[PATCH] drm/amdgpu: Transfer fences to dmabuf importer

[Huang Rui]

On Tue, Aug 07, 2018 at 11:45:00AM +0100, Chris Wilson wrote:
> amdgpu only uses shared-fences internally, but dmabuf importers rely on
> implicit write hazard tracking via the reservation_object.fence_excl.
> For example, the importer use the write hazard for timing a page flip to
> only occur after the exporter has finished flushing its write into the
> surface. As such, on exporting a dmabuf, we must either flush all
> outstanding fences (for we do not know which are writes and should have
> been exclusive) or alternatively create a new exclusive fence that is
> the composite of all the existing shared fences, and so will only be
> signaled when all earlier fences are signaled (ensuring that we can not
> be signaled before the completion of any earlier write).
> 
> Testcase: igt/amd_prime/amd-to-i915
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Alex Deucher <alexander.deucher at amd.com>
> Cc: "Christian König" <christian.koenig at amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c | 70 ++++++++++++++++++++---
>  1 file changed, 62 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
> index 1c5d97f4b4dd..47e6ec5510b6 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
> @@ -37,6 +37,7 @@
>  #include "amdgpu_display.h"
>  #include <drm/amdgpu_drm.h>
>  #include <linux/dma-buf.h>
> +#include <linux/dma-fence-array.h>
>  
>  static const struct dma_buf_ops amdgpu_dmabuf_ops;
>  
> @@ -188,6 +189,57 @@ amdgpu_gem_prime_import_sg_table(struct drm_device *dev,
>  	return ERR_PTR(ret);
>  }
>  
> +static int
> +__reservation_object_make_exclusive(struct reservation_object *obj)
> +{

Why not you move the helper to reservation.c, and then export symbol for
this file?

Thanks,
Ray

> +	struct reservation_object_list *fobj;
> +	struct dma_fence_array *array;
> +	struct dma_fence **fences;
> +	unsigned int count, i;
> +
> +	fobj = reservation_object_get_list(obj);
> +	if (!fobj)
> +		return 0;
> +
> +	count = !!rcu_access_pointer(obj->fence_excl);
> +	count += fobj->shared_count;
> +
> +	fences = kmalloc_array(sizeof(*fences), count, GFP_KERNEL);
> +	if (!fences)
> +		return -ENOMEM;
> +
> +	for (i = 0; i < fobj->shared_count; i++) {
> +		struct dma_fence *f =
> +			rcu_dereference_protected(fobj->shared[i],
> +						  reservation_object_held(obj));
> +
> +		fences[i] = dma_fence_get(f);
> +	}
> +
> +	if (rcu_access_pointer(obj->fence_excl)) {
> +		struct dma_fence *f =
> +			rcu_dereference_protected(obj->fence_excl,
> +						  reservation_object_held(obj));
> +
> +		fences[i] = dma_fence_get(f);
> +	}
> +
> +	array = dma_fence_array_create(count, fences,
> +				       dma_fence_context_alloc(1), 0,
> +				       false);
> +	if (!array)
> +		goto err_fences_put;
> +
> +	reservation_object_add_excl_fence(obj, &array->base);
> +	return 0;
> +
> +err_fences_put:
> +	for (i = 0; i < count; i++)
> +		dma_fence_put(fences[i]);
> +	kfree(fences);
> +	return -ENOMEM;
> +}
> +
>  /**
>   * amdgpu_gem_map_attach - &dma_buf_ops.attach implementation
>   * @dma_buf: shared DMA buffer
> @@ -219,16 +271,18 @@ static int amdgpu_gem_map_attach(struct dma_buf *dma_buf,
>  
>  	if (attach->dev->driver != adev->dev->driver) {
>  		/*
> -		 * Wait for all shared fences to complete before we switch to future
> -		 * use of exclusive fence on this prime shared bo.
> +		 * We only create shared fences for internal use, but importers
> +		 * of the dmabuf rely on exclusive fences for implicitly
> +		 * tracking write hazards. As any of the current fences may
> +		 * correspond to a write, we need to convert all existing
> +		 * fences on the resevation object into a single exclusive
> +		 * fence.
>  		 */
> -		r = reservation_object_wait_timeout_rcu(bo->tbo.resv,
> -							true, false,
> -							MAX_SCHEDULE_TIMEOUT);
> -		if (unlikely(r < 0)) {
> -			DRM_DEBUG_PRIME("Fence wait failed: %li\n", r);
> +		reservation_object_lock(bo->tbo.resv, NULL);
> +		r = __reservation_object_make_exclusive(bo->tbo.resv);
> +		reservation_object_unlock(bo->tbo.resv);
> +		if (r)
>  			goto error_unreserve;
> -		}
>  	}
>  
>  	/* pin buffer into GTT */
> -- 
> 2.18.0
> 
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