Add documentation about centrally managed subdev state. Signed-off-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx> Reviewed-by: Jacopo Mondi <jacopo+renesas@xxxxxxxxxx> Reviewed-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx> --- .../driver-api/media/v4l2-subdev.rst | 60 +++++++++++++++++++ 1 file changed, 60 insertions(+) diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst index 08ea2673b19e..6b14cc97afa3 100644 --- a/Documentation/driver-api/media/v4l2-subdev.rst +++ b/Documentation/driver-api/media/v4l2-subdev.rst @@ -518,6 +518,66 @@ 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 sub-device driver can access and modify the configuration + stored in the provided state after having locked the state by calling +:c:func:`v4l2_subdev_lock_state()`. The driver must then call +:c:func:`v4l2_subdev_unlock_state()` to unlock the state when done. + +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. + V4L2 sub-device functions and data structures --------------------------------------------- -- 2.25.1
