41 lines
1.8 KiB
ReStructuredText
41 lines
1.8 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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.. _media-controller-intro:
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Introduction
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============
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Media devices increasingly handle multiple related functions. Many USB
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cameras include microphones, video capture hardware can also output
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video, or SoC camera interfaces also perform memory-to-memory operations
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similar to video codecs.
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Independent functions, even when implemented in the same hardware, can
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be modelled as separate devices. A USB camera with a microphone will be
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presented to userspace applications as V4L2 and ALSA capture devices.
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The devices' relationships (when using a webcam, end-users shouldn't
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have to manually select the associated USB microphone), while not made
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available directly to applications by the drivers, can usually be
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retrieved from sysfs.
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With more and more advanced SoC devices being introduced, the current
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approach will not scale. Device topologies are getting increasingly
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complex and can't always be represented by a tree structure. Hardware
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blocks are shared between different functions, creating dependencies
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between seemingly unrelated devices.
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Kernel abstraction APIs such as V4L2 and ALSA provide means for
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applications to access hardware parameters. As newer hardware expose an
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increasingly high number of those parameters, drivers need to guess what
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applications really require based on limited information, thereby
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implementing policies that belong to userspace.
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The media controller API aims at solving those problems.
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