[PATCH v3 2/2] drm/i915: Don't use BAR mappings for ring buffers with LLC

[Ceraolo Spurio, Daniele]

On 2/15/2023 5:11 PM, John.C.Harrison at Intel.com wrote:
> From: John Harrison <John.C.Harrison at Intel.com>
>
> Direction from hardware is that ring buffers should never be mapped
> via the BAR on systems with LLC. There are too many caching pitfalls
> due to the way BAR accesses are routed. So it is safest to just not
> use it.
>
> Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> Fixes: 9d80841ea4c9 ("drm/i915: Allow ringbuffers to be bound anywhere")
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Cc: intel-gfx at lists.freedesktop.org
> Cc: <stable at vger.kernel.org> # v4.9+

I've double-checked the original patches to make sure the 4.9 fixes tag 
was correct for both (which dim confirmed), because if we backport this 
fix without the previous one then the driver would break. Also, the 
original patches were merged as part of the same series 
(https://patchwork.freedesktop.org/series/11278/), so we should be 
guaranteed that they're always there as a pair.

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>

Daniele

> ---
>   drivers/gpu/drm/i915/gt/intel_ring.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_ring.c b/drivers/gpu/drm/i915/gt/intel_ring.c
> index fb1d2595392ed..fb99143be98e7 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ring.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ring.c
> @@ -53,7 +53,7 @@ int intel_ring_pin(struct intel_ring *ring, struct i915_gem_ww_ctx *ww)
>   	if (unlikely(ret))
>   		goto err_unpin;
>   
> -	if (i915_vma_is_map_and_fenceable(vma)) {
> +	if (i915_vma_is_map_and_fenceable(vma) && !HAS_LLC(vma->vm->i915)) {
>   		addr = (void __force *)i915_vma_pin_iomap(vma);
>   	} else {
>   		int type = i915_coherent_map_type(vma->vm->i915, vma->obj, false);
> @@ -98,7 +98,7 @@ void intel_ring_unpin(struct intel_ring *ring)
>   		return;
>   
>   	i915_vma_unset_ggtt_write(vma);
> -	if (i915_vma_is_map_and_fenceable(vma))
> +	if (i915_vma_is_map_and_fenceable(vma) && !HAS_LLC(vma->vm->i915))
>   		i915_vma_unpin_iomap(vma);
>   	else
>   		i915_gem_object_unpin_map(vma->obj);