Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/media/uapi/v4l/app-pri.rst b/Documentation/media/uapi/v4l/app-pri.rst
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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _app-pri:
+
+********************
+Application Priority
+********************
+
+When multiple applications share a device it may be desirable to assign
+them different priorities. Contrary to the traditional "rm -rf /" school
+of thought a video recording application could for example block other
+applications from changing video controls or switching the current TV
+channel. Another objective is to permit low priority applications
+working in background, which can be preempted by user controlled
+applications and automatically regain control of the device at a later
+time.
+
+Since these features cannot be implemented entirely in user space V4L2
+defines the :ref:`VIDIOC_G_PRIORITY <VIDIOC_G_PRIORITY>` and
+:ref:`VIDIOC_S_PRIORITY <VIDIOC_G_PRIORITY>` ioctls to request and
+query the access priority associate with a file descriptor. Opening a
+device assigns a medium priority, compatible with earlier versions of
+V4L2 and drivers not supporting these ioctls. Applications requiring a
+different priority will usually call :ref:`VIDIOC_S_PRIORITY
+<VIDIOC_G_PRIORITY>` after verifying the device with the
+:ref:`VIDIOC_QUERYCAP` ioctl.
+
+Ioctls changing driver properties, such as
+:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>`, return an ``EBUSY`` error code
+after another application obtained higher priority.