[Intel-gfx] [5.10.y regression] i915 clear-residuals mitigation is causing gfx issues

[Chris Wilson]

Quoting Hans de Goede (2021-02-10 10:37:19)
> Hi,
> 
> On 2/10/21 12:07 AM, Chris Wilson wrote:
> > Quoting Hans de Goede (2021-02-09 11:46:46)
> >> Hi,
> >>
> >> On 2/9/21 12:27 AM, Chris Wilson wrote:
> >>> Quoting Hans de Goede (2021-02-08 20:38:58)
> >>>> Hi All,
> >>>>
> >>>> We (Fedora) have been receiving reports from multiple users about gfx issues / glitches
> >>>> stating with 5.10.9. All reporters are users of Ivy Bridge / Haswell iGPUs and all
> >>>> reporters report that adding i915.mitigations=off to the cmdline fixes things, see:
> >>>
> >>> I tried to reproduce this on the w/e on hsw-gt1, to no avail; and piglit
> >>> did not report any differences with and without mitigations. I have yet
> >>> to test other platforms. So I don't yet have an alternative.
> >>
> >> Note the original / first reporter of:
> >>
> >> https://bugzilla.redhat.com/show_bug.cgi?id=1925346
> >>
> >> Is using hsw-gt2, so it seems that the problem is not just the enabling of
> >> the mitigations on ivy-bridge / bay-trail but that there actually is
> >> a regression on devices where the WA worked fine before...
> > 
> > There have been 3 crashes uploaded related to v5.10.9, and in all 3
> > cases the ACTHD has been in the first page. This strongly suggests that
> > the w/a is scribbling over address 0. And there's then a very good
> > chance that
> > 
> > commit 29d35b73ead4e41aa0d1a954c9bfbdce659ec5d6
> > Author: Chris Wilson <chris at chris-wilson.co.uk>
> > Date:   Mon Jan 25 12:50:33 2021 +0000
> > 
> >     drm/i915/gt: Always try to reserve GGTT address 0x0
> >     
> >     commit 489140b5ba2e7cc4b853c29e0591895ddb462a82 upstream.
> > 
> > in v5.10.14 is sufficient to hide the issue.
> 
> That one actually is already in v5.10.13 and the various reportes of these
> issues have already tested 5.10.13. They did mention that it took longer
> to reproduce with 5.10.13 then with 5.10.10, but that could also be due to:
> 
> "drm/i915/gt: Clear CACHE_MODE prior to clearing residuals"
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.10.y&id=520d05a77b2866eb4cb9e548e1d8c8abcfe60ec5

Started looking for scratch page overwrites, and found this little gem:
https://patchwork.freedesktop.org/patch/420436/?series=86947&rev=1

Looks promising wrt the cause of overwriting random addresses -- and
I hope that is the explanation for the glitches/hangs. I have a hsw gt2
with gnome shell, piglit is happy, but I suspect it is all due to
placement and so will only occur at random.
-Chris