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diff --git a/Documentation/userspace-api/media/v4l/app-pri.rst b/Documentation/userspace-api/media/v4l/app-pri.rst new file mode 100644 index 000000000000..5018ede2706f --- /dev/null +++ b/Documentation/userspace-api/media/v4l/app-pri.rst @@ -0,0 +1,37 @@ +.. Permission is granted to copy, distribute and/or modify this +.. document under the terms of the GNU Free Documentation License, +.. Version 1.1 or any later version published by the Free Software +.. Foundation, with no Invariant Sections, no Front-Cover Texts +.. and no Back-Cover Texts. A copy of the license is included at +.. Documentation/userspace-api/media/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _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. |