[RFC PATCH 150/162] drm/i915: need consider system BO snoop for dgfx

[Chris Wilson]

Quoting Matthew Auld (2020-11-27 12:07:06)
> From: CQ Tang <cq.tang at intel.com>
> 
> When cache_level is NONE, we check HAS_LLC(i915).
> But additionally for DGFX, we also need to check
> HAS_SNOOP(i915) on system memory object to use
> I915_BO_CACHE_COHERENT_FOR_READ. on dg1, has_llc=0, and
> has_snoop=1. Otherwise, we set obj->cache_choerent=0 and
> have performance impact.
> 
> Cc: Chris P Wilson <chris.p.wilson at intel.com>
> Cc: Ramalingam C <ramalingam.c at intel.com>
> Cc: Sudeep Dutt <sudeep.dutt at intel.com>
> Cc: Matthew Auld <matthew.auld at intel.com>
> Signed-off-by: CQ Tang <cq.tang at intel.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_object.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> index ddb448f275eb..be603171c444 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> @@ -95,6 +95,20 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
>         mutex_init(&obj->mm.get_dma_page.lock);
>  }
>  
> +static bool i915_gem_object_use_llc(struct drm_i915_gem_object *obj)
> +{
> +       struct drm_i915_private *i915 = to_i915(obj->base.dev);
> +
> +       if (HAS_LLC(i915))
> +               return true;
> +
> +       if (IS_DGFX(i915) && HAS_SNOOP(i915) &&
> +           !i915_gem_object_is_lmem(obj))
> +               return true;
> +
> +       return false;
> +}
> +
>  /**
>   * Mark up the object's coherency levels for a given cache_level
>   * @obj: #drm_i915_gem_object
> @@ -108,7 +122,7 @@ void i915_gem_object_set_cache_coherency(struct drm_i915_gem_object *obj,
>         if (cache_level != I915_CACHE_NONE)
>                 obj->cache_coherent = (I915_BO_CACHE_COHERENT_FOR_READ |
>                                        I915_BO_CACHE_COHERENT_FOR_WRITE);
> -       else if (HAS_LLC(to_i915(obj->base.dev)))
> +       else if (i915_gem_object_use_llc(obj))
>                 obj->cache_coherent = I915_BO_CACHE_COHERENT_FOR_READ;
>         else
>                 obj->cache_coherent = 0;

You must also define obj->cache_level correctly. You can not just assume
the object will be snooped.
-Chris
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