Greg Kroah-Hartman
598cf27219
Link: https://lore.kernel.org/r/20260217200002.683975158@linuxfoundation.org Tested-by: Florian Fainelli <florian.fainelli@broadcom.com> Tested-by: Takeshi Ogasawara <takeshi.ogasawara@futuring-girl.com> Tested-by: Peter Schneider <pschneider1968@googlemail.com> Tested-by: Jon Hunter <jonathanh@nvidia.com> Tested-by: Salvatore Bonaccorso <carnil@debian.org> Tested-by: Brett A C Sheffield <bacs@librecast.net> Tested-by: Mark Brown <broonie@kernel.org> Tested-by: Luna Jernberg <droidbittin@gmail.com> Tested-by: Ronald Warsow <rwarsow@gmx.de> Tested-by: Justin M. Forbes <jforbes@fedoraproject.org> Tested-by: Ron Economos <re@w6rz.net> Tested-by: Miguel Ojeda <ojeda@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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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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