Report: Feature reports are used for specific static device features + and never reported spontaneously. A host can read and/or write them to access + data like battery-state or device-settings. + Feature reports are never sent without requests. A host must explicitly set + or retrieve a feature report. This also means, feature reports are never sent + on the intr channel as this channel is asynchronous. + +INPUT and OUTPUT reports can be sent as pure data reports on the intr channel. +For INPUT reports this is the usual operational mode. But for OUTPUT reports, +this is rarely done as OUTPUT reports are normally quite scarce. But devices are +free to make excessive use of asynchronous OUTPUT reports (for instance, custom +HID audio speakers make great use of it). + +Plain reports must not be sent on the ctrl channel, though. Instead, the ctrl +channel provides synchronous GET/SET_REPORT requests. Plain reports are only +allowed on the intr channel and are the only means of data there. + + - GET_REPORT: A GET_REPORT request has a report ID as payload and is sent + from host to device. The device must answer with a data report for the + requested report ID on the ctrl channel as a synchronous acknowledgement. + Only one GET_REPORT request can be pending for each device. This restriction + is enforced by HID core as several transport drivers don't allow multiple + simultaneous GET_REPORT requests. + Note that data reports which are sent as answer to a GET_REPORT request are + not handled as generic device events. That is, if a device does not operate + in continuous data reporting mode, an answer to GET_REPORT does not replace + the raw data report on the intr channel on state change. + GET_REPORT is only used by custom HID device drivers to query device state. + Normally, HID core caches any device state so this request is not necessary + on devices that follow the HID specs except during device initialization to + retrieve the current state. + GET_REPORT requests can be sent for any of the 3 report types and shall + return the current report state of the device. However, OUTPUT reports as + payload may be blocked by the underlying transport driver if the + specification does not allow them. + - SET_REPORT: A SET_REPORT request has a report ID plus data as payload. It is + sent from host to device and a device must update it's current report state + according to the given data. Any of the 3 report types can be used. However, + INPUT reports as payload might be blocked by the underlying transport driver + if the specification does not allow them. + A device must answer with a synchronous acknowledgement. However, HID core + does not require transport drivers to forward this acknowledgement to HID + core. + Same as for GET_REPORT, only one SET_REPORT can be pending at a time. This + restriction is enforced by HID core as some transport drivers do not support + multiple synchronous SET_REPORT requests. + +Other ctrl-channel requests are supported by USB-HID but are not available +(or deprecated) in most other transport level specifications: + + - GET/SET_IDLE: Only used by USB-HID and I2C-HID. + - GET/SET_PROTOCOL: Not used by HID core. + - RESET: Used by I2C-HID, not hooked up in HID core. + - SET_POWER: Used by I2C-HID, not hooked up in HID core. + +2) HID API +========== + +2.1) Initialization +------------------- + +Transport drivers normally use the following procedure to register a new device +with HID core: + + struct hid_device *hid; + int ret; + + hid = hid_allocate_device(); + if (IS_ERR(hid)) { + ret = PTR_ERR(hid); + goto err_<...>; + } + + strlcpy(hid->name, <device-name-src>, 127); + strlcpy(hid->phys, <device-phys-src>, 63); + strlcpy(hid->uniq, <device-uniq-src>, 63); + + hid->ll_driver = &custom_ll_driver; + hid->bus = <device-bus>; + hid->vendor = <device-vendor>; + hid->product = <device-product>; + hid->version = <device-version>; + hid->country = <device-country>; + hid->dev.parent = <pointer-to-parent-device>; + hid->driver_data = <transport-driver-data-field>; + + ret = hid_add_device(hid); + if (ret) + goto err_<...