HDMI-CEC
High-Definition Multimedia Interface - Consumer Electronics Control is an additional low-speed (50 B/s) bus in the HDMI connection that a "network" of HDMI devices can use to communicate with each other. It allows HDMI devices to notify each other that they should be turning on or off, that the TV has switched input or that a remote control button is being pressed, among other things. In PC setups it is usually encountered in an HTPC (home-theater PC) setup.
For a variety of reasons almost no PC GPU has hardware support for CEC. Video game consoles and set-top boxes usually have to include an external chipset to drive the CEC pin. While there are devices with native CEC support (such as the VideoCore GPU found on a Raspberry Pi), most hardware configurations need additional hardware.
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Features
The main purpose of CEC is to grant a television insight and control over the state of the devices plugged into it. As such, it is split into a dozen of "features" that each target specific use cases, and which devices can opt to support or not based on their role as initiator/follower, their capabilities, as well as user configuration.
The standardized features are:
- One Touch Play
- Lets a device signal that it wishes to immediately become the active source, which can automatically turn on the TV
- Routing Control
- Allows the TV to control HDMI switches and lets devices check what source is currently active
- Remote Control Passthrough
- Lets devices send remote control signals to each other, usually from the TV to the active source
- Deck Control
- To control a movie/music player and query its playback status
- Standby/System Standby
- Lets a device request that another specific device be turned off, or broadcast that all devices on the system should now turn off.
- Power Status
- Lets devices be probed to see if they are in standby mode or turned on, or if they are in the process of turning on.
- System Audio Control
- Grants control of an AV receiver connected over the TV's Audio Return Channel, allowing the volume to be changed and the receiver to be turned on or off.
- Tuner Control
- Lets any device step through the list of TV channels known to a tuner device and query information on the active channel, like the channel number for analog TV or DVB/ATSC/ARIB transport stream information for digital TV
- One Touch Record
- Enables a recorder to query what channel the TV is currently showing so that said recorder can tune to the same channel and begin recording, or know that it should record itself or a downstream device if it already is the currently active HDMI source
- Timer Programming
- Allows a TV to configure a timer on a recorder to start recording a given source at a specific time
- OSD display
- Allows a device to print a message on the TV, between 1 and 13 ASCII characters long
- Dynamic Auto Lipsync
- Allows a TV to broadcast changes in presentation latency to the the audio sink, which a source that has its own speakers (like a PC) can use for latency compensation with the image
For a device like a PC, the most useful one among these is going to be Remote Control Passthrough. System Standby may be useful for HTPCs, but would be of questionable use on more general-purpose machines, which are not usually expected to go to sleep when the screen turns off. Routing Control could be used to wake up the system when the the TV attempts to display that input, provided the connected PC has a way to listen to CEC traffic while suspended. System Audio Control would be convenient for some HDMI sound outputs, but does not currently work as a mean of volume control with either PipeWire or PulseAudio.
Hardware setup
The Linux Kernel already has a built-in subsystem to automatically respond to queries and handle CEC events, but the hardware may need to be configured first in order to work.
Native CEC
Native CEC is mostly encountered in ARM devices. In x86 world the easiest option is tunneling over DisplayPort, otherwise only some Chrome OS devices and SECO UDOO single-board computers offer CEC.
Tunneling over DisplayPort
This works with i915, nouveau and amdgpu drivers. The DisplayPort 1.3 standard (introduced in 2014) allows DisplayPort-to-HDMI adapters to use the auxiliary channel to forward CEC signals both ways. This is the sort of feature that adapters do not usually support unless mentioned, and is not commonly found, but it can counter-intuitively be cheaper and easier to use CEC tunneling over DisplayPort than a USB-CEC adapter. The kernel documentation page for the CEC submodule has a list of adapters which have been confirmed to work.
