1366x768 only works without fkms

Start by reproducing the display failure on a Raspberry Pi 4 or Compute Module 4 with the AOpen 20CH1Q, using the listed /boot/config.txt settings and KMS or FKMS enabled. Compare the behavior with hdmi_safe=1, the tvservice mode output, and the supplied raspinfo.txt. Done means the monitor displays video during boot with KMS or FKMS enabled without requiring a second monitor or disabling the overlay.

Describe the bug

dtoverlay=vc4-fkms-v3d in /boot/config.txt prevents video on a 1366x768 monitor ( AOpen 20CH1Q ) from working.

Tested on both a Raspberry Pi 4 and a Raspberry Pi compute module 4 (with Waveshare IO Board with POE).

Eventually we determined that commenting out the dtoverlay line in /boot/config.txt resolved the problem (albeit with much slower video without fkms rendering acceleration).

Note: The same problem occurs with kms as with fkms.

In more detail:

When using an AOpen 20CH1Q monitor (native resolution 1366 x 768), no video on boot up.

Managed to get video (640x480) by setting hdmi_safe=1 (aka hdmi_group=1 and hdmi_mode=1) in /boot/config.txt.

No other settings of hdmi_group/hdmi_mode made video work.

If booted with the HDMI plugged into another monitor (Dell U24), video on the Dell works, and if the HDMI cable is then unplugged from the Dell and plugged into the AOpen, video on the AOpen works.

When booted plugged into the Dell monitor, the tvservice query shows:

pi@pi192:~ $ tvservice --mode=DMT
Group DMT has 9 modes:
           mode 4: 640x480 @ 60Hz 4:3, clock:25MHz progressive
           mode 9: 800x600 @ 60Hz 4:3, clock:40MHz progressive
           mode 16: 1024x768 @ 60Hz 4:3, clock:65MHz progressive
           mode 32: 1280x960 @ 60Hz 4:3, clock:108MHz progressive
           mode 35: 1280x1024 @ 60Hz 5:4, clock:108MHz progressive
           mode 51: 1600x1200 @ 60Hz 4:3, clock:162MHz progressive
           mode 58: 1680x1050 @ 60Hz 16:10, clock:146MHz progressive
  (prefer) mode 68: 1920x1200 @ 60Hz 16:10, clock:154MHz progressive
           mode 82: 1920x1080 @ 60Hz 16:9, clock:148MHz progressive
pi@pi192:~ $ tvservice --mode=CEA
Group CEA has 0 modes:
pi@pi192:~ $

When the following settings were used in /boot/config.txt, and the pi cm4&IO Board, booted with the HDMI plugged into the AOpen monitor, video worked:

a) the dtoverlay line commented out
b) hdmi_group=2
c) hdmi_mode=68

Movie playing was jerky of course, due to the lack of fkms/kms rendering acceleration.

To reproduce

1. Install Raspberry Pi OS into a pi 4 or pi compute module 4 w/IO board
2. Plug the HDMI out from the pi 4 into an AOpen 20CH1Q monitor
3. Boot up the pi.

Expected behaviour

For the video to work.

Actual behaviour

Screen backlight comes on, blinking underline cursor in upper left corner.
Rasperry Pi splash screen displays for a second or two.
Screen goes dark.
Screen lights up again.
Screen goes dark again.
Screen lights up again.
Screen goes dark again.
Screen displays "No Signal" briefly, then goes dark.

System

See attached raspinfo.txt

raspinfo.txt

• Which model of Raspberry Pi? e.g. Pi3B+, PiZeroW

Both pi 4 and pi compute module 4 with a waveshare IO Board with POE.

• Which OS and version (cat /etc/rpi-issue)?

pi@pi192:~ $ cat /etc/rpi-issue 
Raspberry Pi reference 2021-05-07
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, dcfd74d7d1fa293065ac6d565711e9ff891fe2b8, stage5
pi@pi192:~ $ 

• Which firmware version (vcgencmd version)?

pi@pi192:~ $ vcgencmd version
May 27 2021 14:02:15 
Copyright (c) 2012 Broadcom
version 7d9a298cda813f747b51fe17e1e417e7bf5ca94d (clean) (release) (start)
pi@pi192:~ $ 

• Which kernel version (uname -a)?

pi@pi192:~ $ uname -a
Linux pi192.168.66.69 5.10.17-v7l+ #1421 SMP Thu May 27 14:00:13 BST 2021 armv7l GNU/Linux
pi@pi192:~ $ 

Logs

No logs applicable.

Additional context

For completeness, here's the monitor details:

https://www.bestbuy.com/site/acer-aopen-20ch1q-bi-19-5-hd-widescreen-tn-led-lcd-flat-panel-monitor-hdmi/6435025.p?skuId=6435025
Acer - AOPEN 20CH1Q BI 19.5" HD Widescreen TN LED LCD Flat Panel Monitor (HDMI)
Model:20CH1Q BI
SKU:6435025