.. SPDX-License-Identifier: GPL-2.0

========
HDMI CEC
========

Supported hardware in mainline
==============================

HDMI Transmitters:

- Exynos4
- Exynos5
- STIH4xx HDMI CEC
- V4L2 adv7511 (same HW, but a different driver from the drm adv7511)
- stm32
- Allwinner A10 (sun4i)
- Raspberry Pi
- dw-hdmi (Synopsis IP)
- amlogic (meson ao-cec and ao-cec-g12a)
- drm adv7511/adv7533
- omap4
- tegra
- rk3288, rk3399
- tda998x
- DisplayPort CEC-Tunneling-over-AUX on i915, nouveau and amdgpu
- ChromeOS EC CEC
- CEC for SECO boards (UDOO x86).
- Chrontel CH7322


HDMI Receivers:

- adv7604/11/12
- adv7842
- tc358743

USB Dongles (see below for additional information on how to use these
dongles):

- Pulse-Eight: the pulse8-cec driver implements the following module option:
  ``persistent_config``: by default this is off, but when set to 1 the driver
  will store the current settings to the device's internal eeprom and restore
  it the next time the device is connected to the USB port.
- RainShadow Tech. Note: this driver does not support the persistent_config
  module option of the Pulse-Eight driver. The hardware supports it, but I
  have no plans to add this feature. But I accept patches :-)

Miscellaneous:

- vivid: emulates a CEC receiver and CEC transmitter.
  Can be used to test CEC applications without actual CEC hardware.

- cec-gpio. If the CEC pin is hooked up to a GPIO pin then
  you can control the CEC line through this driver. This supports error
  injection as well.


Utilities
=========

Utilities are available here: https://git.linuxtv.org/v4l-utils.git

``utils/cec-ctl``: control a CEC device

``utils/cec-compliance``: test compliance of a remote CEC device

``utils/cec-follower``: emulate a CEC follower device

Note that ``cec-ctl`` has support for the CEC Hospitality Profile as is
used in some hotel displays. See http://www.htng.org.

Note that the libcec library (https://github.com/Pulse-Eight/libcec) supports
the linux CEC framework.

If you want to get the CEC specification, then look at the References of
the HDMI wikipedia page: https://en.wikipedia.org/wiki/HDMI. CEC is part
of the HDMI specification. HDMI 1.3 is freely available (very similar to
HDMI 1.4 w.r.t. CEC) and should be good enough for most things.


DisplayPort to HDMI Adapters with working CEC
=============================================

Background: most adapters do not support the CEC Tunneling feature,
and of those that do many did not actually connect the CEC pin.
Unfortunately, this means that while a CEC device is created, it
is actually all alone in the world and will never be able to see other
CEC devices.

This is a list of known working adapters that have CEC Tunneling AND
that properly connected the CEC pin. If you find adapters that work
but are not in this list, then drop me a note.

To test: hook up your DP-to-HDMI adapter to a CEC capable device
(typically a TV), then run::

	cec-ctl --playback	# Configure the PC as a CEC Playback device
	cec-ctl -S		# Show the CEC topology

The ``cec-ctl -S`` command should show at least two CEC devices,
ourselves and the CEC device you are connected to (i.e. typically the TV).

General note: I have only seen this work with the Parade PS175, PS176 and
PS186 chipsets and the MegaChips 2900. While MegaChips 28x0 claims CEC support,
I have never seen it work.

USB-C to HDMI
-------------

Samsung Multiport Adapter EE-PW700: https://www.samsung.com/ie/support/model/EE-PW700BBEGWW/

Kramer ADC-U31C/HF: https://www.kramerav.com/product/ADC-U31C/HF

Club3D CAC-2504: https://www.club-3d.com/en/detail/2449/usb_3.1_type_c_to_hdmi_2.0_uhd_4k_60hz_active_adapter/

DisplayPort to HDMI
-------------------

Club3D CAC-1080: https://www.club-3d.com/en/detail/2442/displayport_1.4_to_hdmi_2.0b_hdr/

CableCreation (SKU: CD0712): https://www.cablecreation.com/products/active-displayport-to-hdmi-adapter-4k-hdr

HP DisplayPort to HDMI True 4k Adapter (P/N 2JA63AA): https://www.hp.com/us-en/shop/pdp/hp-displayport-to-hdmi-true-4k-adapter

Mini-DisplayPort to HDMI
------------------------

Club3D CAC-1180: https://www.club-3d.com/en/detail/2443/mini_displayport_1.4_to_hdmi_2.0b_hdr/

Note that passive adapters will never work, you need an active adapter.

The Club3D adapters in this list are all MegaChips 2900 based. Other Club3D adapters
are PS176 based and do NOT have the CEC pin hooked up, so only the three Club3D
adapters above are known to work.

