[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
b6b6e678 MCC	6	.. note::
	7
	8	This documents the proposed CEC API. This API is not yet finalized
706f8a99	9	and is currently only available as a staging kernel module.
da98a79b MCC	10
	11	HDMI connectors provide a single pin for use by the Consumer Electronics
	12	Control protocol. This protocol allows different devices connected by an
	13	HDMI cable to communicate. The protocol for CEC version 1.4 is defined
	14	in supplements 1 (CEC) and 2 (HEAC or HDMI Ethernet and Audio Return
	15	Channel) of the HDMI 1.4a (:ref:`hdmi`) specification and the
	16	extensions added to CEC version 2.0 are defined in chapter 11 of the
	17	HDMI 2.0 (:ref:`hdmi2`) specification.
	18
	19	The bitrate is very slow (effectively no more than 36 bytes per second)
	20	and is based on the ancient AV.link protocol used in old SCART
	21	connectors. The protocol closely resembles a crazy Rube Goldberg
	22	contraption and is an unholy mix of low and high level messages. Some
	23	messages, especially those part of the HEAC protocol layered on top of
	24	CEC, need to be handled by the kernel, others can be handled either by
	25	the kernel or by userspace.
	26
	27	In addition, CEC can be implemented in HDMI receivers, transmitters and
	28	in USB devices that have an HDMI input and an HDMI output and that
	29	control just the CEC pin.
	30
	31	Drivers that support CEC will create a CEC device node (/dev/cecX) to
	32	give userspace access to the CEC adapter. The
	33	:ref:`CEC_ADAP_G_CAPS` ioctl will tell userspace what it is allowed to do.