Re: [RFC PATCH 5/8] media: Document the media request API

[Alexandre Courbot]

On Tue, Jan 30, 2018 at 1:04 AM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
> On 01/26/2018 07:02 AM, Alexandre Courbot wrote:
>> From: Laurent Pinchart <laurent.pinchart+renesas@xxxxxxxxxxxxxxxx>
>>
>> The media request API is made of a new ioctl to implement request
>> management. Document it.
>>
>> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@xxxxxxxxxxxxxxxx>
>> [acourbot@xxxxxxxxxxxx: adapt for newest API]
>> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
>> ---
>>  Documentation/media/uapi/mediactl/media-funcs.rst  |   1 +
>>  .../media/uapi/mediactl/media-ioc-request-cmd.rst  | 140 +++++++++++++++++++++
>>  2 files changed, 141 insertions(+)
>>  create mode 100644 Documentation/media/uapi/mediactl/media-ioc-request-cmd.rst
>>
>> diff --git a/Documentation/media/uapi/mediactl/media-funcs.rst b/Documentation/media/uapi/mediactl/media-funcs.rst
>> index 076856501cdb..e3a45d82ffcb 100644
>> --- a/Documentation/media/uapi/mediactl/media-funcs.rst
>> +++ b/Documentation/media/uapi/mediactl/media-funcs.rst
>> @@ -15,4 +15,5 @@ Function Reference
>>      media-ioc-g-topology
>>      media-ioc-enum-entities
>>      media-ioc-enum-links
>> +    media-ioc-request-cmd
>>      media-ioc-setup-link
>> diff --git a/Documentation/media/uapi/mediactl/media-ioc-request-cmd.rst b/Documentation/media/uapi/mediactl/media-ioc-request-cmd.rst
>> new file mode 100644
>> index 000000000000..17223e8170e9
>> --- /dev/null
>> +++ b/Documentation/media/uapi/mediactl/media-ioc-request-cmd.rst
>> @@ -0,0 +1,140 @@
>> +.. -*- coding: utf-8; mode: rst -*-
>> +
>> +.. _media_ioc_request_cmd:
>> +
>> +***************************
>> +ioctl MEDIA_IOC_REQUEST_CMD
>> +***************************
>> +
>> +Name
>> +====
>> +
>> +MEDIA_IOC_REQUEST_CMD - Manage media device requests
>> +
>> +
>> +Synopsis
>> +========
>> +
>> +.. c:function:: int ioctl( int fd, MEDIA_IOC_REQUEST_CMD, struct media_request_cmd *argp )
>> +    :name: MEDIA_IOC_REQUEST_CMD
>> +
>> +
>> +Arguments
>> +=========
>> +
>> +``fd``
>> +    File descriptor returned by :ref:`open() <media-func-open>`.
>> +
>> +``argp``
>> +
>> +
>> +Description
>> +===========
>> +
>> +The MEDIA_IOC_REQUEST_CMD ioctl allow applications to manage media device
>> +requests. A request is an object that can group media device configuration
>> +parameters, including subsystem-specific parameters, in order to apply all the
>> +parameters atomically. Applications are responsible for allocating and
>> +deleting requests, filling them with configuration parameters submitting them.
>> +
>> +Request operations are performed by calling the MEDIA_IOC_REQUEST_CMD ioctl
>> +with a pointer to a struct :c:type:`media_request_cmd` with the cmd field set
>> +to the appropriate command. :ref:`media-request-command` lists the commands
>> +supported by the ioctl.
>> +
>> +The struct :c:type:`media_request_cmd` request field contains the file
>> +descriptorof the request on which the command operates. For the
>> +``MEDIA_REQ_CMD_ALLOC`` command the field is set to zero by applications and
>> +filled by the driver. For all other commands the field is set by applications
>> +and left untouched by the driver.
>> +
>> +To allocate a new request applications use the ``MEDIA_REQ_CMD_ALLOC``
>> +command. The driver will allocate a new request and return its ID in the
>
> ID -> FD

Indeed, this is a leftover from the original documentation.

