Re: [Linaro-mm-sig] [PATCHv2 5/6] common: DMA-mapping: add DMA_ATTR_SKIP_CPU_SYNC attribute

Daniel Vetter
2012-06-13T18:44:20Z

On Wed, Jun 13, 2012 at 01:50:17PM +0200, Marek Szyprowski wrote:
> This patch adds DMA_ATTR_SKIP_CPU_SYNC attribute to the DMA-mapping
> subsystem.
>
> By default dma_map_{single,page,sg} functions family transfer a given
> buffer from CPU domain to device domain. Some advanced use cases might
> require sharing a buffer between more than one device. This requires
> having a mapping created separately for each device and is usually
> performed by calling dma_map_{single,page,sg} function more than once
> for the given buffer with device pointer to each device taking part in
> the buffer sharing. The first call transfers a buffer from 'CPU' domain
> to 'device' domain, what synchronizes CPU caches for the given region
> (usually it means that the cache has been flushed or invalidated
> depending on the dma direction). However, next calls to
> dma_map_{single,page,sg}() for other devices will perform exactly the
> same sychronization operation on the CPU cache. CPU cache sychronization
> might be a time consuming operation, especially if the buffers are
> large, so it is highly recommended to avoid it if possible.
> DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
> the CPU cache for the given buffer assuming that it has been already
> transferred to 'device' domain. This attribute can be also used for
> dma_unmap_{single,page,sg} functions family to force buffer to stay in
> device domain after releasing a mapping for it. Use this attribute with
> care!
>
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>

Curious question: What's the use-case for this? Is this just to
work-around the fact that dma-buf atm doesn't support streaming dma so
that we could optimize this all (and keep around the mappings)? Or is
there a different use-case that I don't see?
-Daniel

> ---
> Documentation/DMA-attributes.txt | 24 ++++++++++++++++++++++++
> include/linux/dma-attrs.h | 1 +
> 2 files changed, 25 insertions(+), 0 deletions(-)
>
> diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
> index 725580d..f503090 100644
> --- a/Documentation/DMA-attributes.txt
> +++ b/Documentation/DMA-attributes.txt
> @@ -67,3 +67,27 @@ set on each call.
> Since it is optional for platforms to implement
> DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
> attribute and exhibit default behavior.
> +
> +DMA_ATTR_SKIP_CPU_SYNC
> +----------------------
> +
> +By default dma_map_{single,page,sg} functions family transfer a given
> +buffer from CPU domain to device domain. Some advanced use cases might
> +require sharing a buffer between more than one device. This requires
> +having a mapping created separately for each device and is usually
> +performed by calling dma_map_{single,page,sg} function more than once
> +for the given buffer with device pointer to each device taking part in
> +the buffer sharing. The first call transfers a buffer from 'CPU' domain
> +to 'device' domain, what synchronizes CPU caches for the given region
> +(usually it means that the cache has been flushed or invalidated
> +depending on the dma direction). However, next calls to
> +dma_map_{single,page,sg}() for other devices will perform exactly the
> +same sychronization operation on the CPU cache. CPU cache sychronization
> +might be a time consuming operation, especially if the buffers are
> +large, so it is highly recommended to avoid it if possible.
> +DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
> +the CPU cache for the given buffer assuming that it has been already
> +transferred to 'device' domain. This attribute can be also used for
> +dma_unmap_{single,page,sg} functions family to force buffer to stay in
> +device domain after releasing a mapping for it. Use this attribute with
> +care!
> diff --git a/include/linux/dma-attrs.h b/include/linux/dma-attrs.h
> index a37c10c..f83f793 100644
> --- a/include/linux/dma-attrs.h
> +++ b/include/linux/dma-attrs.h
> @@ -16,6 +16,7 @@ enum dma_attr {
> DMA_ATTR_WRITE_COMBINE,
> DMA_ATTR_NON_CONSISTENT,
> DMA_ATTR_NO_KERNEL_MAPPING,
> + DMA_ATTR_SKIP_CPU_SYNC,
> DMA_ATTR_MAX,
> };
>
> --
> 1.7.1.569.g6f426
>
>
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--
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48