Add documentation about centrally managed subdev state. Signed-off-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx> --- .../driver-api/media/v4l2-subdev.rst | 58 +++++++++++++++++++ 1 file changed, 58 insertions(+) diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst index 08ea2673b19e..f0ba04c80563 100644 --- a/Documentation/driver-api/media/v4l2-subdev.rst +++ b/Documentation/driver-api/media/v4l2-subdev.rst @@ -518,6 +518,64 @@ 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 +configuration for the subdev. This is often implemented e.g. as an array of +struct v4l2_mbus_framefmt, one entry for each pad, and similarly for cropping +and composition using struct v4l2_rect. + +In addition to the active configuration, each subdev filehandle has an array of +struct v4l2_subdev_pad_config, managed by 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 associated with the sub-device itself as part of +the :c:type:`v4l2_subdev` structure, while the core associates to each open +file handle a try state, which contains the configuration valid in the +file-handle context only. + +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 acquired resources before unregistering the sub-device. +The core automatically initializes a state for each open file handle where to +store the try configurations and releases them at file handle closing time. + +V4L2 sub-device operations that operate on both the :ref:`ACTIVE and TRY formats +<v4l2-subdev-format-whence>` receive the correct state to operate on an +operation parameter. The sub-device driver can access and modify the +configuration stored in the provided state after having locked it 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 +should equally be released by calling +:c:func:`v4l2_subdev_unlock_state()`. + +In no occasions driver should try to manually access the state stored +in the :c:type:`v4l2_subdev` or in the file handle without going +through the designated helpers. + +The V4L2 core will pass either the try- or active-state to various subdev ops. +Unfortunately not all the callers comply with this yet, and may pass NULL as +the active-state. This is only a problem for subdev drivers which use the +centrally managed active-state and are used in media pipelines with older +subdev drivers. In these cases the called subdev ops must also handle the NULL +case. This can be easily managed by the use of +v4l2_subdev_validate_and_lock_state() helper. + +v4l2_subdev_validate_and_lock_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
