Quoting Wang, Zhi A (2017-08-29 12:13:26)
> Hi Chris:
> There is mapping between i915 cache level -> PPAT index. Currently it's:
>
> static gen8_pte_t gen8_pte_encode(dma_addr_t addr,
> enum i915_cache_level level)
> {
> ...
> switch (level) {
> case I915_CACHE_NONE:
> pte |= PPAT_UNCACHED_INDEX;
> break;
> case I915_CACHE_WT:
> pte |= PPAT_DISPLAY_ELLC_INDEX;
> break;
> default:
> pte |= PPAT_CACHED_INDEX;
> break;
> }
> ...
>
> According to bspec, the PPAT index filled in the page table is calculated as:
>
> PPAT index = 4 * PAT + 2 * PCD + PWT
>
> In the i915_gem_gtt.c
>
> #define PPAT_UNCACHED_INDEX (_PAGE_PWT | _PAGE_PCD) // PPAT INDEX = 1 + 2 * 1 = 3
> #define PPAT_CACHED_PDE_INDEX 0 /* WB LLC */ // PPAT INDEX = 0
> #define PPAT_CACHED_INDEX _PAGE_PAT /* WB LLCeLLC */ // PPAT INDEX = 4 * 1 = 4
> #define PPAT_DISPLAY_ELLC_INDEX _PAGE_PCD /* WT eLLC */ // PPAT INDEX = 2 * 1 = 2
>
> Actually the PPAT index used by i915 are: 0 2 3 4, which has already been reserved in the RFC patches.
So we can use these values in alloc_ppat, right? Still very concerned
about the hole -- it kind of implies there is an entry we should be
using but have forgotten!
> > > @@ -2864,17 +3038,22 @@ static void bdw_setup_private_ppat(struct drm_i915_private *dev_priv)
> > > * So we can still hold onto all our assumptions wrt cpu
> > > * clflushing on LLC machines.
> > > */
> > > - pat = GEN8_PPAT(0, GEN8_PPAT_UC);
> > > + alloc_ppat_entry(ppat, 0, GEN8_PPAT_UC);
> > > + return;
> > > + }
> > >
> > > - /* XXX: spec defines this as 2 distinct registers. It's unclear if a 64b
> > > - * write would work. */
> > > - I915_WRITE(GEN8_PRIVATE_PAT_LO, pat);
> > > - I915_WRITE(GEN8_PRIVATE_PAT_HI, pat >> 32);
> > > + alloc_ppat_entry(ppat, 0, GEN8_PPAT_WB | GEN8_PPAT_LLC); /* for normal objects, no eLLC */
> > > + alloc_ppat_entry(ppat, 2, GEN8_PPAT_WT | GEN8_PPAT_LLCELLC); /* for scanout with eLLC */
> > > + alloc_ppat_entry(ppat, 3, GEN8_PPAT_UC); /* Uncached objects, mostly for scanout */
> > > + alloc_ppat_entry(ppat, 4, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC |
/* See gen8_pte_encode() for the mapping from cache-level to ppat */
alloc_ppage_entry(ppat, PPAT_CACHED_PDE_INDEX, GEN8_PPAT_WB | GEN8_PPAT_LLC);
alloc_ppage_entry(ppat, PPAT_DISPLAY_ELLC_INDEX, GEN8_PPAT_WT | GEN8_PPAT_LLCELLC);
alloc_ppage_entry(ppat, PPAT_UNCACHED_INDEX, GEN8_PPAT_UC);
alloc_ppage_entry(ppat, PPAT_CACHED_INDEX, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC | GEN8_PPAT_AGE(0));
etc.
-Chris
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