1*4882a593Smuzhiyun.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2*4882a593Smuzhiyun 3*4882a593Smuzhiyun.. _cec-intro: 4*4882a593Smuzhiyun 5*4882a593SmuzhiyunIntroduction 6*4882a593Smuzhiyun============ 7*4882a593Smuzhiyun 8*4882a593SmuzhiyunHDMI connectors provide a single pin for use by the Consumer Electronics 9*4882a593SmuzhiyunControl protocol. This protocol allows different devices connected by an 10*4882a593SmuzhiyunHDMI cable to communicate. The protocol for CEC version 1.4 is defined 11*4882a593Smuzhiyunin supplements 1 (CEC) and 2 (HEAC or HDMI Ethernet and Audio Return 12*4882a593SmuzhiyunChannel) of the HDMI 1.4a (:ref:`hdmi`) specification and the 13*4882a593Smuzhiyunextensions added to CEC version 2.0 are defined in chapter 11 of the 14*4882a593SmuzhiyunHDMI 2.0 (:ref:`hdmi2`) specification. 15*4882a593Smuzhiyun 16*4882a593SmuzhiyunThe bitrate is very slow (effectively no more than 36 bytes per second) 17*4882a593Smuzhiyunand is based on the ancient AV.link protocol used in old SCART 18*4882a593Smuzhiyunconnectors. The protocol closely resembles a crazy Rube Goldberg 19*4882a593Smuzhiyuncontraption and is an unholy mix of low and high level messages. Some 20*4882a593Smuzhiyunmessages, especially those part of the HEAC protocol layered on top of 21*4882a593SmuzhiyunCEC, need to be handled by the kernel, others can be handled either by 22*4882a593Smuzhiyunthe kernel or by userspace. 23*4882a593Smuzhiyun 24*4882a593SmuzhiyunIn addition, CEC can be implemented in HDMI receivers, transmitters and 25*4882a593Smuzhiyunin USB devices that have an HDMI input and an HDMI output and that 26*4882a593Smuzhiyuncontrol just the CEC pin. 27*4882a593Smuzhiyun 28*4882a593SmuzhiyunDrivers that support CEC will create a CEC device node (/dev/cecX) to 29*4882a593Smuzhiyungive userspace access to the CEC adapter. The 30*4882a593Smuzhiyun:ref:`CEC_ADAP_G_CAPS` ioctl will tell userspace what it is allowed to do. 31*4882a593Smuzhiyun 32*4882a593SmuzhiyunIn order to check the support and test it, it is suggested to download 33*4882a593Smuzhiyunthe `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_ package. It 34*4882a593Smuzhiyunprovides three tools to handle CEC: 35*4882a593Smuzhiyun 36*4882a593Smuzhiyun- cec-ctl: the Swiss army knife of CEC. Allows you to configure, transmit 37*4882a593Smuzhiyun and monitor CEC messages. 38*4882a593Smuzhiyun 39*4882a593Smuzhiyun- cec-compliance: does a CEC compliance test of a remote CEC device to 40*4882a593Smuzhiyun determine how compliant the CEC implementation is. 41*4882a593Smuzhiyun 42*4882a593Smuzhiyun- cec-follower: emulates a CEC follower. 43