1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3.. _cec-intro: 4 5Introduction 6============ 7 8HDMI connectors provide a single pin for use by the Consumer Electronics 9Control protocol. This protocol allows different devices connected by an 10HDMI cable to communicate. The protocol for CEC version 1.4 is defined 11in supplements 1 (CEC) and 2 (HEAC or HDMI Ethernet and Audio Return 12Channel) of the HDMI 1.4a (:ref:`hdmi`) specification and the 13extensions added to CEC version 2.0 are defined in chapter 11 of the 14HDMI 2.0 (:ref:`hdmi2`) specification. 15 16The bitrate is very slow (effectively no more than 36 bytes per second) 17and is based on the ancient AV.link protocol used in old SCART 18connectors. The protocol closely resembles a crazy Rube Goldberg 19contraption and is an unholy mix of low and high level messages. Some 20messages, especially those part of the HEAC protocol layered on top of 21CEC, need to be handled by the kernel, others can be handled either by 22the kernel or by userspace. 23 24In addition, CEC can be implemented in HDMI receivers, transmitters and 25in USB devices that have an HDMI input and an HDMI output and that 26control just the CEC pin. 27 28Drivers that support CEC will create a CEC device node (/dev/cecX) to 29give userspace access to the CEC adapter. The 30:ref:`CEC_ADAP_G_CAPS` ioctl will tell userspace what it is allowed to do. 31 32In order to check the support and test it, it is suggested to download 33the `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_ package. It 34provides three tools to handle CEC: 35 36- cec-ctl: the Swiss army knife of CEC. Allows you to configure, transmit 37 and monitor CEC messages. 38 39- cec-compliance: does a CEC compliance test of a remote CEC device to 40 determine how compliant the CEC implementation is. 41 42- cec-follower: emulates a CEC follower. 43