CEC adapter
PulseEight USB adapter
The PulseEight USB-CEC adapter works by passively extending all the pins of the HDMI connector on from the "PC side" connector to the "TV side" connector, save for the CEC pin, which is intercepted. The data going through that pin is instead exposed over a USB serial interface to let a PC control and monitor CEC traffic. The serial device needs to have its line discipline (a flag to signal to the kernel that a TTY is of a specific known type and requires a driver to work) configured manually before the kernel takes over and acknowledges it as a CEC adapter. This cannot be done automatically due to limitations around serial device APIs, so it is currently best achieved with a udev rule paired with a systemd unit (as udev rules cannot launch long-running or forking processes) to run inputattach --pulse8-cec ... when the device is plugged.
This serial interface appears as device node /dev/ttyACMX, and the inputattach utility is needed to set the line discipline and let the kernel drivers take over to create the /dev/cecX device that will be needed later. This requires the linuxconsole package.
/etc/udev/rules.d/pulse8-cec-autoattach.rules
SUBSYSTEM=="tty" ACTION=="add" ATTRS{manufacturer}=="Pulse-Eight" ATTRS{product}=="CEC Adapter" TAG+="systemd" ENV{SYSTEMD_WANTS}="pulse8-cec-attach@$devnode.service"
/etc/systemd/system/pulse8-cec-attach@.service
[Unit] # Should be called as "pulse8-cec-attach@-dev-ttyACM0.service" or similar Description=Configure USB Pulse-Eight serial device at %I ConditionPathExists=%I [Service] Type=forking # inputattach is built without systemd daemon support by default, so systemd will have to guess the PID. # https://sourceforge.net/p/linuxconsole/code/ci/a3366c0d5f82485e6aae7b005ec7a2d9a93bf458/tree/utils/inputattach.c#l1233 ExecStart=/usr/bin/inputattach --daemon --pulse8-cec %I
However, USB device connections are usually reset when the system wakes up from sleep (a step known as reset-resume) , meaning the serial connection will be lost if the computer is ever suspended, on top of serial connections usually hanging up on resume anyway. This means the above rule has to be triggered again somehow.
Unfortunately, the cdc_acm driver in charge of the ttyACM* object that the above rule reacts to does not raise any uevent about the connection being reset and the line discipline being lost, and the rule cannot be hooked on the USB device directly. Instead, the most reliable way to get the used rule above to trigger again at the right time is to delete and recreate the ttyACM* object by forcing the USB device to be reconfigured when it resets. In order to react to this and ensure the the connection is reopened, udev can keep track of when the USB device is reset and enumerated, as evidenced by the DEVNUM property being zeroed and later restored, and touching the bConfigurationValue sysfs attribute.
/etc/udev/rules.d/pulse8-cec-autoattach.rules
SUBSYSTEM=="tty" ACTION=="add" ATTRS{manufacturer}=="Pulse-Eight" ATTRS{product}=="CEC Adapter" TAG+="systemd" ENV{SYSTEMD_WANTS}="pulse8-cec-attach@$devnode.service"
# Force device to be reconfigured when reset after suspend, otherwise the ttyACM link is lost but udev will not notice.
# A usb_dev_uevent with DEVNUM=000 is a sign that the device is being reset before enumeration.
# Re-configuring causes ttyACM to be removed and re-added instead.
SUBSYSTEM=="usb" ACTION=="change" ATTR{manufacturer}=="Pulse-Eight" ATTR{product}=="CEC Adapter" ENV{DEVNUM}=="000" ATTR{bConfigurationValue}=="1" ATTR{bConfigurationValue}="1"
This essentially acts as if the USB adapter had been unplugged and re-plugged immediately upon coming out of sleep, ensuring the SUBSYSTEM=="tty" ACTION=="add" rule from before gets to run again. This ensures that the systemd service will be restarted as soon as the device is back to a usable state.
Software setup
CEC subsystem configuration
Now that the CEC subsystem has something to bind on and that /dev/cec0 has been created, it is now possible to configure the PC so other CEC devices know about it. When using the command-line, CEC devices are normally controlled via cec-ctl, which is part of v4l-utils.
One thing to be aware of is that the CEC pin alone does not have enough information on its own to send a valid CEC message. A CEC adapter that only monitors pin 13 (CEC) cannot know its "physical address" (its position in terms of port numbers in the "tree" of HDMI devices, such as 3.1.0.0), which it needs to be aware of in order to complete the logical address allocation procedure. Without a logical address, a device can only receive and send broadcast messages. The physical address is communicated over pin 16 (DDC/EDID), so configuring the CEC subsystem includes specifying which display output port is supposed to be associated with with that CEC object, in order for the physical address to be extracted from the display's EDID.
