[Bug 101998] New: X server 1.19.3 failure with amdgpu (radeon M295X) on Ubuntu HWE16.04 kernel 4.10.0-28

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101998

            Bug ID: 101998
           Summary: X server 1.19.3 failure with amdgpu (radeon M295X) on
                    Ubuntu HWE16.04 kernel 4.10.0-28
           Product: xorg
           Version: 7.7 (2012.06)
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: blocker
          Priority: medium
         Component: Driver/AMDgpu
          Assignee: xorg-driver-ati at lists.x.org
          Reporter: tech at meikneubauer.de
        QA Contact: xorg-team at lists.x.org

Created attachment 133168
  --> https://bugs.freedesktop.org/attachment.cgi?id=133168&action=edit
log file showing segmentation fault

Note: The problem only occurs if an external monitor is attached!

Kernel initialization is ok. Seeing boot msgs on all ext. monitors driven by
AMD.

Subsequent Greeter load failures are accompanied by these messages:

Aug  1 13:24:59 mnmaster kernel: [   40.455942]
[drm:amdgpu_atombios_dp_link_train [amdgpu]] *ERROR* channel eq failed: 5 tries
Aug  1 13:24:59 mnmaster kernel: [   40.455954]
[drm:amdgpu_atombios_dp_link_train [amdgpu]] *ERROR* channel eq failed

Greeter is then stable after a few failures and login is possible.
Xorg.0.log shows segmentation fault:

[   119.868] randr: falling back to unsynchronized pixmap sharing
[   120.087] (EE) 
[   120.087] (EE) Backtrace:
[   120.087] (EE) 0: /usr/lib/xorg/Xorg (xorg_backtrace+0x4e) [0x55cb016cbf7e]
[   120.087] (EE) 1: /usr/lib/xorg/Xorg (0x55cb0151a000+0x1b5ce9)
[0x55cb016cfce9]
[   120.087] (EE) 2: /lib/x86_64-linux-gnu/libpthread.so.0
(0x7f1587a1d000+0x11390) [0x7f1587a2e390]
[   120.087] (EE) 
[   120.087] (EE) Segmentation fault at address 0x0
[   120.087] (EE) 
Fatal server error:
[   120.087] (EE) Caught signal 11 (Segmentation fault). Server aborting


I will add the current Xorg.0.log and also an older Xorg.0.log from April that
used the 4.8.0-36 kernel. Please compare these logs to see that the root cause
seems to be that amdgpu cannot allocate a frame buffer.

I should also mention that kern.log is flooded with these messages (but that
may not be related to the error):
Aug  1 14:44:19 mnmaster kernel: [ 2759.820005] [drm:drm_mode_addfb2 [drm]]
[FB:74]
Aug  1 14:44:19 mnmaster kernel: [ 2759.847678] [drm:drm_mode_addfb2 [drm]]
[FB:77]

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