[Intel-gfx] [PATCH 09/66] drm/i915: Provide a fastpath for waiting on vma bindings

[Tvrtko Ursulin]

On 15/07/2020 12:50, Chris Wilson wrote:
> Before we can execute a request, we must wait for all of its vma to be
> bound. This is a frequent operation for which we can optimise away a
> few atomic operations (notably a cmpxchg) in lieu of the RCU protection.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_active.h | 15 +++++++++++++++
>   drivers/gpu/drm/i915/i915_vma.c    |  9 +++++++--
>   2 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h
> index b9e0394e2975..fb165d3f01cf 100644
> --- a/drivers/gpu/drm/i915/i915_active.h
> +++ b/drivers/gpu/drm/i915/i915_active.h
> @@ -231,4 +231,19 @@ struct i915_active *i915_active_create(void);
>   struct i915_active *i915_active_get(struct i915_active *ref);
>   void i915_active_put(struct i915_active *ref);
>   
> +static inline int __i915_request_await_exclusive(struct i915_request *rq,
> +						 struct i915_active *active)
> +{
> +	struct dma_fence *fence;
> +	int err = 0;
> +
> +	fence = i915_active_fence_get(&active->excl);
> +	if (fence) {
> +		err = i915_request_await_dma_fence(rq, fence);
> +		dma_fence_put(fence);
> +	}
> +
> +	return err;
> +}
> +
>   #endif /* _I915_ACTIVE_H_ */
> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> index bc64f773dcdb..cd12047c7791 100644
> --- a/drivers/gpu/drm/i915/i915_vma.c
> +++ b/drivers/gpu/drm/i915/i915_vma.c
> @@ -1167,6 +1167,12 @@ void i915_vma_revoke_mmap(struct i915_vma *vma)
>   		list_del(&vma->obj->userfault_link);
>   }
>   
> +static int
> +__i915_request_await_bind(struct i915_request *rq, struct i915_vma *vma)
> +{
> +	return __i915_request_await_exclusive(rq, &vma->active);
> +}
> +
>   int __i915_vma_move_to_active(struct i915_vma *vma, struct i915_request *rq)
>   {
>   	int err;
> @@ -1174,8 +1180,7 @@ int __i915_vma_move_to_active(struct i915_vma *vma, struct i915_request *rq)
>   	GEM_BUG_ON(!i915_vma_is_pinned(vma));
>   
>   	/* Wait for the vma to be bound before we start! */
> -	err = i915_request_await_active(rq, &vma->active,
> -					I915_ACTIVE_AWAIT_EXCL);
> +	err = __i915_request_await_bind(rq, vma);
>   	if (err)
>   		return err;
>   
> 

Looks like for like, apart from missing i915_active_acquire_if_busy 
across the operation. Remind me please what is acquire/release 
protecting against? :)

Regards,

Tvrtko