[Intel-gfx] [PATCH 3/4] drm/i915: peel dma-fence-chains wait fences

[Lionel Landwerlin]

On 10/04/2020 19:51, Venkata Sandeep Dhanalakota wrote:
> From: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>
> To allow faster engine to engine synchronization, peel the layer of
> dma-fence-chain to expose potential i915 fences so that the
> i915-request code can emit HW semaphore wait/signal operations in the
> ring which is faster than waking up the host to submit unblocked
> workloads after interrupt notification.
>
> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> ---
>   .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 39 +++++++++++++++++--
>   1 file changed, 35 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index 8dd651cdca39..e43b76d7e9fd 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -2508,6 +2508,7 @@ await_fence_array(struct i915_execbuffer *eb,
>   
>   	for (n = 0; n < nfences; n++) {
>   		struct drm_syncobj *syncobj;
> +		struct dma_fence_chain *chain;
>   		unsigned int flags;
>   
>   		syncobj = ptr_unpack_bits(fences[n].syncobj, &flags, 2);
> @@ -2515,10 +2516,40 @@ await_fence_array(struct i915_execbuffer *eb,
>   		if (!fences[n].dma_fence)
>   			continue;
>   
> -		err = i915_request_await_dma_fence(eb->request,
> -						   fences[n].dma_fence);
> -		if (err < 0)
> -			return err;
> +		/*
> +		 * If we're dealing with a dma-fence-chain, peel the chain by
> +		 * adding all of the unsignaled fences
> +		 * (dma_fence_chain_for_each does that for us) the chain
> +		 * points to.
> +		 *
> +		 * This enables us to identify waits on i915 fences and allows
> +		 * for faster engine-to-engine synchronization using HW
> +		 * semaphores.
> +		 */
> +		chain = to_dma_fence_chain(fences[n].dma_fence);
> +		if (chain) {
> +			struct dma_fence *iter;
> +
> +			dma_fence_chain_for_each(iter, fences[n].dma_fence) {


The kbuild bot made me think of an interesting case.

It is possible to build a chain where the first element isn't a 
dma_fence_chain.


We should handle this here like this :


if (iter_chain)

     err = i915_request_await_dma_fence(eb->request, iter_chain->fence);

else

     err = i915_request_await_dma_fence(eb->request, iter);

if (err < 0) {

     dma_fence_put(iter);

     return err;

}


> +				struct dma_fence_chain *iter_chain =
> +					to_dma_fence_chain(iter);
> +
> +				GEM_BUG_ON(!iter_chain);
> +
> +				err = i915_request_await_dma_fence(eb->request,
> +								   iter_chain->fence);
> +				if (err < 0) {
> +					dma_fence_put(iter);
> +					return err;
> +				}
> +			}
> +
> +		} else {
> +			err = i915_request_await_dma_fence(eb->request,
> +							   fences[n].dma_fence);
> +			if (err < 0)
> +				return err;
> +		}
>   	}
>   
>   	return 0;