50 lines
2.1 KiB
ReStructuredText
50 lines
2.1 KiB
ReStructuredText
.. 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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.. _cec-intro:
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Introduction
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============
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HDMI connectors provide a single pin for use by the Consumer Electronics
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Control protocol. This protocol allows different devices connected by an
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HDMI cable to communicate. The protocol for CEC version 1.4 is defined
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in supplements 1 (CEC) and 2 (HEAC or HDMI Ethernet and Audio Return
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Channel) of the HDMI 1.4a (:ref:`hdmi`) specification and the
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extensions added to CEC version 2.0 are defined in chapter 11 of the
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HDMI 2.0 (:ref:`hdmi2`) specification.
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The bitrate is very slow (effectively no more than 36 bytes per second)
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and is based on the ancient AV.link protocol used in old SCART
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connectors. The protocol closely resembles a crazy Rube Goldberg
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contraption and is an unholy mix of low and high level messages. Some
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messages, especially those part of the HEAC protocol layered on top of
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CEC, need to be handled by the kernel, others can be handled either by
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the kernel or by userspace.
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In addition, CEC can be implemented in HDMI receivers, transmitters and
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in USB devices that have an HDMI input and an HDMI output and that
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control just the CEC pin.
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Drivers that support CEC will create a CEC device node (/dev/cecX) to
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give userspace access to the CEC adapter. The
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:ref:`CEC_ADAP_G_CAPS` ioctl will tell userspace what it is allowed to do.
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In order to check the support and test it, it is suggested to download
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the `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_ package. It
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provides three tools to handle CEC:
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- cec-ctl: the Swiss army knife of CEC. Allows you to configure, transmit
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and monitor CEC messages.
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- cec-compliance: does a CEC compliance test of a remote CEC device to
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determine how compliant the CEC implementation is.
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- cec-follower: emulates a CEC follower.
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