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