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

[Christian König]

Am 30.01.19 um 11:55 schrieb Chris Wilson:
> 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).
>
> v2: reservation_object is already locked by amdgpu_bo_reserve()
> v3: Replace looping with get_fences_rcu and special case the promotion
> of a single shared fence directly to an exclusive fence, bypassing the
> fence array.
> v4: Drop the fence array ref after assigning to reservation_object
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107341
> Testcase: igt/amd_prime/amd-to-i915
> References: 8e94a46c1770 ("drm/amdgpu: Attach exclusive fence to prime exported bo's. (v5)")
> 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>
> Reviewed-by: "Christian König" <christian.koenig at amd.com>
> ---
> We may disagree on the best long term strategy for fence semantics, but
> I think this is still a nice short term solution to the blocking
> behaviour on exporting amdgpu to prime.

Yeah, I can agree on that. And just pushed the patch to 
amd-staging-drm-next.

Christian.

> -Chris
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c | 59 ++++++++++++++++++++---
>   1 file changed, 51 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
> index 71913a18d142..a38e0fb4a6fe 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
> @@ -38,6 +38,7 @@
>   #include "amdgpu_gem.h"
>   #include <drm/amdgpu_drm.h>
>   #include <linux/dma-buf.h>
> +#include <linux/dma-fence-array.h>
>   
>   /**
>    * amdgpu_gem_prime_get_sg_table - &drm_driver.gem_prime_get_sg_table
> @@ -187,6 +188,48 @@ amdgpu_gem_prime_import_sg_table(struct drm_device *dev,
>   	return ERR_PTR(ret);
>   }
>   
> +static int
> +__reservation_object_make_exclusive(struct reservation_object *obj)
> +{
> +	struct dma_fence **fences;
> +	unsigned int count;
> +	int r;
> +
> +	if (!reservation_object_get_list(obj)) /* no shared fences to convert */
> +		return 0;
> +
> +	r = reservation_object_get_fences_rcu(obj, NULL, &count, &fences);
> +	if (r)
> +		return r;
> +
> +	if (count == 0) {
> +		/* Now that was unexpected. */
> +	} else if (count == 1) {
> +		reservation_object_add_excl_fence(obj, fences[0]);
> +		dma_fence_put(fences[0]);
> +		kfree(fences);
> +	} else {
> +		struct dma_fence_array *array;
> +
> +		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);
> +		dma_fence_put(&array->base);
> +	}
> +
> +	return 0;
> +
> +err_fences_put:
> +	while (count--)
> +		dma_fence_put(fences[count]);
> +	kfree(fences);
> +	return -ENOMEM;
> +}
> +
>   /**
>    * amdgpu_gem_map_attach - &dma_buf_ops.attach implementation
>    * @dma_buf: Shared DMA buffer
> @@ -218,16 +261,16 @@ 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 reservation 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);
> +		r = __reservation_object_make_exclusive(bo->tbo.resv);
> +		if (r)
>   			goto error_unreserve;
> -		}
>   	}
>   
>   	/* pin buffer into GTT */