[REGRESSION] Black screen on HDMI power-cycle after commit 3471b9a31ce3 (7900XTX + LG C3)

[Mikhail Gavrilov]

Hello,

I am reporting a regression in the AMD display driver where the HDMI
output fails to wake up (black screen) specifically after the TV is
power-cycled.

#regzbot introduced: 3471b9a31ce352ffb343cf02a991261880aac3a7

System information:
GPU: AMD Radeon RX 7900 XTX
Display: LG OLED42C3RLA
Kernel version: 6.18.0-37bb2e7217b0 (and later)

Detailed description:
The HDMI signal is lost every time the TV is power-cycled. When the TV
is turned off for the night and turned back on in the morning, the
display shows "No Signal". This has been tested with physically
disconnecting the cable (hotplug) as well as power-cycling the TV;
both result in a persistent black screen. The system remains
accessible via SSH, but the GPU fails to establish a signal with the
display.

Bisect results:
I have bisected this issue and identified the following commit as the cause:
3471b9a31ce352ffb343cf02a991261880aac3a7 "drm/amd/display: Rework HDMI
data channel reads"

Verification:
I successfully verified this by manually reverting the mentioned
commit on top of 37bb2e7217b0. With the revert applied, the HDMI
signal correctly resumes when the TV is powered on. It seems the new
scdc_present check in the rework is causing the driver to skip
necessary HDMI channel initialization on the 7900XTX/LG C3 combo.

Attached files:
dmesg-6.18.0-37bb2e7217b0.tar.xz: Kernel log during the failed wake-up attempt.
.config.tar.xz: The kernel .config used for the build.

I am ready to test any proposed patches

-- 
Best Regards,
Mike Gavrilov.

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