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Diffstat (limited to 'Documentation/linux_tv/media/v4l/app-pri.rst')
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diff --git a/Documentation/linux_tv/media/v4l/app-pri.rst b/Documentation/linux_tv/media/v4l/app-pri.rst new file mode 100644 index 000000000000..9716437ae14d --- /dev/null +++ b/Documentation/linux_tv/media/v4l/app-pri.rst @@ -0,0 +1,39 @@ +.. -*- 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 ``VIDIOC_S_PRIORITY`` after +verifying the device with the +:ref:`VIDIOC_QUERYCAP <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. + + +.. ------------------------------------------------------------------------------ +.. This file was automatically converted from DocBook-XML with the dbxml +.. library (https://github.com/return42/sphkerneldoc). The origin XML comes +.. from the linux kernel, refer to: +.. +.. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook +.. ------------------------------------------------------------------------------ |