Documentation/admin-guide/media/cec.rst - linux - Git at Google

 .. 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-gpio@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-gpio@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``.
	.. 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-gpio@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-gpio@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``.