38 lines
1.7 KiB
ReStructuredText
38 lines
1.7 KiB
ReStructuredText
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.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/media/uapi/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _app-pri:
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********************
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Application Priority
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********************
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When multiple applications share a device it may be desirable to assign
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them different priorities. Contrary to the traditional "rm -rf /" school
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of thought, a video recording application could for example block other
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applications from changing video controls or switching the current TV
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channel. Another objective is to permit low priority applications
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working in background, which can be preempted by user controlled
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applications and automatically regain control of the device at a later
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time.
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Since these features cannot be implemented entirely in user space V4L2
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defines the :ref:`VIDIOC_G_PRIORITY <VIDIOC_G_PRIORITY>` and
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:ref:`VIDIOC_S_PRIORITY <VIDIOC_G_PRIORITY>` ioctls to request and
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query the access priority associate with a file descriptor. Opening a
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device assigns a medium priority, compatible with earlier versions of
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V4L2 and drivers not supporting these ioctls. Applications requiring a
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different priority will usually call :ref:`VIDIOC_S_PRIORITY
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<VIDIOC_G_PRIORITY>` after verifying the device with the
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:ref:`VIDIOC_QUERYCAP` ioctl.
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Ioctls changing driver properties, such as
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:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>`, return an ``EBUSY`` error code
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after another application obtained higher priority.
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