[PATCH 23/26] drm/i915: Make request conflict tracking understand parallel submits

[John Harrison]

On 10/4/2021 15:06, Matthew Brost wrote:
> If an object in the excl or shared slot is a composite fence from a
> parallel submit and the current request in the conflict tracking is from
> the same parallel context there is no need to enforce ordering as the
> ordering already implicit. Make the request conflict tracking understand
ordering already -> ordering is already

> this by comparing the parents parallel fence values and skipping the
parents -> parent's

> conflict insertion if the values match.
Presumably, this is to cope with the fact that the parallel submit 
fences do not look like regular submission fences. And hence the 
existing code that says 'new fence belongs to same context as old fence, 
so safe to ignore' does not work with parallel submission. However, this 
change does not appear to be adding parallel submit support to an 
existing 'same context' check. It seems to be a brand new check that 
does not exist for single submission. What makes parallel submit 
different? If we aren't skipping same context fences for single submits, 
why do we need it for parallel? Conversely, if we need it for parallel 
then why don't we need it for single?

And if the single submission version is simply somewhere else in the 
code, why do the parallel version here instead of at the same place?

John.

>
> Signed-off-by: Matthew Brost <matthew.brost at intel.com>
> ---
>   drivers/gpu/drm/i915/i915_request.c | 43 +++++++++++++++++++----------
>   1 file changed, 29 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index e9bfa32f9270..cf89624020ad 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -1325,6 +1325,25 @@ i915_request_await_external(struct i915_request *rq, struct dma_fence *fence)
>   	return err;
>   }
>   
> +static inline bool is_parallel_rq(struct i915_request *rq)
> +{
> +	return intel_context_is_parallel(rq->context);
> +}
> +
> +static inline struct intel_context *request_to_parent(struct i915_request *rq)
> +{
> +	return intel_context_to_parent(rq->context);
> +}
> +
> +static bool is_same_parallel_context(struct i915_request *to,
> +				     struct i915_request *from)
> +{
> +	if (is_parallel_rq(to))
Should this not say '&& is_parallel_rq(from)'?

> +		return request_to_parent(to) == request_to_parent(from);
> +
> +	return false;
> +}
> +
>   int
>   i915_request_await_execution(struct i915_request *rq,
>   			     struct dma_fence *fence)
> @@ -1356,11 +1375,14 @@ i915_request_await_execution(struct i915_request *rq,
>   		 * want to run our callback in all cases.
>   		 */
>   
> -		if (dma_fence_is_i915(fence))
> +		if (dma_fence_is_i915(fence)) {
> +			if (is_same_parallel_context(rq, to_request(fence)))
> +				continue;
>   			ret = __i915_request_await_execution(rq,
>   							     to_request(fence));
> -		else
> +		} else {
>   			ret = i915_request_await_external(rq, fence);
> +		}
>   		if (ret < 0)
>   			return ret;
>   	} while (--nchild);
> @@ -1461,10 +1483,13 @@ i915_request_await_dma_fence(struct i915_request *rq, struct dma_fence *fence)
>   						 fence))
>   			continue;
>   
> -		if (dma_fence_is_i915(fence))
> +		if (dma_fence_is_i915(fence)) {
> +			if (is_same_parallel_context(rq, to_request(fence)))
> +				continue;
>   			ret = i915_request_await_request(rq, to_request(fence));
> -		else
> +		} else {
>   			ret = i915_request_await_external(rq, fence);
> +		}
>   		if (ret < 0)
>   			return ret;
>   
> @@ -1539,16 +1564,6 @@ i915_request_await_object(struct i915_request *to,
>   	return ret;
>   }
>   
> -static inline bool is_parallel_rq(struct i915_request *rq)
> -{
> -	return intel_context_is_parallel(rq->context);
> -}
> -
> -static inline struct intel_context *request_to_parent(struct i915_request *rq)
> -{
> -	return intel_context_to_parent(rq->context);
> -}
> -
>   static struct i915_request *
>   __i915_request_ensure_parallel_ordering(struct i915_request *rq,
>   					struct intel_timeline *timeline)