[Bug 100440] [GLK] [IGT] testdisplay -a shows corruption on HDMI 4k display

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100440

--- Comment #19 from Ander Conselvan de Oliveira <conselvan2 at gmail.com> ---
(In reply to Luis Botello from comment #18)
> After testing testdisplay on HDMI I am getting "No Signal" on HDMI screen
> while test sets CRTC(36):[7]  2560x1440 60 2560 2608 2640 2720 1440 1443
> 1448 1481 0x9 0x40.

This seems related to HDMI 2.0 high TMDS clock ratio. If I force the driver to
not enable the tmds clock ratio, that mode works.

I tested custom modes based on the failing one but changing the clock. If the
port clock was below 340, then the driver doesn't enable scrambling and the
monitor is able to display image. With port clock between [340,405) I got no
signal. With port clock >= 405, there was image on the screen.

Curiously, if I forced 8 bpc and let scrambling and high tmds clock ratio be
enabled normally, the following mode worked:

./testdisplay -f "362.25,2560,2608,2640,4080,1440,1443,1448,1481" -o 68

I also tested enabling the given mode with only scrambling and only high tmds. 

scrambling      high tmds       result
yes             yes             fail
yes             no              pass    
no              yes             fail
no              no              pass

So this seems to be a combination of 12 bpc, a port clock in the range
[340,405) and high tmds ratios. But as far as I can tell, we are following the
spec with respect to those. Shashank, any ideas?

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