[Bug 107390] [BISECTED] EDID read failure breaks display mirroring

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107390

            Bug ID: 107390
           Summary: [BISECTED] EDID read failure breaks display mirroring
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/AMDgpu
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: junaru at gmail.com

Created attachment 140839
  --> https://bugs.freedesktop.org/attachment.cgi?id=140839&action=edit
Boot log showing EDID read failure

Two displays are connected to a POLARIS 10 GPU:

DVI-D-1   BenQ G2420HDBL (monitor)
HDMI-A-1  Philips 55PUS6401 (4k TV)

The displays worked fine in mirror mode on 1920x1080 at 60 until commit
ac916c914c3156e53505e9ea3a9d1495518bf873: see 416.png - gnome display settings
working as expected on mainline 4.16.9 kernel.

As far as i can tell ac916c914c3156e53505e9ea3a9d1495518bf873 introduces 3
issues (listing the later two because they are probably side effects of the
first one):

#### 1st issue ####
ac916c914c3156e53505e9ea3a9d1495518bf873 and later builds only allow desktop to
be extended and not mirrored leaving the impression AMDGPU thinks the displays
have no compatible output modes for mirroring: see 417.png - gnome display
settings on mainline 4.17+ no longer allowing the two displays to be mirrored,
the tabline present at the top of 416.png is missing in 417.png.

Grepping through kernel logs indeed shows AMDGPU failing to read EDID (full
boot log attached):

[drm:dm_logger_write [amdgpu]] *ERROR* No EDID read.

* The error message is present with only the monitor connected.
* The error message is also present with only the TV connected.
This leaves me to believe that EDID is bad on both of my displays and AMDGPU
was tolerating it until now or there might be some issues on AMDGPU's side too.

#### 2nd issue (probably related) ####
If using the "Join Displays" (extended desktop) mode it was previously possible
to have have an extended desktop span 3840x2160 on TV and 1920x1080 on monitor. 

After ac916c914c3156e53505e9ea3a9d1495518bf873 gnome display settings no longer
allows choosing 3840x2160 on TV when monitor is also plugged in. Both displays
are capped to 1920x1080 at 60.
The 4k resolutions return when only TV is connected.

#### 3rd issue (probably related) ####
After ac916c914c3156e53505e9ea3a9d1495518bf873 a third erroneous "Unknown
display" is found and put in 'enabled' state on what appears to be HDMI-A-2.
with following modes:
$ cat /sys/class/drm/card0-HDMI-A-2/modes 
1024x768
800x600
800x600
848x480
640x480

There's nothing connected to HDMI-A-2 physically.

#### SUMMARY ####
Since broken EDID is probably the root cause of all of this i'm attaching both
displays EDID dumps as produced by read-edid 3.0.2
I have no experience in kernel development but would gladly test patches if
anyone has ideas on what could be wrong.
I'll also understand if this will be filed under woun't fix due to display EDID
issues.

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