Regression: no display with Vega 56 AMD GPU bisected to commit bfdec2340478

From: Jeremy Cline
Date: Mon Jul 02 2018 - 14:33:49 EST

Hi folks,

A Fedora user, Mikhail (CCed), reported[0] a regression with their Vega
56 AMD GPU introduced by bfdec2340478 ("drm/amd/display: Implement
dm_pp_get_clock_levels_by_type_with_latency"). The symptom is that
nothing is displayed and looking at the logs (attached to the Bugzilla),
I see gnome-shell is failing with "Failed to set CRTC mode 3840x2160:
Invalid argument". The problem persists through 4.18 RC2 so it's not
fixed by 10dd2b865393 ("drm/amd/display: Fix wrong latency assignment
for VEGA clock levels").

I see a couple "if (!dm_pp_get_clock_levels_by_type_with_latency)"
blocks which would no longer be unconditionally run, but I don't know
enough to even guess at the problem.