[Bug 200645] New: 4.18-rc regression bisected to e03fd3f30: amdgpu polaris11/rx460 only activates on one output/monitor of two

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https://bugzilla.kernel.org/show_bug.cgi?id=200645

            Bug ID: 200645
           Summary: 4.18-rc regression bisected to e03fd3f30: amdgpu
                    polaris11/rx460 only activates on one output/monitor
                    of two
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.18-rc1-4.18-rc6+
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: 1i5t5.duncan@cox.net
        Regression: No

Created attachment 277487
  --> https://bugzilla.kernel.org/attachment.cgi?id=277487&action=edit
dmesg booting with the bad commit

I have a dual-monitor setup connected to my polaris11/rx460, both actually TVs,
an older 48" full-HD 1920x1080 connected to DVI-D-0, and a newer 65" 4k
3840x2160 connected to HDMI-A-0 (outputs as reported by xrandr).

In drmfb mode I want them both at 1920x1080 resolution, so I have this set as
part of the kernel-config built-in commandline to force 1920x1080 on the 4k:
video=HDMI-A-1:1920x1080 (note that here it's HDMI-A-1, while X calls it
HDMI-A-0)

On 4.17.0, both outputs/monitors work well.

On 4.18-rcs, only the 4k monitor (on the HDMI output) works.  The full-HD on
the DVI output displays the BIOS messages, grub, and early-pre-fb-kernel
messages, but gets no signal when the framebuffer loads.  If I start
X/kde/plasma and run xrandr, it lists the output as connected but with no
modes.  If I try to activate it via xrandr, it says there's no modes.

Bisecting gets me this:

commit e03fd3f300f6184c1264186a4c815e93bf658abb (refs/bisect/bad)
Author: Mikita Lipski <mikita.lipski@amd.com>
Date:   Wed May 16 16:46:18 2018 -0400

drm/amd/display: Do not limit color depth to 8bpc

Delete if statement that would force any display's color depth higher
than 8 bpc to 8

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 1ce10bc2d..52e57b52c 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -2095,12 +2095,6 @@ convert_color_depth_from_display_info(const struct
drm_connector *connector)
{
uint32_t bpc = connector->display_info.bpc;
-       /* Limited color depth to 8bit
-        * TODO: Still need to handle deep color
-        */
-       if (bpc > 8)
-               bpc = 8;
-
switch (bpc) {
case 0:
/* Temporary Work around, DRM don't parse color depth for

--- snip ---

So presumably the full-HD on the DVI has the wrong color depth for some reason
and forcing it to 8bpc works, while allowing it to run what it has without that
if breaks it.

dmesg booting the bad commit is attached.

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