[Bug 112008] eDP -> Dual Channel LVDS bridge unable to accept any modelines: Corrupt display!

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=112008

            Bug ID: 112008
           Summary: eDP -> Dual Channel LVDS bridge unable to accept any
                    modelines: Corrupt display!
           Product: DRI
           Version: DRI git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: major
          Priority: not set
         Component: DRM/AMDgpu
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: Babblebones at gmail.com

Created attachment 145745
  --> https://bugs.freedesktop.org/attachment.cgi?id=145745&action=edit
kernel5.3-drm.debug=0xe

At my wits end on this bug. My bisecting have proven useless and I wanted to
get some help from you guys. Several reverted regressions reproduce the same
behavior along the way and I have spent hours sifting and booting to see where
it's at.

The short of it is that the monitor in my HP laptop boots to a 640x480 modeline
on mainline kernels newer than 4.17-rc3.
Would not be a colossal problem, as I can just force a working modeline from
userspace, if it didn't mean without the modeset the bridge utterly scrambles
the display by lining up 640x480 as a wrapping 2D length in the very top of my
screen as a jumble of colors.

The issue is GPU independent but is AMD related. Both W7170M and WX7100
affected.
Somewhere after kernel 4.17-rc3 the the rodeo begins.
My dmesg is probably going to be much much more helpful when it comes to
debugging and I will GLADLY test any kernel patches if you need more info &
detailed kernel/ custom dmesg dumps.

Notable stuff in the dump:

[    4.773991] [drm:amdgpu_vm_init [amdgpu]] VM update mode is SDMA
[    4.774149] [drm:drm_client_modeset_probe] 
[    4.774156] [drm:drm_helper_probe_single_connector_modes]
[CONNECTOR:59:eDP-1]
[    4.774159] [drm:drm_helper_probe_single_connector_modes]
[CONNECTOR:59:eDP-1] status updated from unknown to connected
[    4.774268] [drm:update_stream_scaling_settings [amdgpu]] Destination
Rectangle x:0  y:0  width:640  height:480
[    4.774278] [drm:drm_helper_probe_single_connector_modes]
[CONNECTOR:59:eDP-1] probed modes :
[    4.774282] [drm:drm_mode_debug_printmodeline] Modeline "640x480": 60 25175
640 656 752 800 480 490 492 525 0x40 0xa
[    4.774284] [drm:drm_helper_probe_single_connector_modes]
[CONNECTOR:63:DP-1]

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