Re: [PATCH v2] dma-buf: Wait on the reservation object when sync'ing before CPU access

[Daniel Vetter]

On Mon, Aug 15, 2016 at 04:42:18PM +0100, Chris Wilson wrote:
> Rendering operations to the dma-buf are tracked implicitly via the
> reservation_object (dmabuf->resv). This is used to allow poll() to
> wait upon outstanding rendering (or just query the current status of
> rendering). The dma-buf sync ioctl allows userspace to prepare the
> dma-buf for CPU access, which should include waiting upon rendering.
> (Some drivers may need to do more work to ensure that the dma-buf mmap
> is coherent as well as complete.)
> 
> v2: Always wait upon the reservation object implicitly. We choose to do
> it after the native handler in case it can do so more efficiently.
> 
> Testcase: igt/prime_vgem
> Testcase: igt/gem_concurrent_blit # *vgem*
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Eric Anholt <eric@xxxxxxxxxx>
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  drivers/dma-buf/dma-buf.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index ddaee60ae52a..cf04d249a6a4 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -586,6 +586,22 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
>  
> +static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
> +				      enum dma_data_direction direction)
> +{
> +	bool write = (direction == DMA_BIDIRECTIONAL ||
> +		      direction == DMA_TO_DEVICE);
> +	struct reservation_object *resv = dmabuf->resv;
> +	long ret;
> +
> +	/* Wait on any implicit rendering fences */
> +	ret = reservation_object_wait_timeout_rcu(resv, write, true,
> +						  MAX_SCHEDULE_TIMEOUT);
> +	if (ret < 0)
> +		return ret;
> +
> +	return 0;
> +}
>  
>  /**
>   * dma_buf_begin_cpu_access - Must be called before accessing a dma_buf from the
> @@ -608,6 +624,13 @@ int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
>  	if (dmabuf->ops->begin_cpu_access)
>  		ret = dmabuf->ops->begin_cpu_access(dmabuf, direction);
>  
> +	/* Ensure that all fences are waited upon - but we first allow
> +	 * the native handler the chance to do so more efficiently if it
> +	 * chooses. A double invocation here will be reasonably cheap no-op.
> +	 */
> +	if (ret == 0)
> +		ret = __dma_buf_begin_cpu_access(dmabuf, direction);

Not sure we should wait first and the flush or the other way round. But I
don't think it'll matter for any current dma-buf exporter, so meh.

Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

Sumits, can you pls pick this one up and put into drm-misc?
-Daniel

> +
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_begin_cpu_access);
> -- 
> 2.8.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch