No video output on 4K HDMI 2.0 TV if booted with no screen attached [Gemini Lake]
On Gemini Lake platforms with an HDMI 2.0 output, the video output on a 4K TV works fine as long as the TV is connected at boot, I get the login prompt on fbdev console at the end of boot. However, if I start with the display cable unplugged, wait for the end of boot and then plug the display I get no video output, the TV stays black and goes to power save after a couple of minutes. This is systematic, it has occurred of all times that I have tried and occurs with two different Gemini Lake platforms (ECS Liva Q2 and Seneca XK-FLX).
- machine x86_64
- kernel 5.6.0-rc1-intel-pk-standard+ (from drm-tip @ cceb0c30a3 2020-02-14)
- Yocto distro
- machine is ECS Liva Q2 (but also occurs with the other Gemini Lake platform I have at hand, a Seneca XK-FLX)
- HDMI 2.0 connector, no LSPCON (HDMI-A-1)
- no graphical environment running, just the fbdev console (with the drm fbdev emulation)
- attaching dmesg output of failure, booted without the display plugged and plugging later at 150s
- dmesg output booted with the display, all correct
- dmidecode
- could not get dump of VBIOS, I get a
cat: '/sys/devices/pci0000:00/0000:00:02.0/rom': Input/output error
and dmesg saysInvalid PCI ROM header signature: expecting 0xaa55, got 0x7d08
I have looked far and wide various logs and the code, but I must admit I have a hard time following the i915 code, so I am at a loss.