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.
cec-gpio and Allwinner A10 (or any other driver that uses the CEC pin framework to drive the CEC pin directly): the CEC pin framework uses high-resolution timers. These timers are affected by NTP daemons that speed up or slow down the clock to sync with the official time. The chronyd server will by default increase or decrease the clock by 1/12th. This will cause the CEC timings to go out of spec. To fix this, add a ‘maxslewrate 40000’ line to chronyd.conf. This limits the clock frequency change to 1/25th, which keeps the CEC timings within spec.

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 4B and some cheap components you can make your own low-level CEC debugger.

The critical component is one 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

The video quality is variable and certainly not enough to pass-through 4kp60 (594 MHz) video. You might be able to support 4kp30, but more likely you will be limited to 1080p60 (148.5 MHz). But for CEC testing that is fine.

You need 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

If you want to monitor the HPD and/or 5V lines as well, then you need one of these 5V to 3.3V 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 ground pin of the HDMI connector needs to be connected to a ground pin of the Raspberry Pi, of course.

The CEC pin of the HDMI connector needs to be connected to these pins: GPIO 6 and GPIO 7. The optional HPD pin of the HDMI connector should be connected via the level shifter to these pins: GPIO 23 and GPIO 12. The optional 5V pin of the HDMI connector should be connected via the level shifter to these pins: GPIO 25 and GPIO 22. Monitoring the HPD and 5V lines is not necessary, but it is helpful.

This device tree addition in arch/arm/boot/dts/bcm2711-rpi-4-b.dts will hook up the cec-gpio driver correctly:

cec@6 {
        compatible = "cec-gpio";
        cec-gpios = <&gpio 6 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
        hpd-gpios = <&gpio 23 GPIO_ACTIVE_HIGH>;
        v5-gpios = <&gpio 25 GPIO_ACTIVE_HIGH>;
};

cec@7 {
        compatible = "cec-gpio";
        cec-gpios = <&gpio 7 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
        hpd-gpios = <&gpio 12 GPIO_ACTIVE_HIGH>;
        v5-gpios = <&gpio 22 GPIO_ACTIVE_HIGH>;
};

If you haven’t hooked up the HPD and/or 5V lines, then just delete those lines.

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.

If you just want to monitor traffic, then a single instance is sufficient. The minimum configuration is one HDMI female-female passthrough connector and two female-female breadboard wires: one for connecting the HDMI ground pin to a ground pin on the Raspberry Pi, and the other to connect the HDMI CEC pin to GPIO 6 on the Raspberry Pi.

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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