[Intel-gfx] [PATCH] drm/i915/cnl: Avoid ioremap_wc on Cannonlake as well.

[Chris Wilson]

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 at linux.intel.com>
> > Suggested-by: Ben Widawsky <benjamin.widawsky at intel.com>
> > Signed-off-by: Rodrigo Vivi <rodrigo.vivi at 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.
-Chris