Re: [PATCH] drm/i915: Disable caches for Global GTT.

[Chris Wilson <chris@chris-wilson.co.uk>]

On Thu, Oct 30, 2014 at 09:18:18AM -0700, Rodrigo Vivi 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"
> 
> Reference: https://bugs.freedesktop.org/show_bug.cgi?id=85576
> Cc: James Ausmus <james.ausmus@intel.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem_gtt.c | 29 +++++++++++++++++++++--------
>  1 file changed, 21 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index cb7adab..24b4f27 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -1911,14 +1911,27 @@ static void bdw_setup_private_ppat(struct drm_i915_private *dev_priv)
>  {
>  	uint64_t pat;
>  
> -	pat = GEN8_PPAT(0, GEN8_PPAT_WB | GEN8_PPAT_LLC)     | /* for normal objects, no eLLC */
> -	      GEN8_PPAT(1, GEN8_PPAT_WC | GEN8_PPAT_LLCELLC) | /* for something pointing to ptes? */
> -	      GEN8_PPAT(2, GEN8_PPAT_WT | GEN8_PPAT_LLCELLC) | /* for scanout with eLLC */
> -	      GEN8_PPAT(3, GEN8_PPAT_UC)                     | /* Uncached objects, mostly for scanout */
> -	      GEN8_PPAT(4, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC | GEN8_PPAT_AGE(0)) |
> -	      GEN8_PPAT(5, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC | GEN8_PPAT_AGE(1)) |
> -	      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));

Instead of deleting this and making a rather ugly if,

Just
> +	if (!USES_PPGTT(dev_priv))
> +		/* 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);

to override in the unlikely case of disabling ppgtt.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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