[Intel-gfx] [RFCv5 2/2] drm/i915: Introduce private PAT management

[Chris Wilson]

Quoting Wang, Zhi A (2017-08-29 18:54:51)
> Another finding during the re-factoring are:
> 
> a)It looks like that the PPAT_CACHE_INDEX on BDW/SKL is mapped to:
> GEN8_PPAT_WB | GEN8_PPAT_LLCELLC | GEN8_PPAT_AGE(0);
> 
> But the PPAT_CACHE_INDEX on CNL is mapped to 
> GEN8_PPAT_LLCELLC | GEN8_PPAT_AGE(0);
> 
> GEN8_PPAT_WB is missing here and by default the cache attribute is UC.
> 
> Is this set intentionally?

That sounds like a nasty little bug.

> b) Looks like all the ages of PPAT in windows driver is AGE(3) because of some performance gains, is there any reason that i915 has to set it to AGE(0)?

Nope, it's never been rigorously tested. On occasion, we've swapped it
around (at least for the older gen) and never found a significant
difference; I haven't even heard if anyone has tried such experiments
on gen8+. Off the top of my head, the age should only matter when you
have PTE with different ages (unless there's some automatic clock
algorithm tracking the age on each page in a shadow, the challenge
being then when you decide to refresh the age from the PTE.)
-Chris