31 lines
1.3 KiB
ReStructuredText
31 lines
1.3 KiB
ReStructuredText
.. -*- 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.
|