1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _app-pri:
4
5********************
6Application Priority
7********************
8
9When multiple applications share a device it may be desirable to assign
10them different priorities. Contrary to the traditional "rm -rf /" school
11of thought, a video recording application could for example block other
12applications from changing video controls or switching the current TV
13channel. Another objective is to permit low priority applications
14working in background, which can be preempted by user controlled
15applications and automatically regain control of the device at a later
16time.
17
18Since these features cannot be implemented entirely in user space V4L2
19defines the :ref:`VIDIOC_G_PRIORITY <VIDIOC_G_PRIORITY>` and
20:ref:`VIDIOC_S_PRIORITY <VIDIOC_G_PRIORITY>` ioctls to request and
21query the access priority associate with a file descriptor. Opening a
22device assigns a medium priority, compatible with earlier versions of
23V4L2 and drivers not supporting these ioctls. Applications requiring a
24different priority will usually call :ref:`VIDIOC_S_PRIORITY
25<VIDIOC_G_PRIORITY>` after verifying the device with the
26:ref:`VIDIOC_QUERYCAP` ioctl.
27
28Ioctls changing driver properties, such as
29:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>`, return an ``EBUSY`` error code
30after another application obtained higher priority.
31