[deliverable/linux.git] / Documentation / media / uapi / cec / cec-intro.rst

.. _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.
Commit	Line	Data
da98a79b MCC	1	.. _cec-intro:
	2
	3	Introduction
	4	============
	5
706f8a99 MCC	6	.. note:: This documents the proposed CEC API. This API is not yet finalized
706f8a99 MCC	7	and is currently only available as a staging kernel module.
da98a79b MCC	8
	9	HDMI connectors provide a single pin for use by the Consumer Electronics
	10	Control protocol. This protocol allows different devices connected by an
	11	HDMI cable to communicate. The protocol for CEC version 1.4 is defined
	12	in supplements 1 (CEC) and 2 (HEAC or HDMI Ethernet and Audio Return
	13	Channel) of the HDMI 1.4a (:ref:`hdmi`) specification and the
	14	extensions added to CEC version 2.0 are defined in chapter 11 of the
	15	HDMI 2.0 (:ref:`hdmi2`) specification.
	16
	17	The bitrate is very slow (effectively no more than 36 bytes per second)
	18	and is based on the ancient AV.link protocol used in old SCART
	19	connectors. The protocol closely resembles a crazy Rube Goldberg
	20	contraption and is an unholy mix of low and high level messages. Some
	21	messages, especially those part of the HEAC protocol layered on top of
	22	CEC, need to be handled by the kernel, others can be handled either by
	23	the kernel or by userspace.
	24
	25	In addition, CEC can be implemented in HDMI receivers, transmitters and
	26	in USB devices that have an HDMI input and an HDMI output and that
	27	control just the CEC pin.
	28
	29	Drivers that support CEC will create a CEC device node (/dev/cecX) to
	30	give userspace access to the CEC adapter. The
	31	:ref:`CEC_ADAP_G_CAPS` ioctl will tell userspace what it is allowed to do.