[Intel-gfx] [PATCH] drm/i915: Don't use stolen memory for ring buffers

[Ceraolo Spurio, Daniele]

On 2/14/2023 3:48 PM, John.C.Harrison at Intel.com wrote:
> From: John Harrison <John.C.Harrison at Intel.com>
>
> Direction from hardware is that stolen memory should never be used for
> ring buffer allocations. There are too many caching pitfalls due to the
> way stolen memory accesses are routed. So it is safest to just not use
> it.

I'm wondering if this applies to machines in ringbuffer mode as well, as 
some of the caching stuff that according to the HW team may not work 
properly with stolen mem accesses from the CS (mocs, ppat) came with 
gen8/gen9.
Maybe limit this change to gen8+, to avoid changing the behavior for 
very old platforms?

>
> Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_ring.c | 2 --
>   1 file changed, 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_ring.c b/drivers/gpu/drm/i915/gt/intel_ring.c
> index 15ec64d881c44..d1a47e1ae6452 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ring.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ring.c
> @@ -116,8 +116,6 @@ static struct i915_vma *create_ring_vma(struct i915_ggtt *ggtt, int size)
>   
>   	obj = i915_gem_object_create_lmem(i915, size, I915_BO_ALLOC_VOLATILE |
>   					  I915_BO_ALLOC_PM_VOLATILE);
> -	if (IS_ERR(obj) && i915_ggtt_has_aperture(ggtt))
> -		obj = i915_gem_object_create_stolen(i915, size);

There is code in ring_pin/unpin() that only applies to rings in stolen 
memory, so you need to remove that as well if you drop stolen for rings 
on all platforms.

Daniele

>   	if (IS_ERR(obj))
>   		obj = i915_gem_object_create_internal(i915, size);
>   	if (IS_ERR(obj))