I suspect that MegaChips 2900 based designs in general are likely to work
whereas with the PS176 it is more hit-and-miss (mostly miss). The PS186 is
likely to have the CEC pin hooked up, it looks like they changed the reference
design for that chipset.


USB CEC Dongles
===============

These dongles appear as ``/dev/ttyACMX`` devices and need the ``inputattach``
utility to create the ``/dev/cecX`` devices. Support for the Pulse-Eight
has been added to ``inputattach`` 1.6.0. Support for the Rainshadow Tech has
been added to ``inputattach`` 1.6.1.

You also need udev rules to automatically start systemd services::

	SUBSYSTEM=="tty", KERNEL=="ttyACM[0-9]*", ATTRS{idVendor}=="2548", ATTRS{idProduct}=="1002", ACTION=="add", TAG+="systemd", ENV{SYSTEMD_WANTS}+="pulse8-cec-inputattach@%k.service"
	SUBSYSTEM=="tty", KERNEL=="ttyACM[0-9]*", ATTRS{idVendor}=="2548", ATTRS{idProduct}=="1001", ACTION=="add", TAG+="systemd", ENV{SYSTEMD_WANTS}+="pulse8-cec-inputattach@%k.service"
	SUBSYSTEM=="tty", KERNEL=="ttyACM[0-9]*", ATTRS{idVendor}=="04d8", ATTRS{idProduct}=="ff59", ACTION=="add", TAG+="systemd", ENV{SYSTEMD_WANTS}+="rainshadow-cec-inputattach@%k.service"

and these systemd services:

For Pulse-Eight make /lib/systemd/system/pulse8-cec-inputattach@.service::

	[Unit]
	Description=inputattach for pulse8-cec device on %I

	[Service]
	Type=simple
	ExecStart=/usr/bin/inputattach --pulse8-cec /dev/%I

For the RainShadow Tech make /lib/systemd/system/rainshadow-cec-inputattach@.service::

	[Unit]
	Description=inputattach for rainshadow-cec device on %I

	[Service]
	Type=simple
	ExecStart=/usr/bin/inputattach --rainshadow-cec /dev/%I


For proper suspend/resume support create: /lib/systemd/system/restart-cec-inputattach.service::

	[Unit]
	Description=restart inputattach for cec devices
	After=suspend.target

	[Service]
	Type=forking
	ExecStart=/bin/bash -c 'for d in /dev/serial/by-id/usb-Pulse-Eight*; do /usr/bin/inputattach --daemon --pulse8-cec $d; done; for d in /dev/serial/by-id/usb-RainShadow_Tech*; do /usr/bin/inputattach --daemon --rainshadow-cec $d; done'

	[Install]
	WantedBy=suspend.target

And run ``systemctl enable restart-cec-inputattach``.

To automatically set the physical address of the CEC device whenever the
EDID changes, you can use ``cec-ctl`` with the ``-E`` option::

	cec-ctl -E /sys/class/drm/card0-DP-1/edid

This assumes the dongle is connected to the card0-DP-1 output (``xrandr`` will tell
you which output is used) and it will poll for changes to the EDID and update
the Physical Address whenever they occur.

To automatically run this command you can use cron. Edit crontab with
``crontab -e`` and add this line::

	@reboot /usr/local/bin/cec-ctl -E /sys/class/drm/card0-DP-1/edid

This only works for display drivers that expose the EDID in ``/sys/class/drm``,
such as the i915 driver.


CEC Without HPD
===============

Some displays when in standby mode have no HDMI Hotplug Detect signal, but
CEC is still enabled so connected devices can send an <Image View On> CEC
message in order to wake up such displays. Unfortunately, not all CEC
adapters can support this. An example is the Odroid-U3 SBC that has a
level-shifter that is powered off when the HPD signal is low, thus
blocking the CEC pin. Even though the SoC can use CEC without a HPD,
the level-shifter will prevent this from functioning.

There is a CEC capability flag to signal this: ``CEC_CAP_NEEDS_HPD``.
If set, then the hardware cannot wake up displays with this behavior.

Note for CEC application implementers: the <Image View On> message must
be the first message you send, don't send any other messages before.
Certain very bad but unfortunately not uncommon CEC implementations
get very confused if they receive anything else but this message and
they won't wake up.

When writing a driver it can be tricky to test this. There are two
ways to do this:

1) Get a Pulse-Eight USB CEC dongle, connect an HDMI cable from your
   device to the Pulse-Eight, but do not connect the Pulse-Eight to
   the display.

   Now configure the Pulse-Eight dongle::

	cec-ctl -p0.0.0.0 --tv

   and start monitoring::

	sudo cec-ctl -M

   On the device you are testing run::

	cec-ctl --playback

   It should report a physical address of f.f.f.f. Now run this
   command::

	cec-ctl -t0 --image-view-on

   The Pulse-Eight should see the <Image View On> message. If not,
   then something (hardware and/or software) is preventing the CEC
   message from going out.

