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 (HDMI) specification and the extensions added to CEC version 2.0 are defined in chapter 11 of the HDMI 2.0 (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 ioctl 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 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.