5.13 i915/PAT regression on Brasswell, adding nopat to the kernel commandline worksaround this

[Hans de Goede]

Hi All,

I'm not sure if this is a i915 bug, or caused by changes elsewhere in the kernel,
so I thought it would be best to just send out an email and then see from there.

With 5.13-rc1 gdm fails to show and dmesg contains:

[   38.504613] x86/PAT: Xwayland:683 map pfn RAM range req write-combining for [mem 0x23883000-0x23883fff], got write-back
<repeated lots of times for different ranges>
[   39.484766] x86/PAT: gnome-shell:632 map pfn RAM range req write-combining for [mem 0x1c6a3000-0x1c6a3fff], got write-back
<repeated lots of times for different ranges>
[   54.314858] Asynchronous wait on fence 0000:00:02.0:gnome-shell[632]:a timed out (hint:intel_cursor_plane_create [i915])
[   58.339769] i915 0000:00:02.0: [drm] GPU HANG: ecode 8:1:86dfdffb, in gnome-shell [632]
[   58.341161] i915 0000:00:02.0: [drm] Resetting rcs0 for stopped heartbeat on rcs0
[   58.341267] i915 0000:00:02.0: [drm] gnome-shell[632] context reset due to GPU hang

Because of the PAT errors I tried adding "nopat" to the kernel commandline
and I'm happy to report that that works around this.

Any hints on how to debug this further (without doing a full git bisect) would be
appreciated.

Regards,

Hans