Re: kern/56562: drm update breaks Sandy Bridge display

[mlelstv%serpens.de@localhost (Michael van Elst)]

The following reply was made to PR kern/56562; it has been noted by GNATS.

From: mlelstv%serpens.de@localhost (Michael van Elst)
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: kern/56562: drm update breaks Sandy Bridge display
Date: Tue, 21 Dec 2021 12:09:12 -0000 (UTC)

 mlelstv%serpens.de@localhost writes:
 
 >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,
 
 The hardware gets misconfigured somewhere around that message.
 
 kern info: [drm] Initialized i915 1.6.0 20200114 for i915drmkms0 on minor 0
 
 Entering DDB before that and rebooting (reboot 4) still keeps a sane display.
 Entering DDB after that and rebooting already gets the corrupted display
 after reboot.
 
 Since that event happens after the kernel asks for a root device, I booted
 with 'boot -a' and entered 'reboot'.
 
 This was sufficient to clear the display corruption and the next boot
 went normally.
 
 The same effect has booting the pre-update kernel (with still corrupted
 display) and booting again.
 
 So something seems to put the hardware into a sane state, but this only
 gets effective after next reboot, and initializing drm with the post-update
 kernel corrupts the state again but this also becomes effective after
 a reboot.