kern/54180: xorg segmentation fault with drm

[dogan%tesmer.org.tr@localhost]

>Number:         54180
>Category:       kern
>Synopsis:       xorg segmentation fault with drm
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu May 09 07:40:00 +0000 2019
>Originator:     Mustafa Dogan
>Release:        NetBSD 8.0_STABLE on amd64
>Organization:
TESMER
>Environment:
NetBSD localhost 8.0_STABLE NetBSD 8.0_STABLE (MuDo) #0: Mon May  6 07:43:31 +03 2019  root%research.net.md@localhost:/root/OBJ/sys/arch/amd64/compile/MuDo amd64
>Description:
After recent src changes (probably dated on 22-23 April 2019) related to drm, xorg starts as expected, but crashes a few minutes with segmentation fault.
I tried several different PCs and laptops (most of them have i915drmkms). My observations are:
- If drm device detected and used at boot time, xorg crashes.
- If PC doesn't have usable drm, vesa driver works well.
- If disabled drm at boot time ("userconf disable i915drmkms" for example) loaded vesa driver and works well.

Last portion of sample output of Xorg.0.log is below:
[    35.742] (II) XINPUT: Adding extended input device "<default keyboard>" (type: KEYBOARD, id 7)
[    42.831] (II) intel(0): EDID vendor "LGD", prod id 699
[    42.831] (II) intel(0): Printing DDC gathered Modelines:
[    42.831] (II) intel(0): Modeline "1366x768"x0.0   71.00  1366 1414 1446 1498  768 771 776 790 +hsync -vsync (47.4 kHz eP)
[    42.831] (II) intel(0): Modeline "1366x768"x0.0   71.00  1366 1414 1446 1833  768 771 776 968 +hsync -vsync (38.7 kHz e)
[  2030.189] (EE) Segmentation fault at address 0x7ee0f7255000
[  2030.190] (EE) 
Fatal server error:
[  2030.191] (EE) Caught signal 11 (Segmentation fault). Server aborting
[  2030.191] (EE) 
[  2030.191] (EE) 
Please consult the The X.Org Foundation support 
         at http://wiki.X.Org
 for help. 
[  2030.192] (EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[  2030.192] (EE) 
[  2030.193] (II) AIGLX: Suspending AIGLX clients for VT switch
[  2030.978] (EE) Server terminated with error (1). Closing log file.

>How-To-Repeat:
Boot NetBSD 8 built with latest source on a pc with i915 (or detected as i915) VGA chipset. Start startx and try to open some programs (I used xfce4, and thunar is enough for trigger this error).
>Fix:
Workaround: 
At boot time:
Go to command prompt before kernel load.
> userconf disable i915drmkms
> boot

After login, you can use xorg as usual.