Ok, running some tests.

First thing is that with fKMS overlay, right on boot (even before the
kernel started) I can see these messages on the serial console:

RPi: BOOTLOADER release VERSION:0b7b6f28 DATE: 2024/04/17 TIME: 13:51:36
BOOTMODE: 0x06 partition 0 build-ts BUILD_TIMESTAMP=1713358296 serial a0940f46 
boardrev b03111 stc
(lots of lines removed)
Boot mode: USB-MSD (04) order e
HDMI0 edid block 0 offset 0
00ffffffffffff0049f7000000000000
011a0103800000000ad7a5a2594a9624
145054afce0081c00101010101010101
0101010101017f2156aa51001e30468f
33003f432100001ea91a00a050001630
3020370020560000001a000000fd0032
4c1e5010000a202020202020000000fc
004141410a20202020202020200a016f
HDMI0 edid block 1 offset 128
020323744f109f140504131216111503
0702060123090f038301000066030c00
100080011d00bc52d01e20b8285540c4
8e2100001e011d80d0721c1620102c25
80c48e2100009e8c0ad08a20e02d1010
3e9600138e210000188c0ad090204031
200c405500138e210000180000000000
0000000000000000000000000000009d
HDMI0: best-mode 1 (limit 1) 1280x720 60 Hz CEA modes 
fe007f80000000000000000000000000 extensions 1
USB2[1] 400202e1 connected


If I switch to the KMS overlay, the message is exactly the same, so it means 
the EDID is accessible from the monitor in either case. I compared both outputs 
and they are the same.

But with KMS, as expected, the monitor went blank again by the end of
the boot messages.

Another interesting thing is that I *could* dump edid even with the KMS
overlay right after boot, once. When I tried again, it did not work
anymore, saying it was empty:

ubuntu@ubuntu:~$ edid-decode /sys/class/drm/card0-HDMI-A-1/edid 
edid-decode (hex):

00 ff ff ff ff ff ff 00 49 f7 00 00 00 00 00 00
01 1a 01 03 80 00 00 00 0a d7 a5 a2 59 4a 96 24
14 50 54 af ce 00 81 c0 01 01 01 01 01 01 01 01
01 01 01 01 01 01 7f 21 56 aa 51 00 1e 30 46 8f
33 00 3f 43 21 00 00 1e a9 1a 00 a0 50 00 16 30
30 20 37 00 20 56 00 00 00 1a 00 00 00 fd 00 32
4c 1e 50 10 00 0a 20 20 20 20 20 20 00 00 00 fc
00 41 41 41 0a 20 20 20 20 20 20 20 20 0a 01 6f

02 03 23 74 4f 10 9f 14 05 04 13 12 16 11 15 03
07 02 06 01 23 09 0f 03 83 01 00 00 66 03 0c 00
10 00 80 01 1d 00 bc 52 d0 1e 20 b8 28 55 40 c4
8e 21 00 00 1e 01 1d 80 d0 72 1c 16 20 10 2c 25
80 c4 8e 21 00 00 9e 8c 0a d0 8a 20 e0 2d 10 10
3e 96 00 13 8e 21 00 00 18 8c 0a d0 90 20 40 31
20 0c 40 55 00 13 8e 21 00 00 18 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 9d