>; + +Once hid_add_device() is entered, HID core might use the callbacks provided in +"custom_ll_driver". Note that fields like "country" can be ignored by underlying +transport-drivers if not supported. + +To unregister a device, use: + + hid_destroy_device(hid); + +Once hid_destroy_device() returns, HID core will no longer make use of any +driver callbacks. + +2.2) hid_ll_driver operations +----------------------------- + +The available HID callbacks are: + - int (*start) (struct hid_device *hdev) + Called from HID device drivers once they want to use the device. Transport + drivers can choose to setup their device in this callback. However, normally + devices are already set up before transport drivers register them to HID core + so this is mostly only used by USB-HID. + + - void (*stop) (struct hid_device *hdev) + Called from HID device drivers once they are done with a device. Transport + drivers can free any buffers and deinitialize the device. But note that + ->start() might be called again if another HID device driver is loaded on the + device. + Transport drivers are free to ignore it and deinitialize devices after they + destroyed them via hid_destroy_device(). + + - int (*open) (struct hid_device *hdev) + Called from HID device drivers once they are interested in data reports. + Usually, while user-space didn't open any input API/etc., device drivers are + not interested in device data and transport drivers can put devices asleep. + However, once ->open() is called, transport drivers must be ready for I/O. + ->open() calls are nested for each client that opens the HID device. + + - void (*close) (struct hid_device *hdev) + Called from HID device drivers after ->open() was called but they are no + longer interested in device reports. (Usually if user-space closed any input + devices of the driver). + Transport drivers can put devices asleep and terminate any I/O of all + ->open() calls have been followed by a ->close() call. However, ->start() may + be called again if the device driver is interested in input reports again. + + - int (*parse) (struct hid_device *hdev) + Called once during device setup after ->start() has been called. Transport + drivers must read the HID report-descriptor from the device and tell HID core + about it via hid_parse_report(). + + - int (*power) (struct hid_device *hdev, int level) + Called by HID core to give PM hints to transport drivers. Usually this is + analogical to the ->open() and ->close() hints and redundant. + + - void (*request) (struct hid_device *hdev, struct hid_report *report, + int reqtype) + Send an HID request on the ctrl channel. "report" contains the report that + should be sent and "reqtype" the request type. Request-type can be + HID_REQ_SET_REPORT or HID_REQ_GET_REPORT. + This callback is optional. If not provided, HID core will assemble a raw + report following the HID specs and send it via the ->raw_request() callback. + The transport driver is free to implement this asynchronously. + + - int (*wait) (struct hid_device *hdev) + Used by HID core before calling ->request() again. A transport driver can use + it to wait for any pending requests to complete if only one request is + allowed at a time. + + - int (*raw_request) (struct hid_device *hdev, unsigned char reportnum, + __u8 *buf, size_t count, unsigned char rtype, + int reqtype) + Same as ->request() but provides the report as raw buffer. This request shall + be synchronous. A transport driver must not use ->wait() to complete such + requests. + + - int (*output_report) (struct hid_device *hdev, __u8 *buf, size_t len) + Send raw output report via intr channel. Used by some HID device drivers + which require high throughput for outgoing requests on the intr channel. This + must not cause SET_REPORT calls! This must be implemented as asynchronous + output report on the intr channel! + + - int (*idle) (struct hid_device *hdev, int report, int idle, int reqtype) + Perform SET/GET_IDLE request. Only used by USB-HID, do not implement! + +2.3) Data Path +-------------- + +Transport drivers are responsible of reading data from I/O devices. They must +handle any I/O-related state-tracking themselves. HID core does not implement +protocol handshakes or other management commands which can be required by the +given HID transport specification. + +Every raw data packet read from a device must be fed into HID core via +hid_input_report(). You must specify the channel-type (intr or ctrl) and report +type (input/output/feature). Under normal conditions, only input reports are +provided via this API. + +Responses to GET_REPORT requests via ->request() must also be provided via this +API. Responses to ->raw_request() are synchronous and must be intercepted by the +transport driver and not passed to hid_input_report(). +Acknowledgements to SET_REPORT requests are not of interest to HID core. + +---------------------------------------------------- +Written 2013, David Herrmann <dh.herrmann@gmail.com> |