| .. _cec-intro: |
| |
| Introduction |
| ============ |
| |
| .. note:: |
| |
| This documents the proposed CEC API. This API is not yet finalized |
| and is currently only available as a staging kernel module. |
| |
| 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. |