----------------

Block 0, Base EDID:
  EDID Structure Version & Revision: 1.3
  Vendor & Product Identification:
    Manufacturer: ROW
    Model: 0
    Made in: week 1 of 2016
  Basic Display Parameters & Features:
    Digital display
    Image size is variable
    Gamma: 1.00
    RGB color display
    First detailed timing is the preferred timing
  Color Characteristics:
    Red  : 0.6357, 0.3486
    Green: 0.2900, 0.5888
    Blue : 0.1425, 0.0800
    White: 0.3134, 0.3291
  Established Timings I & II:
    IBM     :   720x400    70.081663 Hz   9:5     31.467 kHz     28.320000 MHz
    DMT 0x04:   640x480    59.940476 Hz   4:3     31.469 kHz     25.175000 MHz
    DMT 0x05:   640x480    72.808802 Hz   4:3     37.861 kHz     31.500000 MHz
    DMT 0x06:   640x480    75.000000 Hz   4:3     37.500 kHz     31.500000 MHz
    DMT 0x08:   800x600    56.250000 Hz   4:3     35.156 kHz     36.000000 MHz
    DMT 0x09:   800x600    60.316541 Hz   4:3     37.879 kHz     40.000000 MHz
    DMT 0x0a:   800x600    72.187572 Hz   4:3     48.077 kHz     50.000000 MHz
    DMT 0x0b:   800x600    75.000000 Hz   4:3     46.875 kHz     49.500000 MHz
    DMT 0x10:  1024x768    60.003840 Hz   4:3     48.363 kHz     65.000000 MHz
    DMT 0x11:  1024x768    70.069359 Hz   4:3     56.476 kHz     75.000000 MHz
    DMT 0x12:  1024x768    75.028582 Hz   4:3     60.023 kHz     78.750000 MHz
  Standard Timings:
    DMT 0x55:  1280x720    60.000000 Hz  16:9     45.000 kHz     74.250000 MHz
  Detailed Timing Descriptors:
    DTD 1:  1366x768    59.964364 Hz 683:384   47.852 kHz     85.750000 MHz 
(575 mm x 323 mm)
                 Hfront   70 Hsync 143 Hback  213 Hpol P
                 Vfront    3 Vsync   3 Vback   24 Vpol P
    DTD 2:  1280x768    59.994726 Hz   5:3     47.396 kHz     68.250000 MHz (32 
mm x 86 mm)
                 Hfront   48 Hsync  32 Hback   80 Hpol P
                 Vfront    3 Vsync   7 Vback   12 Vpol N
    Display Range Limits:
      Monitor ranges (GTF): 50-76 Hz V, 30-80 kHz H, max dotclock 160 MHz
    Display Product Name: 'AAA'
  Extension blocks: 1
Checksum: 0x6f

----------------

Block 1, CTA-861 Extension Block:
  Revision: 3
  Basic audio support
  Supports YCbCr 4:4:4
  Supports YCbCr 4:2:2
  Native detailed modes: 4
  Video Data Block:
    VIC  16:  1920x1080   60.000000 Hz  16:9     67.500 kHz    148.500000 MHz
    VIC  31:  1920x1080   50.000000 Hz  16:9     56.250 kHz    148.500000 MHz 
(native)
    VIC  20:  1920x1080i  50.000000 Hz  16:9     28.125 kHz     74.250000 MHz
    VIC   5:  1920x1080i  60.000000 Hz  16:9     33.750 kHz     74.250000 MHz
    VIC   4:  1280x720    60.000000 Hz  16:9     45.000 kHz     74.250000 MHz
    VIC  19:  1280x720    50.000000 Hz  16:9     37.500 kHz     74.250000 MHz
    VIC  18:   720x576    50.000000 Hz  16:9     31.250 kHz     27.000000 MHz
    VIC  22:  1440x576i   50.000000 Hz  16:9     15.625 kHz     27.000000 MHz
    VIC  17:   720x576    50.000000 Hz   4:3     31.250 kHz     27.000000 MHz
    VIC  21:  1440x576i   50.000000 Hz   4:3     15.625 kHz     27.000000 MHz
    VIC   3:   720x480    59.940060 Hz  16:9     31.469 kHz     27.000000 MHz
    VIC   7:  1440x480i   59.940060 Hz  16:9     15.734 kHz     27.000000 MHz
    VIC   2:   720x480    59.940060 Hz   4:3     31.469 kHz     27.000000 MHz
    VIC   6:  1440x480i   59.940060 Hz   4:3     15.734 kHz     27.000000 MHz
    VIC   1:   640x480    59.940476 Hz   4:3     31.469 kHz     25.175000 MHz
  Audio Data Block:
    Linear PCM:

ubuntu@ubuntu:~$ edid-decode /sys/class/drm/card0-HDMI-A-1/edid 
EDID of '/sys/class/drm/card0-HDMI-A-1/edid' was empty.

This seems very odd.

I'm still gonna test force_hotplug and overriding edid.

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https://bugs.launchpad.net/bugs/2064370

Title:
  Blank screen after boot in raspberry pi 4

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