HID: Add HID transport driver documentation

                          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>