[Bug 199917] New: Stack dump with amdgpu.dc=1 the instant FreeSync is enabled in the connected display.

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=199917

            Bug ID: 199917
           Summary: Stack dump with amdgpu.dc=1 the instant FreeSync is
                    enabled in the connected display.
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.17.0-041700-generic
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri at kernel-bugs.osdl.org
          Reporter: nicholas.johnson at outlook.com.au
        Regression: No

Created attachment 276315
  --> https://bugzilla.kernel.org/attachment.cgi?id=276315&action=edit
dmesg output and the error starts on the line with "Jun 4 13:26" in it

My hardware:

Dell XPS 9370 (i7-8650U, 16GB RAM) - although should be reproducible on any
system with two Thunderbolt 3 ports.
Gigabyte Aorus Thunderbolt Gaming Box external graphics with AMD Radeon R9 Nano
installed - should be reproducible with any R9 Fury (Fiji) series.
VBIOS is UEFI GOP, laptop BIOS up to date
Monitor is Samsung U32E850R with FreeSync support - although any FreeSync
monitor should be sufficient.
4.17 kernel enables amdgpu.dc=1 by default, but on previous kernels, you have
to pass it explicitly.

There is a separate bug with getting the GPU posted when on Thunderbolt - ATOM
BIOS loop. However, trying again with the other Thunderbolt port on the
computer than the one with which you first attempted, works.

Once approved, I can mess around with Xorg conf (seriously, why does Xorg have
to be so difficult?) and get the lightdm / Cinnamon running on the external GPU
with Intel graphics ignored. This is with FreeSync disabled in the monitor
settings. No issues, no errors in dmesg. However, when I enable FreeSync in the
monitor menu, when the video signal returns, dmesg has spewed a barrage of
errors, as attached in the file (starting from the line containing "Jun 4
13:26" - search without quotes. That line was when I first enabled FreeSync.

Passing amdgpu.dc=0 eliminates this particular problem. There are a lot more
problems for AMD to fix, though.

It is unknown of yet if this error is specific to Thunderbolt only, or if it
also happens with the same Radeon GPU installed inside of a desktop. Either
way, the AMDGPU DC is not ready for prime time, and should not have been
enabled by default until tested properly.

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