[Intel-gfx] [PATCH] drm/i915: Disable caches for Global GTT.

[Ausmus, James]

On Fri, Oct 31, 2014 at 5:27 AM, Rodrigo Vivi <rodrigo.vivi at intel.com>
wrote:
>
> Global GTT doesn't have pat_sel[2:0] so it always point to pat_sel = 000;
> So the only way to avoid screen corruptions is setting PAT 0 to Uncached.
>
> MOCS can still be used though. But if userspace is trusting PTE for
> cache selection the safest thing to do is to let caches disabled.
>
> BSpec: "For GGTT, there is NO pat_sel[2:0] from the entry,
> so RTL will always use the value corresponding to pat_sel = 000"
>
> v2: Cleaner patch as suggested by Chris.
>
> Reference: https://bugs.freedesktop.org/show_bug.cgi?id=85576
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: James Ausmus <james.ausmus at intel.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>

Matches my read of BSpec.

Reviewed-by: James Ausmus <james.ausmus at intel.com>


> ---
>  drivers/gpu/drm/i915/i915_gem_gtt.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c
b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index cb7adab..ae568a2 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -1920,6 +1920,15 @@ static void bdw_setup_private_ppat(struct
drm_i915_private *dev_priv)
>               GEN8_PPAT(6, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC |
GEN8_PPAT_AGE(2)) |
>               GEN8_PPAT(7, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC |
GEN8_PPAT_AGE(3));
>
> +       if (!USES_PPGTT(dev_priv->dev))
> +               /* Spec: "For GGTT, there is NO pat_sel[2:0] from the
entry,
> +                * so RTL will always use the value corresponding to
> +                * pat_sel = 000".
> +                * So let's disable cache for GGTT to avoid screen
corruptions.
> +                * MOCS still can be used though.
> +                */
> +               pat = GEN8_PPAT(0, GEN8_PPAT_UC);
> +
>         /* XXX: spec defines this as 2 distinct registers. It's unclear
if a 64b
>          * write would work. */
>         I915_WRITE(GEN8_PRIVATE_PAT, pat);
> --
> 1.9.3
>



--


James Ausmus
Sr. Software Engineer
SSG-OTC ChromeOS Integration
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