| .. _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. |