   To make sure you have the wiring correct just connect the
   Pulse-Eight to a CEC-enabled display and run the same command
   on your device: now there is a HPD, so you should see the command
   arriving at the Pulse-Eight.

2) If you have another linux device supporting CEC without HPD, then
   you can just connect your device to that device. Yes, you can connect
   two HDMI outputs together. You won't have a HPD (which is what we
   want for this test), but the second device can monitor the CEC pin.

   Otherwise use the same commands as in 1.

If CEC messages do not come through when there is no HPD, then you
need to figure out why. Typically it is either a hardware restriction
or the software powers off the CEC core when the HPD goes low. The
first cannot be corrected of course, the second will likely required
driver changes.


Microcontrollers & CEC
======================

We have seen some CEC implementations in displays that use a microcontroller
to sample the bus. This does not have to be a problem, but some implementations
have timing issues. This is hard to discover unless you can hook up a low-level
CEC debugger (see the next section).

You will see cases where the CEC transmitter holds the CEC line high or low for
a longer time than is allowed. For directed messages this is not a problem since
if that happens the message will not be Acked and it will be retransmitted.
For broadcast messages no such mechanism exists.

It's not clear what to do about this. It is probably wise to transmit some
broadcast messages twice to reduce the chance of them being lost. Specifically
<Standby> and <Active Source> are candidates for that.


Making a CEC debugger
=====================

By using a Raspberry Pi 2B/3/4 and some cheap components you can make
your own low-level CEC debugger.

Here is a picture of my setup:

https://hverkuil.home.xs4all.nl/rpi3-cec.jpg

It's a Raspberry Pi 3 together with a breadboard and some breadboard wires:

http://www.dx.com/p/diy-40p-male-to-female-male-to-male-female-to-female-dupont-line-wire-3pcs-356089#.WYLOOXWGN7I

Finally on of these HDMI female-female passthrough connectors (full soldering type 1):

https://elabbay.myshopify.com/collections/camera/products/hdmi-af-af-v1a-hdmi-type-a-female-to-hdmi-type-a-female-pass-through-adapter-breakout-board?variant=45533926147

We've tested this and it works up to 4kp30 (297 MHz). The quality is not high
enough to pass-through 4kp60 (594 MHz).

I also added an RTC and a breakout shield:

https://www.amazon.com/Makerfire%C2%AE-Raspberry-Module-DS1307-Battery/dp/B00ZOXWHK4

https://www.dx.com/p/raspberry-pi-gpio-expansion-board-breadboard-easy-multiplexing-board-one-to-three-with-screw-for-raspberry-pi-2-3-b-b-2729992.html#.YGRCG0MzZ7I

These two are not needed but they make life a bit easier.

If you want to monitor the HPD line as well, then you need one of these
level shifters:

https://www.adafruit.com/product/757

(This is just where I got these components, there are many other places you
can get similar things).

The CEC pin of the HDMI connector needs to be connected to these pins:
CE0/IO8 and CE1/IO7 (pull-up GPIOs). The (optional) HPD pin of the HDMI
connector should be connected (via a level shifter to convert the 5V
to 3.3V) to these pins: IO17 and IO27. The (optional) 5V pin of the HDMI
connector should be connected (via a level shifter) to these pins: IO22
and IO24. Monitoring the HPD an 5V lines is not necessary, but it is helpful.

This kernel patch will hook up the cec-gpio driver correctly to
e.g. ``arch/arm/boot/dts/bcm2837-rpi-3-b-plus.dts``::

	cec@7 {
		compatible = "cec-gpio";
		cec-gpios = <&gpio 7 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
		hpd-gpios = <&gpio 17 GPIO_ACTIVE_HIGH>;
		v5-gpios = <&gpio 22 GPIO_ACTIVE_HIGH>;
	};

	cec@8 {
		compatible = "cec-gpio";
		cec-gpios = <&gpio 8 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
		hpd-gpios = <&gpio 27 GPIO_ACTIVE_HIGH>;
		v5-gpios = <&gpio 24 GPIO_ACTIVE_HIGH>;
	};

This dts change will enable two cec GPIO devices: I typically use one to
send/receive CEC commands and the other to monitor. If you monitor using
an unconfigured CEC adapter then it will use GPIO interrupts which makes
monitoring very accurate.

The documentation on how to use the error injection is here: :ref:`cec_pin_error_inj`.

``cec-ctl --monitor-pin`` will do low-level CEC bus sniffing and analysis.
You can also store the CEC traffic to file using ``--store-pin`` and analyze
it later using ``--analyze-pin``.

You can also use this as a full-fledged CEC device by configuring it
using ``cec-ctl --tv -p0.0.0.0`` or ``cec-ctl --playback -p1.0.0.0``.
