Found the exact scenario to reproduce my problem:
When it works:
1. We have a laptop completely turned off. First we plug-in external monitor
and then we turn it on.
2. Boot and try to get EDID - it works.
How to make it fail:
3. Suspend my laptop. And unplug external monitor.
4. Wake it up. Adjust laptop screen resolution via System->Preferences->Display.
5. Suspend it again. Plug external monitor.
6. Wake it up. Try to get EDID - failure, it doesn't reads CRT edid anymore
(but still reads LVDS edid).
r...@my-laptop:/home/me# i2cdetect "intel drm CRTDDC_A"
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-0.
I will probe address range 0x03-0x77.
Continue? [Y/n]
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
r...@my-laptop:/home/me# i2cdetect "intel drm LVDSDDC_C"
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1.
I will probe address range 0x03-0x77.
Continue? [Y/n]
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: 30 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: 50 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
--
[i915gm] Karmic Koala, i915, Unknown monitor (fails to get EDID)
https://bugs.launchpad.net/bugs/422406
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