4. HDMI CEC¶

4.1. 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.

4.2. 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.

4.3. 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.

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.

4.4. 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.

4.5. 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:

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.
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.

4.6. 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.

4.7. 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: CEC Pin Framework Error Injection.

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.

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