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[media] buffer.rst: clarify how V4L2_CTRL_FLAG_MODIFY_LAYOUT/GRABBER are used 

Explain when the V4L2_CTRL_FLAG_MODIFY_LAYOUT and
V4L2_CTRL_FLAG_MODIFY_GRABBER flags should be used.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
This commit is contained in:
Hans Verkuil 2017-04-10 16:17:06 -03:00 committed by Mauro Carvalho Chehab
parent c0e681f5b0
commit aacfed858f
1 changed files with 13 additions and 4 deletions
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17
Documentation/media/uapi/v4l/buffer.rst
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@ -48,10 +48,16 @@ The set of information needed to interpret the content of a buffer (e.g. the
pixel format, the line stride, the tiling orientation or the rotation) is
collectively referred to in the rest of this section as the buffer layout.
Controls that can modify the buffer layout shall set the
``V4L2_CTRL_FLAG_MODIFY_LAYOUT`` flag.
Modifying formats or controls that influence the buffer size or layout require
the stream to be stopped. Any attempt at such a modification while the stream
is active shall cause the ioctl setting the format or the control to return
the ``EBUSY`` error code.
the ``EBUSY`` error code. In that case drivers shall also set the
``V4L2_CTRL_FLAG_GRABBED`` flag when calling
:c:func:`VIDIOC_QUERYCTRL` or :c:func:`VIDIOC_QUERY_EXT_CTRL` for such a
control while the stream is active.
.. note::
@ -67,7 +73,8 @@ the ``EBUSY`` error code.
Controls that only influence the buffer layout can be modified at any time
when the stream is stopped. As they don't influence the buffer size, no
special handling is needed to synchronize those controls with buffer
allocation.
allocation and the ``V4L2_CTRL_FLAG_GRABBED`` flag is cleared once the
stream is stopped.
Formats and controls that influence the buffer size interact with buffer
allocation. The simplest way to handle this is for drivers to always require
@ -75,8 +82,10 @@ buffers to be reallocated in order to change those formats or controls. In
that case, to perform such changes, userspace applications shall first stop
the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it is running
and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if they are
allocated. The format or controls can then be modified, and buffers shall then
be reallocated and the stream restarted. A typical ioctl sequence is
allocated. After freeing all buffers the ``V4L2_CTRL_FLAG_GRABBED`` flag
for controls is cleared. The format or controls can then be modified, and
buffers shall then be reallocated and the stream restarted. A typical ioctl
sequence is
#. VIDIOC_STREAMOFF
#. VIDIOC_REQBUFS(0)