But to get it permanent, I had to enhance /boot/config-user.txt by
hdmi_force_hotplug=1
Of course there is the HDMI wiring pi3–>Denon AVR–>Hisense TV, and my pi3 is running 24/7 while the other devices are in standby. In theory even standby devices should provide their HDMI capabilities. But from my experience this additional option is mandatory.
My first try BTW failed in pretending on 1080p@50Hz by
hdmi_group=1
hdmi_mode=31
Those settings (from original raspberry documentation) fail to enable that mode (error message [ 20.319942] vc4-drm soc:gpu: [drm] Cannot find any crtc or sizes).
When I did my last (successful) test during boot, AVR and TV were both running. But I might sometimes boot my raspberry even if both HDMI devices are in standby. Then I might try to enforce the usage of an edid.dat file - I saved the currently working one by
cat /sys/devices/platform/soc/soc:gpu/drm/card0/card0-HDMI-A-1/edid > edid.dat
Not tested till now, and hopefully maybe not necessary.
Regards, Michael
PS: Sadly only enabling hdmi_force_hotplug was not enough. Now I’ve stored the above edid.dat into /boot and enabled hdmi_edid_file. To my surprise there is no message within dmesg reporting succsessful loading /boot/edid.dat - hope the best:-)