1.. Permission is granted to copy, distribute and/or modify this
2.. document under the terms of the GNU Free Documentation License,
3.. Version 1.1 or any later version published by the Free Software
4.. Foundation, with no Invariant Sections, no Front-Cover Texts
5.. and no Back-Cover Texts. A copy of the license is included at
6.. Documentation/userspace-api/media/fdl-appendix.rst.
7..
8.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
9
10.. _app-pri:
11
12********************
13Application Priority
14********************
15
16When multiple applications share a device it may be desirable to assign
17them different priorities. Contrary to the traditional "rm -rf /" school
18of thought, a video recording application could for example block other
19applications from changing video controls or switching the current TV
20channel. Another objective is to permit low priority applications
21working in background, which can be preempted by user controlled
22applications and automatically regain control of the device at a later
23time.
24
25Since these features cannot be implemented entirely in user space V4L2
26defines the :ref:`VIDIOC_G_PRIORITY <VIDIOC_G_PRIORITY>` and
27:ref:`VIDIOC_S_PRIORITY <VIDIOC_G_PRIORITY>` ioctls to request and
28query the access priority associate with a file descriptor. Opening a
29device assigns a medium priority, compatible with earlier versions of
30V4L2 and drivers not supporting these ioctls. Applications requiring a
31different priority will usually call :ref:`VIDIOC_S_PRIORITY
32<VIDIOC_G_PRIORITY>` after verifying the device with the
33:ref:`VIDIOC_QUERYCAP` ioctl.
34
35Ioctls changing driver properties, such as
36:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>`, return an ``EBUSY`` error code
37after another application obtained higher priority.
38