Hello, On Tue, Dec 14, 2021 at 09:31:51AM +0100, Jacopo Mondi wrote: > 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 s/operates/operate/ > TRY formats <v4l2-subdev-format-whence>` receive by the core the s/by the/from the/ (or just s/by the core //) > 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 s/exclusively acquired/locked/ > :c:func:`v4l2_subdev_lock_state()` and released it with > :c:func:`v4l2_subdev_unlock_state()` when done. "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 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 ? Or expanded with a note that this option is only possible when transitioning existing subdev drivers, and that the function will eventually be removed. And maybe that it prints a warning, and that host drivers must be converted as a result. Otherwise this looks good to me. On a side note, I think we should prepare to allow subdev drivers to subclass the state structure at some point :-) > > + > > V4L2 sub-device functions and data structures > > --------------------------------------------- > > -- Regards, Laurent Pinchart