>
>> +request field.
>> +
>> +Requests are reference-counted. A newly allocated request is referenced
>> +by the returned file descriptor, and can be later referenced by
>> +subsystem-specific operations. Requests will thus be automatically deleted
>> +when they're not longer used after the returned file descriptor is closed.
>> +
>> +If a request isn't needed applications can delete it by calling ``close()``
>> +on it. The driver will drop the file handle reference. The request will not
>> +be usable through the MEDIA_IOC_REQUEST_CMD ioctl anymore, but will only be
>> +deleted when the last reference is released. If no other reference exists when
>> +``close()`` is invoked the request will be deleted immediately.
>> +
>> +After creating a request applications should fill it with configuration
>> +parameters. This is performed through subsystem-specific request APIs outside
>> +the scope of the media controller API. See the appropriate subsystem APIs for
>> +more information, including how they interact with the MEDIA_IOC_REQUEST_CMD
>> +ioctl.
>> +
>> +Once a request contains all the desired configuration parameters it can be
>> +submitted using the ``MEDIA_REQ_CMD_SUBMIT`` command. This will let the
>> +buffers queued for the request be passed to their respective drivers, which
>> +will then apply the request's parameters before processing them.
>> +
>> +Once a request has been queued applications are not allowed to modify its
>> +configuration parameters until the request has been fully processed. Any
>> +attempt to do so will result in the related subsystem API returning an error.
>> +The application that submitted the request can wait for its completion by
>> +polling on the request's file descriptor.
>> +
>> +Once a request has completed, it can be reused. The ``MEDIA_REQ_CMD_REINIT``
>> +command will bring it back to its initial state, so it can be prepared and
>> +submitted again.
>> +
>> +.. c:type:: media_request_cmd
>> +
>> +.. flat-table:: struct media_request_cmd
>> +    :header-rows:  0
>> +    :stub-columns: 0
>> +    :widths: 1 2 8
>> +
>> +    * - __u32
>> +      - ``cmd``
>> +      - Command, set by the application. See below for the list of supported
>> +        commands.
>> +    * - __u32
>> +      - ``fd``
>> +      - Request FD, set by the driver for the MEDIA_REQ_CMD_ALLOC command and
>> +        by the application for all other commands.
>> +
>> +
>> +.. _media-request-command:
>> +
>> +.. cssclass:: longtable
>> +
>> +.. flat-table:: Media request commands
>> +    :header-rows:  0
>> +    :stub-columns: 0
>> +
>> +    * .. _MEDIA-REQ-CMD-ALLOC:
>> +
>> +      - ``MEDIA_REQ_CMD_ALLOC``
>> +      - Allocate a new request.
>> +    * .. _MEDIA-REQ-CMD-SUBMIT:
>> +
>> +      - ``MEDIA_REQ_CMD_SUBMIT``
>> +      - Submit a request to be processed.
>> +    * .. _MEDIA-REQ-CMD-QUEUE:
>> +
>> +      - ``MEDIA_REQ_CMD_REINIT``
>> +      - Reinitializes a completed request.
>> +
>> +
>> +Return Value
>> +============
>> +
>> +On success 0 is returned, on error -1 and the ``errno`` variable is set
>> +appropriately. The generic error codes are described at the
>> +:ref:`Generic Error Codes <gen-errors>` chapter.
>> +
>> +EINVAL
>> +    The struct :c:type:`media_request_cmd` specifies an invalid command or
>> +    references a non-existing request.
>> +
>> +ENOSYS
>> +    The struct :c:type:`media_request_cmd` specifies the
>> +    ``MEDIA_REQ_CMD_QUEUE`` or ``MEDIA_REQ_CMD_APPLY`` command and that
>
> MEDIA_REQ_CMD_APPLY isn't defined in the table above.

Same. :)

>
>> +    command isn't implemented by the device.
>>
>
>
> So what is missing from this documentation is whether a newly created request
> has a copy of the current state or if it is empty. And didn't we allow a
> request to be based on an other existing request? Unfortunately I have no
> time to dig up that information at the moment.

Will specify the behavior better.

>
> More comments in my review for 6/8. Probably best to read that review
> first before this one.
>
> Regards,
>
>         Hans