[RFC 2/2] drm/i915: Remove PAT hack from i915_gem_object_can_bypass_llc

[Yang, Fei]

> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>
> According to the comment in i915_gem_object_can_bypass_llc the
> purpose of the function is to return false if the platform/object
> has a caching mode where GPU can bypass the LLC.
>
> So far the only platforms which allegedly can do this are Jasperlake
> and Elkhartlake, and that via MOCS (not PAT).
>
> Instead of blindly assuming that objects where userspace has set the
> PAT index can (bypass the LLC), question is is there a such PAT index
> on a platform. Probably starting with Meteorlake since that one is the
> only one where set PAT extension can be currently used. Or if there is
> a MOCS entry which can achieve the same thing on Meteorlake.
>
> If there is such PAT, now that i915 can be made to understand them
> better, we can make the check more fine grained. Or if there is a MOCS
> entry then we probably should apply the blanket IS_METEORLAKE condition.
>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Fixes: 9275277d5324 ("drm/i915: use pat_index instead of cache_level")
> Cc: Chris Wilson <chris.p.wilson at linux.intel.com>
> Cc: Fei Yang <fei.yang at intel.com>
> Cc: Andi Shyti <andi.shyti at linux.intel.com>
> Cc: Matt Roper <matthew.d.roper at intel.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_object.c | 6 ------
>  1 file changed, 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> index 33a1e97d18b3..1e34171c4162 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> @@ -229,12 +229,6 @@ bool i915_gem_object_can_bypass_llc(struct drm_i915_gem_object *obj)
>       if (!(obj->flags & I915_BO_ALLOC_USER))
>               return false;
>
> -     /*
> -      * Always flush cache for UMD objects at creation time.
> -      */
> -     if (obj->pat_set_by_user)

I'm afraid this is going to break MESA. Can we run MESA tests with this patch?

>       /*
>        * EHL and JSL add the 'Bypass LLC' MOCS entry, which should make it
>        * possible for userspace to bypass the GTT caching bits set by the
> --
> 2.39.2