On Thu, Jul 27, 2023 at 03:54:57PM +0100, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
>
> On Meteorlake CPU cache will not contain stale data after GPU access since
> write-invalidate protocol is used, which means there is no need to flush
> before potentially transitioning the buffer to a non-coherent domain.
>
> Use the opportunity to documet the situation on discrete too.
>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Matt Roper <matthew.d.roper@xxxxxxxxx>
> Cc: Fei Yang <fei.yang@xxxxxxxxx>
> Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Reviewed-by: Matt Roper <matthew.d.roper@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/gem/i915_gem_domain.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_domain.c b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> index ffddec1d2a76..57db9c581bf6 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> @@ -24,9 +24,22 @@ static bool gpu_write_needs_clflush(struct drm_i915_gem_object *obj)
> {
> struct drm_i915_private *i915 = to_i915(obj->base.dev);
>
> + /*
> + * Discrete GPUs never dirty the CPU cache.
> + */
> if (IS_DGFX(i915))
> return false;
>
> + /*
> + * Cache snooping on Meteorlake is using write-invalidate so GPU writes
> + * never end up in the CPU cache.
> + *
> + * QQQ: Do other snooping platforms behave identicaly and could we
> + * therefore write this as "if !HAS_LLC(i915) && HAS_SNOOP(i915)"?
> + */
> + if (IS_METEORLAKE(i915))
> + return false;
> +
> /*
> * For objects created by userspace through GEM_CREATE with pat_index
> * set by set_pat extension, i915_gem_object_has_cache_level() will
> --
> 2.39.2
>
--
Matt Roper
Graphics Software Engineer
Linux GPU Platform Enablement
Intel Corporation
