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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. + +- 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. + + +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 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: :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``. |