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media: Documentation: dv-timings.rst: explain basic approach

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Explain how the DV Timings API and the V4L2_EVENT_SOURCE_CHANGE
event work together to handle video source changes.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
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Hans Verkuil 2023-09-07 10:54:31 +02:00
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Documentation/userspace-api/media/v4l/dv-timings.rst
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@ -33,6 +33,27 @@ current DV timings they use the
the DV timings as seen by the video receiver applications use the
:ref:`VIDIOC_QUERY_DV_TIMINGS` ioctl.
When the hardware detects a video source change (e.g. the video
signal appears or disappears, or the video resolution changes), then
it will issue a `V4L2_EVENT_SOURCE_CHANGE` event. Use the
:ref:`ioctl VIDIOC_SUBSCRIBE_EVENT <VIDIOC_SUBSCRIBE_EVENT>` and the
:ref:`VIDIOC_DQEVENT` to check if this event was reported.
If the video signal changed, then the application has to stop
streaming, free all buffers, and call the :ref:`VIDIOC_QUERY_DV_TIMINGS`
to obtain the new video timings, and if they are valid, it can set
those by calling the :ref:`ioctl VIDIOC_S_DV_TIMINGS <VIDIOC_G_DV_TIMINGS>`.
This will also update the format, so use the :ref:`ioctl VIDIOC_G_FMT <VIDIOC_G_FMT>`
to obtain the new format. Now the application can allocate new buffers
and start streaming again.
The :ref:`VIDIOC_QUERY_DV_TIMINGS` will just report what the
hardware detects, it will never change the configuration. If the
currently set timings and the actually detected timings differ, then
typically this will mean that you will not be able to capture any
video. The correct approach is to rely on the `V4L2_EVENT_SOURCE_CHANGE`
event so you know when something changed.
Applications can make use of the :ref:`input-capabilities` and
:ref:`output-capabilities` flags to determine whether the digital
video ioctls can be used with the given input or output.