[Bug 107668] Blank screen since Kernel 4.17 (AMDGPU.DC) on Amd Radeon RX460 and RX380 on HDMI Ultrawidescreen Monitor

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107668

            Bug ID: 107668
           Summary: Blank screen since Kernel 4.17 (AMDGPU.DC) on Amd
                    Radeon RX460 and RX380 on HDMI Ultrawidescreen Monitor
           Product: DRI
           Version: XOrg git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/AMDgpu
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: naheemzaffar at gmail.com

Created attachment 141265
  --> https://bugs.freedesktop.org/attachment.cgi?id=141265&action=edit
Dmesg from boot with Kernel 4.19  prerelease

I have already reported this on Redhat Bugzilla, so if more information is
needed, that may be a good place to look at comments:
https://bugzilla.redhat.com/show_bug.cgi?id=1594488

Since the change to using dc in the kernel I am getting a blank screen with
trying to boot my ultrawidescreen monitor.

As a workaround I can use amdgpu.dc=0, but this hits al earlier big where at
native resolution the display output is garbled (links in Redhat Bugzilla
attach to another big report, youtube video of the same).

I am attaching dmesg output from
4.19.0-0.rc0.git8.1.vanilla.knurd.1.fc28.x86_64 with and without amdgpu.dc=0

I have noticed a couple of entries in the dmesg without the amdgpu.dc=0:

kernel: [drm] add ip block number 4 <dm>

with amdgpu.dc=0 it is:

kernel: [drm] add ip block number 4 <dce_v11_0>

The other ip address block entries are identical.

with amdgpu.dc=0 I get this section:

Aug 22 23:46:22 kernel: [drm] PCIE GART of 256M enabled (table at
0x000000F400900000).
Aug 22 23:46:22 kernel: [drm] Supports vblank timestamp caching Rev 2
(21.10.2013).
Aug 22 23:46:22 kernel: [drm] Driver supports precise vblank timestamp query.
Aug 22 23:46:22 kernel: [drm] AMDGPU Display Connectors
Aug 22 23:46:22 kernel: [drm] Connector 0:
Aug 22 23:46:22 kernel: [drm]   DP-1
Aug 22 23:46:22 kernel: [drm]   HPD5
Aug 22 23:46:22 kernel: [drm]   DDC: 0x4868 0x4868 0x4869 0x4869 0x486a 0x486a
0x486b 0x486b
Aug 22 23:46:22 kernel: [drm]   Encoders:
Aug 22 23:46:22 kernel: [drm]     DFP1: INTERNAL_UNIPHY1
Aug 22 23:46:22 kernel: [drm] Connector 1:
Aug 22 23:46:22 kernel: [drm]   HDMI-A-1
Aug 22 23:46:22 kernel: [drm]   HPD3
Aug 22 23:46:22 kernel: [drm]   DDC: 0x4874 0x4874 0x4875 0x4875 0x4876 0x4876
0x4877 0x4877
Aug 22 23:46:22 kernel: [drm]   Encoders:
Aug 22 23:46:22 kernel: [drm]     DFP2: INTERNAL_UNIPHY1
Aug 22 23:46:22 kernel: [drm] Connector 2:
Aug 22 23:46:22 kernel: [drm]   DVI-D-1
Aug 22 23:46:22 kernel: [drm]   HPD4
Aug 22 23:46:22 kernel: [drm]   DDC: 0x4878 0x4878 0x4879 0x4879 0x487a 0x487a
0x487b 0x487b
Aug 22 23:46:22 kernel: [drm]   Encoders:
Aug 22 23:46:22 kernel: [drm]     DFP3: INTERNAL_UNIPHY
Aug 22 23:46:22 kernel: [drm] Chained IB support enabled!

but without amdgpu.dc=0 only the first and last line of that section appear.

Later down without the amdgpu.dc=0 I also get the following message:

kernel: [drm] Cannot find any crtc or sizes

This may also be similar to bug
https://bugs.freedesktop.org/show_bug.cgi?id=105880 but that is for laptop
panels on LVDS while this issue is with a discrete RX460 graphics card on HDMI

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