[Bug 93980] blank screen at 4k at 60Hz DP 1.2

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93980

--- Comment #4 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
(In reply to Alin M Elena from comment #3)
> Created attachment 121489 [details]
> log when live changing from dp1.1 to 1.2 and from 30Hz to60Hz

Your log starts 60 seconds after boot which means it lacks all the interesting
bits from when the driver was loaded. You may need to increase the kernel
log_buf_len= knob to catch it all.

Anyways, there's an underrun after the modeset, and the clock is rather high,
so I suspect this is due to cdclk being set too low. This can also be confirmed
by: cat /sys/kernel/debug/dri/0/i915_frequency_info

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