To switch on and off a TV for example you could use cec.

cec-ctl fails to open /dev/cec0, because there isn't one.

I am using the 6.1.16-v7l-sarpi4 kernel.

locate cec.ko returns
/lib/modules/6.1.16-v7l-sarpi4/kernel/drivers/media/cec/core/cec.ko.xz

modprobe cec doesn't help

Under Raspberry Pi OS Bullseye you can find /dev/cec0

Is it a kernel problem? Should I replace Sarpi's kernel by the one from Bullseye?

would be nice to post exactly what did you try and what's happened. "doesn't help" is not really helpful.
for me kodi on RPi Bullseye works perfectly (including CEC), but with an older kernel.
It also depends on the TV, sometimes it connects its CEC interface to another device (like STB).

"doesn't help" -> it loaded the cec module but still no /dev/cec0

For RPi Bullseye I compiled kernel 6.1 and Kodi runs perfectly with CEC on the same television.

I used rpi-update to update the kernel to version 6.1.19-v7l+

Still no /dev/cec0

you need to check the logs if there was any related message somewhere.

I checked dmesg for the words "cec" and "CEC". No hit.

On https://forums.raspberrypi.com/viewtopic.php?t=327247#p1974181 I found to use dtoverlay=vc4-kms-v3d in config.txt instead of dtoverlay=vc4-fkms-v3d

After adjusting config.txt and a reboot:
ls /dev/cec*
/dev/cec0 /dev/cec1

cec-ctl -d/dev/cec1 --test-power-cycle | head -n 2 gives
Adapter is unconfigured, please configure it first.
cec-ctl 1.22.1

How to configure?

According to man page of cec-ctl:
cec-ctl -d/dev/cec1 --tv -S

TV will be switched on and off in a cycle by
cec-ctl -d/dev/cec1 --test-power-cycle

Remark: Unfortunately cec-ctl -d/dev/cec1 -m does not monitor any key presses from the remote control of the television

Tried rasberry pi´s kernel 6.1.19 and had issues when trying to compile large packages like llvm and chromium-ungoogled.
That kernel does not seam to read changes done in config.txt.

So i reverted kernel to 5.15.88

I could not compile Kodi and Libreoffice due to a kernel panic with deadlocked memory. I appreciate you are sharing you also had problems with the latest kernel.

It does read config.txt -> I replaced Sarpi's 6.1.16 kernel by using rpi-update, because I thought it was the reason I could not find /dev/cecX.

I also will replace it by a 5.15 kernel.