Re: [PATCH] HID: Add HID transport driver documentation

[David Herrmann <dh.herrmann@gmail.com>]

Hi

On Wed, Jan 29, 2014 at 4:28 PM, Frank Praznik <frank.praznik@oh.rr.com> wrote:
> Add David Herrmann's documentation for the new low-level HID transport driver
> functions.
>
> Signed-off-by: Frank Praznik <frank.praznik@oh.rr.com>

If you copy code, you really should keep the signed-off-by chain. A
signed-off-by in kernel context means that you either wrote the code
or have permission to copy it. See here:
http://developercertificate.org/ (which is a public copy of the
kernel's signed-off-by practice).
If you copy code unchanged, it's common practice to even keep the
"Author" field via "git commit --author", but that's optional.

Anyhow, patch is good, thanks for picking it up!

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

Putting Benjamin on CC as he reviewed the patch last time and might
have some more comments (or his final reviewed-by).

Thanks!
David

> ---
>
>  Sorry, I forgot to include this in the original patch set.
>
>  Documentation/hid/hid-transport.txt | 324 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 324 insertions(+)
>  create mode 100644 Documentation/hid/hid-transport.txt
>
> diff --git a/Documentation/hid/hid-transport.txt b/Documentation/hid/hid-transport.txt
> new file mode 100644
> index 0000000..14b1c18
> --- /dev/null
> +++ b/Documentation/hid/hid-transport.txt
> @@ -0,0 +1,324 @@
> +                          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 (*hidinput_input_event) (struct input_dev *idev, unsigned int type,
> +                                unsigned int code, int value)
> +   Obsolete callback used by logitech converters. It is called when userspace
> +   writes input events to the input device (eg., EV_LED). A driver can use this
> +   callback to convert it into an output report and send it to the device. If
> +   this callback is not provided, HID core will use ->request() or
> +   ->raw_request() respectively.
> +
> + - 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>
> --
> 1.8.3.2
>