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