I am currently using a Pi CM3 on a custom PCB.
The board boots fine with a desktop using fake KMS, but when using full KMS, no image is present. Boot image appears but once at desktop there is no image. The system appears to be running as I can ssh into it.
Testing the composite video output from the Pi reveals that that is active so it appears that the Pi is not detecting the HDMI device after booting the OS.
Digging further into the problem I found that the Pi was unable to read EDID or detect the presence of the monitor. By hacking around with xrandr, it was possible to intermittently get display to work. But even trying this, it was never guaranteed that a display would be detected. An error regarding
crtc unavailable was present instead. Fake KMS has no issue reading EDID.
I'm using a Pi CM3 using the latest, fully upgraded Raspbian, kernel
GPU version 6820edeee4ef3891b95fc01cf02a7abd7ca52f17 (clean) (release) (start_x).
I would like to know if anyone has encountered this before and how it can be fixed. I don't have the same issue with a dedicated Pi 3, so I suspect that it is related to the different IO configuration of the Pi CM3's IO, although I am using the suggested configuration.