Re: [PATCH] drm/i915/cnl: Avoid ioremap_wc on Cannonlake as well.

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

On Wed, 2017-08-30 at 12:26 +0100, Chris Wilson wrote:
> Quoting Joonas Lahtinen (2017-08-30 12:13:29)
> > On Tue, 2017-08-29 at 16:09 -0700, Rodrigo Vivi wrote:
> > > Driver’s CPU access to GTT is via the GTTMMADR BAR.
> > > 
> > > The current HW implementation of that BAR is to only
> > > support <= DW (and maybe QW) writes—not 16/32/64B writes
> > > that could occur with WC and/or SSE/AVX moves.
> > > 
> > > GTTMMADR must be marked uncacheable (UC).
> > > Accesses to GTTMMADR(GTT), must be 64 bits or less (ie. 1 GTT entry).
> > > 
> > > v2: Get clarification on the reasons and spec is getting
> > >     updated to reflect it now.
> > > 
> > > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > Suggested-by: Ben Widawsky <benjamin.widawsky@intel.com>
> > > Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> > 
> > Rodrigo, can you double-check how this interacts with the patch from
> > Zhi that adds the WB flag to PPAT_CACHE_INDEX on CNL.
> 
> Different issue (or should be). The ioremap concerns access through the
> PCI BAR, affecting how fast we insert entries into the GGTT (so
> establishing new mmaps following frequent runtime pm, loading of new
> contexts + rings, as well as the stressful GGTT thrashing). PPAT affects
> how the device accesses the physical pages, not the PTE themselves.

Yes, I know it should be :) But Rodrigo also described pretty random
hangs, IIRC not much was pinpointing to either of the issues. With
these two bugs present, device could be operating without write-back on
certain pages, or could be operating on wrong pages altogether.

I'd just like one round of testing to try to avoid this change if we
can.

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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