kern/56562: drm update breaks Sandy Bridge display

[mlelstv%serpens.de@localhost]

>Number:         56562
>Category:       kern
>Synopsis:       drm update breaks Sandy Bridge display
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Dec 20 19:05:00 +0000 2021
>Originator:     Michael van Elst
>Release:        NetBSD 9.99.92
>Organization:
	
>Environment:
	
	
System: NetBSD tazz 9.99.92 NetBSD 9.99.92 (GENERIC) #7: Thu Dec 16 19:44:34 UTC 2021 mlelstv@slowpoke:/scratch2/obj.amd64/scratch/netbsd-current/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

On a Thinkpad T420:

i915drmkms0 at pci0 dev 2 function 0: Intel Sandy Bridge (mobile) GT2+ Integrated Graphics Device (rev. 0x09)

the updated kernel seems to work fine. But after a reboot, the display gets corrupted immediately, starting
with the BIOS messages. The resolution doesn't match the display, colors are wrong and even change and there
are display artefacts that show that the laptop TFT isn't driven correctly.

Example:

http://ftp.netbsd.org/pub/NetBSD/misc/mlelstv/newdrm.jpg

The bar in the lower left is a display artefact that starts dark and "bleeds" to white.

The issue persists if you reboot with a pre-update kernel.

I found two possibilites to recover:

1. interrupt boot process and change the "default boot display" in BIOS. This seems to reset the hardware.
2. Power off and on.

With the next reboot the issue comes back.

>How-To-Repeat:
Boot updated kernel on a Thinkpad T420.

>Fix:
	

>Unformatted: