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

Submitted by Bab..@..il.com

Assigned to Default DRI bug account

Link to original bug (#112008)

Description

Created attachment 145745
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]

Attachment 145745, "kernel5.3-drm.debug=0xe":
kernel5.3-drm.debug_0xe