One way to find the name of the active connectors is to use xrandr --query (which also works on Wayland):
$ xrandr --query
Screen 0: minimum 16 x 16, current 3840 x 2160, maximum 32767 x 32767 DP-1 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 600mm x 340mm 3840x2160 59.98*+ 2048x1536 59.95 ... HDMI-A-1 connected 3840x2160+0+0 (normal left inverted right x axis y axis) 1440mm x 810mm 3840x2160 59.98*+ 2048x1536 59.95 ...
Once the correct port has been identified (for example HDMI-A-1), then the sysfs port name can be found by using ls -1d /sys/class/drm/card*-HDMI-A-1 (such as card1-HDMI-A-1). In this case, the corresponding display's EDID data would be kept at /sys/class/drm/card1-HDMI-A-1/edid.
The physical address can be previewed like this:
$ edid-decode --physical-address /sys/class/drm/card1-DP-3/edid
4.0.0.0
Given how CEC configuration must be performed every time the cec device node is re-created, this is best handled with another udev rule that fires when the cec object appears.
/etc/udev/rules.d/cec-configure-autostart.rules
SUBSYSTEM=="cec" KERNEL=="cec0" ACTION=="add" TAG+="systemd" ENV{SYSTEMD_WANTS}="cec0-configure@card1-HDMI-A-1.service"
/etc/systemd/system/cec0-configure@.service
[Unit] # Should be called as "cec0-configure@card1-HDMI-A-1.service" or similar Description=Configure CEC adapter cec0 assuming it runs on output %i AssertPathExists=/sys/class/drm/%i/edid BindsTo=dev-cec0.device [Service] Type=exec # --phys-addr-from-edid-poll checks EDID every tenth of a second # https://git.linuxtv.org/v4l-utils.git/tree/utils/cec-ctl/cec-ctl.cpp?id=0a195181d771090f3c99d4a6ddb8151352509061#n1977 # Use `Type=oneshot` if using `--phys-addr-from-edid` instead ExecStart=/usr/bin/cec-ctl --device=0 "--osd-name=%H" --playback "--phys-addr-from-edid-poll=/sys/class/drm/%i/edid"
There are three device classes that an HDMI source can try to advertise itself as, which are "Recording device" (3 max), "Tuner" (4 max) and "Playback device" (3 max). This is important because HDMI devices do not use their physical address when communicating with each other over CEC, but a 4 bit "logical address", which identify devices as "Tuner #3" or "Playback Device #1", with a finite number of each. If address allocation fails because too many devices of one type are present, it may be assigned the "Backup" (2 max) role instead. These roles are intended to relate to the CEC features mentioned earlier, namely:
- "Tuner" is supposed to support "Tuner Control"
- "Recording device" is the only type that can use One Touch Record, as TVs are supposed to ignore related messages coming from other addresses
- "Playback device" is for general purpose video sources. Computers, like video game consoles, are considered "Playback devices".
The above cec0-configure@.service unit uses --playback to configure a Playback device. It is, however, generally OK to set the device class to Tuner (--tuner) or Recorder (--record), whether because there are no more unused playback addresses, or simply to have the PC stand out in the list on TVs that visually set apart each device class in their input menus.
Input-handling daemons
Userspace tools
User access to /dev/cec* devices can be granted by enrolling users into the video user group. The basic tool for controlling CEC devices is cec-ctl from v4l-utils. A similar one is cec-client from libcec, for which there are also Python bindings available in python-cecAUR.
See also
- Kernel documentation for the CEC subsystem
- CEC (Consumer Electronics Control) over HDMI, in Embedded Linux Wiki (eLinux.org)
- HDMI CEC: What? Why? How?, by Hans Verkuil, who wrote most of the CEC subsystem
- CEC-O-Matic, which allows to create raw CEC messages and gives an overview of what makes up a valid CEC frame