[v4,7/7] media: Documentation: add documentation about subdev state

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(+)

Hi Tomi,

Thank you for the patch.

On Wed, Feb 16, 2022 at 03:00:49PM +0200, 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..997d76134b50 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 is expected to be locked and unlocked by the

s/is expected to be/must be/

> +caller by calling :c:func:`v4l2_subdev_lock_state()` and
> +:c:func:`v4l2_subdev_unlock_state()`. Alternatively the caller can  use
> +:c:func:`v4l2_subdev_call_state_active()` macro to call the op.

I'd write the last sentence as

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_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
> +used by subdevice drivers to retrieve the active state if a NULL state
> +is passed to the subdevice operation.
> +
> +:c:func:`v4l2_subdev_lock_and_return_state()` should only be used when porting
> +an existing driver to the new state management when it cannot be guaranteed
> +that the current callers will pass the state properly. The function prints a
> +notice when the passed state is NULL to encourage the porting of the callers
> +to the new state management.
> +
> +The whole subdev state is in reality split into three parts: the
> +v4l2_subdev_state, subdev controls and subdev driver's internal state. In the
> +future these parts should be combined into a single state. For the time being
> +we need a way to handle the locking for these parts. This can be accomplished
> +by sharing a lock. The v4l2_ctrl_handler already supports this via it's 'lock'

s/it's/its/

> +pointer and the same model is used with states. The driver can do the following
> +before calling v4l2_subdev_init_finalize():
> +
> +.. code-block:: python
> +
> +	sd->ctrl_handler->lock = &priv->mutex;
> +	sd->state_lock = &priv->mutex;
> +
> +This shares the driver's private mutex between the controls and the states.
> +
>  V4L2 sub-device functions and data structures
>  ---------------------------------------------
>