Re: [PATCH v6 8/8] media: Documentation: add documentation about subdev state

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

On 06/04/2022 17:01, Hans Verkuil wrote:
> 
> 
> On 24/03/2022 09:00, Tomi Valkeinen wrote:
>> Add documentation about centrally managed subdev state.
>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
>> Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
>> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> ---
>>   .../driver-api/media/v4l2-subdev.rst          | 75 +++++++++++++++++++
>>   1 file changed, 75 insertions(+)
>>
>> diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst
>> index 08ea2673b19e..7fa950b1bff0 100644
>> --- a/Documentation/driver-api/media/v4l2-subdev.rst
>> +++ b/Documentation/driver-api/media/v4l2-subdev.rst
>> @@ -518,6 +518,81 @@ The :c:func:`v4l2_i2c_new_subdev` function will call
>>   :c:type:`i2c_board_info` structure using the ``client_type`` and the
>>   ``addr`` to fill it.
>>   
>> +Centrally managed subdev active state
>> +-------------------------------------
>> +
>> +Traditionally V4L2 subdev drivers maintained internal state for the active
>> +device configuration. This is often implemented as e.g. an array of struct
>> +v4l2_mbus_framefmt, one entry for each pad, and similarly for crop and compose
>> +rectangles.
>> +
>> +In addition to the active configuration, each subdev file handle has an array of
>> +struct v4l2_subdev_pad_config, managed by the V4L2 core, which contains the try
>> +configuration.
>> +
>> +To simplify the subdev drivers the V4L2 subdev API now optionally supports a
>> +centrally managed active configuration represented by
>> +:c:type:`v4l2_subdev_state`. One instance of state, which contains the active
>> +device configuration, is stored in the sub-device itself as part of
>> +the :c:type:`v4l2_subdev` structure, while the core associates a try state to
>> +each open file handle, to store the try configuration related to that file
>> +handle.
>> +
>> +Sub-device drivers can opt-in and use state to manage their active configuration
>> +by initializing the subdevice state with a call to v4l2_subdev_init_finalize()
>> +before registering the sub-device. They must also call v4l2_subdev_cleanup()
>> +to release all the allocated resources before unregistering the sub-device.
>> +The core automatically allocates and initializes a state for each open file
>> +handle to store the try configurations and frees it when closing the file
>> +handle.
>> +
>> +V4L2 sub-device operations that use both the :ref:`ACTIVE and TRY formats
>> +<v4l2-subdev-format-whence>` receive the correct state to operate on through
>> +the 'state' parameter. The state must be locked and unlocked by the
>> +caller by calling :c:func:`v4l2_subdev_lock_state()` and
>> +:c:func:`v4l2_subdev_unlock_state()`. The caller can do so by calling the subdev
>> +operation through the :c:func:`v4l2_subdev_call_state_active()` macro.
>> +
>> +Operations that do not receive a state parameter implicitly operate on the
>> +subdevice active state, which drivers can exclusively access by
>> +calling :c:func:`v4l2_subdev_lock_and_get_active_state()`. The sub-device active
>> +state must equally be released by calling :c:func:`v4l2_subdev_unlock_state()`.
>> +
>> +Drivers must never manually access the state stored in the :c:type:`v4l2_subdev`
>> +or in the file handle without going through the designated helpers.
>> +
>> +While the V4L2 core passes the correct try or active state to the
>> +subdevice operations, some existing device drivers pass a NULL state
>> +when calling operations with :c:func:`v4l2_subdev_call()`. This legacy
>> +construct causes issues with subdevice drivers that let the V4L2 core
>> +manage the active state, as they expect to receive the appropriate state
>> +as a parameter. To help the conversion of subdevice drivers to a managed
>> +active state without having to convert all callers at the same time, the
>> +:c:func:`v4l2_subdev_lock_and_return_state()` helper function can be
> 
> This is an old name, right? It's no longer called v4l2_subdev_lock_and_return_state.
> Note that the commit log of patch 4 also refers to this old name.
> 
> Actually, isn't this now replaced by the subdev op wrappers? (I might be wrong,
> I don't have much time to review this patch).

That's correct, good catch. I have removed the parts about the drivers 
having to use v4l2_subdev_lock_and_get_active_state, and instead mention 
the additional call wrappers that automate this:

> To help the
> conversion of subdevice drivers to a managed active state without having to
> convert all callers at the same time, an additional wrapper layer has been
> added to v4l2_subdev_call(), which handles the NULL case by getting and locking
> the callee's active state with :c:func:`v4l2_subdev_lock_and_get_active_state()`,
> and unlocking the state after the call.

  Tomi