[PATCH v4 14/36] [media] v4l2-mc: add a function to inherit controls from a pipeline

[Russell King - ARM Linux]

On Fri, Mar 10, 2017 at 02:22:29PM +0100, Hans Verkuil wrote:
> And nobody of the media core developers has the time to work on the docs,
> utilities and libraries you need to make this all work cleanly and reliably.

Well, talking about docs, and in connection to control inheritence,
this is already documented in at least three separate places:

Documentation/media/uapi/v4l/dev-subdev.rst:

  Controls
  ========
  ...
  Depending on the driver, those controls might also be exposed through
  one (or several) V4L2 device nodes.

Documentation/media/kapi/v4l2-subdev.rst:

  ``VIDIOC_QUERYCTRL``,
  ``VIDIOC_QUERYMENU``,
  ``VIDIOC_G_CTRL``,
  ``VIDIOC_S_CTRL``,
  ``VIDIOC_G_EXT_CTRLS``,
  ``VIDIOC_S_EXT_CTRLS`` and
  ``VIDIOC_TRY_EXT_CTRLS``:
  
          The controls ioctls are identical to the ones defined in V4L2. They
          behave identically, with the only exception that they deal only with
          controls implemented in the sub-device. Depending on the driver, those
          controls can be also be accessed through one (or several) V4L2 device
          nodes.

Then there's Documentation/media/kapi/v4l2-controls.rst, which gives a
step by step approach to the main video device inheriting controls from
its subdevices, and it says:

  Inheriting Controls
  -------------------
  
  When a sub-device is registered with a V4L2 driver by calling
  v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
  and v4l2_device are set, then the controls of the subdev will become
  automatically available in the V4L2 driver as well. If the subdev driver
  contains controls that already exist in the V4L2 driver, then those will be
  skipped (so a V4L2 driver can always override a subdev control).
  
  What happens here is that v4l2_device_register_subdev() calls
  v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
  of v4l2_device.

So, either the docs are wrong, or the advice being mentioned in emails
about subdev control inheritence is misleading.  Whatever, the two are
currently inconsistent.

As I've already mentioned, from talking about this with Mauro, it seems
Mauro is in agreement with permitting the control inheritence... I wish
Mauro would comment for himself, as I can't quote our private discussion
on the subject.

Right now, my view is that v4l2 is currently being screwed up by people
with different opinions - there is no unified concensus on how any of
this stuff is supposed to work, everyone is pulling in different
directions.  That needs solving _really_ quickly, so I suggest that
v4l2 people urgently talk to each other and thrash out some of the
issues that Steve's patch set has brought up, and settle on a way
forward, rather than what is seemingly happening today - which is
everyone working in isolation of everyone else with their own bias on
how things should be done.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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