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- .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
- .. _cec-intro:
- Introduction
- ============
- HDMI connectors provide a single pin for use by the Consumer Electronics
- Control protocol. This protocol allows different devices connected by an
- HDMI cable to communicate. The protocol for CEC version 1.4 is defined
- in supplements 1 (CEC) and 2 (HEAC or HDMI Ethernet and Audio Return
- Channel) of the HDMI 1.4a (:ref:`hdmi`) specification and the
- extensions added to CEC version 2.0 are defined in chapter 11 of the
- HDMI 2.0 (:ref:`hdmi2`) specification.
- The bitrate is very slow (effectively no more than 36 bytes per second)
- and is based on the ancient AV.link protocol used in old SCART
- connectors. The protocol closely resembles a crazy Rube Goldberg
- contraption and is an unholy mix of low and high level messages. Some
- messages, especially those part of the HEAC protocol layered on top of
- CEC, need to be handled by the kernel, others can be handled either by
- the kernel or by userspace.
- In addition, CEC can be implemented in HDMI receivers, transmitters and
- in USB devices that have an HDMI input and an HDMI output and that
- control just the CEC pin.
- Drivers that support CEC will create a CEC device node (/dev/cecX) to
- give userspace access to the CEC adapter. The
- :ref:`CEC_ADAP_G_CAPS` ioctl will tell userspace what it is allowed to do.
- In order to check the support and test it, it is suggested to download
- the `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_ package. It
- provides three tools to handle CEC:
- - cec-ctl: the Swiss army knife of CEC. Allows you to configure, transmit
- and monitor CEC messages.
- - cec-compliance: does a CEC compliance test of a remote CEC device to
- determine how compliant the CEC implementation is.
- - cec-follower: emulates a CEC follower.
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