Re: [PATCH v2 6/6] media: Documentation: add documentation about subdev state

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Laurent,

On Tue, Dec 21, 2021 at 10:36:43AM +0200, Laurent Pinchart wrote:
> Moi Sakari,
> 
> On Mon, Dec 20, 2021 at 07:56:31PM +0200, Sakari Ailus wrote:
> > Moi,
> > 
> > Thanks for the update.
> 
> Kiitos arvostelusta.
> 
> > On Fri, Dec 17, 2021 at 03:50:22PM +0200, Tomi Valkeinen wrote:
> > > Add documentation about centrally managed subdev state.
> > > 
> > > Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> > > Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> > > ---
> > >  .../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
> > > +struct v4l2_mbus_framefmt, one entry for each pad, and similarly for cropping
> > > +and composition rectangles.
> 
> I think we usually write
> 
> s/cropping/crop/
> s/composition/compose/
> 
> > > +
> > > +In addition to the active configuration, each subdev file-handle has an array of
> 
> s/file-handle/file handle/ (through the whole patch)
> 
> > > +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
> 
> s/associated with/stored in/
> 
> > > +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.
> 
> I'd write
> 
> [...] 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 acquired resources before unregistering the sub-device.
> 
> s/acquired/allocated/
> 
> > > +The core automatically initializes a state for each open file-handle where to
> 
> s/initializes/allocates and initializes/
> s/where to/to/
> 
> > > +store the try configurations and releases them at file-handle closing time.
> 
> s/releases it at file-handle closing time/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 as a
> 
> s/as a/through the/
> 
> > > +'state' parameter. The sub-device driver can access and modify the
> > > +configuration stored in the provided state after having locked it by calling
> 
> s/locked it/locked the state/
> 
> > > +: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.
> > 
> > Have you thought how this will interact with controls?
> > 
> > Generally active state information exists for a device in struct
> > v4l2_subdev_state as well as the device's control handler as control
> > values. Controls have dependencies to other state information (active and
> > try).
> > 
> > Until now, drivers have been responsible for serialising access to this
> > state on their own, mostly using a single mutex. Controls require a mutex
> > as well, but it's the same mutex independently of whether a driver is
> > dealing with active or try subdev state.
> > 
> > In other words, if the above is assumed, when you're dealing with try state
> > that has dependencies to controls, you have to hold both that try state's
> > mutex as well as the control handler's mutex.
> 
> Going forward, I think we should store the controls in the subdev state.
> That will require a uAPI extension to pass a `which` parameter to the
> control ioctls, and deprecated the control TRY ioctl on subdevs.
> Interactions between controls and pad formats will be easier to test, as
> applications will be able to set controls in the TRY state, interacting
> with the TRY formats. We will also need to rework the control handler
> operations to split .s_ctrl() in two, with one function to adjust a
> control value and one function to apply it.
> 
> In the meantime, I think we'll need to acquire both locks, or possibly
> use the active state lock as the control handler lock.

Note that also trying controls requires locking the control handler,
meaning that the control handler's mutex may not be the same as the active
state mutex (unless access also to try state is serialised using the same
mutex).

What I'm saying is that to make this better usable with controls, changes
will be needed somewhere as the locking scheme is a poor match with that of
controls currently. Just saying the mutexes are acquired in a certain
order and pushing the problem to drivers is not a great solution.

-- 
Sakari Ailus