On 17/12/2021 14:50, Tomi Valkeinen wrote: > Add documentation about centrally managed subdev state. > > Signed-off-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx> > Reviewed-by: Jacopo Mondi <jacopo+renesas@xxxxxxxxxx> > --- > .../driver-api/media/v4l2-subdev.rst | 57 +++++++++++++++++++ > 1 file changed, 57 insertions(+) > > diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst > index 08ea2673b19e..18b00bd3d6d4 100644 > --- a/Documentation/driver-api/media/v4l2-subdev.rst > +++ b/Documentation/driver-api/media/v4l2-subdev.rst > @@ -518,6 +518,63 @@ 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 e.g. as an array of e.g. as -> as e.g. > +struct v4l2_mbus_framefmt, one entry for each pad, and similarly for cropping > +and composition rectangles. > + > +In addition to the active configuration, each subdev file-handle has an array of > +struct v4l2_subdev_pad_config, managed by V4L2 core, which contains the try by V4L2 -> by the V4L2 > +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 use both the :ref:`ACTIVE and TRY formats > +<v4l2-subdev-format-whence>` receive the correct state to operate on as a > +'state' 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 > +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 will pass the correct try- or active-state to the > +subdevice operations, device drivers might call operations on other > +subdevices by using :c:func:`v4l2_subdev_call()` kAPI and pass NULL as the > +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 > +:c:func:`v4l2_subdev_lock_and_return_state()` helper. > + > +: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 > --------------------------------------------- > > With those tiny changes: Reviewed-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx> Regards, Hans
