Hi Tomi, On Tue, Nov 30, 2021 at 04:15:04PM +0200, Tomi Valkeinen wrote: > Add documentation about centrally managed subdev state. > > Signed-off-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx> > --- > .../driver-api/media/v4l2-subdev.rst | 28 +++++++++++++++++++ > 1 file changed, 28 insertions(+) > > diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst > index 7736da077fb8..db012729f8e3 100644 > --- a/Documentation/driver-api/media/v4l2-subdev.rst > +++ b/Documentation/driver-api/media/v4l2-subdev.rst > @@ -518,6 +518,34 @@ 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 as an array of struct > +v4l2_mbus_framefmt, one entry for each pad. Not only v4l2_mbus_framefmt, but the 'active' configuration also contains cropping rectangles. I would drop the last statement maybe ? > + > +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. A subdev driver must use I would here explain what active and try states are and where they live. "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." > +v4l2_subdev_init_finalize() to initialize the active state between calls to > +media_entity_pads_init() and v4l2_*_register_subdev(), and must call > +v4l2_subdev_cleanup() to free the state. "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() as the last operation 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." > + > +The active state must be locked before access, and that can be done with > +v4l2_subdev_lock_state() or v4l2_subdev_lock_active_state(). I would also explain which state to use and when. "V4L2 sub-device operations that operates on both the :ref:`ACTIVE and TRY formats <v4l2-subdev-format-whence>` receive by the core 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 exclusively acquired it by calling :c:func:`v4l2_subdev_lock_state()` and released it with :c:func:`v4l2_subdev_unlock_state()` when done. Operations that do not receive a state parameter implicitly operates 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 subdev drivers comply with this yet, and may pass If I'm not mistaken it's not the subdev drivers but the top drivers which might not comply. > +NULL for the ACTIVE case. 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. depending on the previous suggestion to fail hard if !state, this might be removed ? Thanks j > + > V4L2 sub-device functions and data structures > --------------------------------------------- > > -- > 2.25.1 >
