Re: [Linaro-mm-sig] [PATCHv2 1/6] common: DMA-mapping: add DMA_ATTR_NO_KERNEL_MAPPING attribute

Daniel Vetter
2012-06-13T18:50:33Z

On Wed, Jun 13, 2012 at 01:50:13PM +0200, Marek Szyprowski wrote:
> This patch adds DMA_ATTR_NO_KERNEL_MAPPING attribute which lets the
> platform to avoid creating a kernel virtual mapping for the allocated
> buffer. On some architectures creating such mapping is non-trivial task
> and consumes very limited resources (like kernel virtual address space
> or dma consistent address space). Buffers allocated with this attribute
> can be only passed to user space by calling dma_mmap_attrs().
>
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>

Looks like a nice little extension to support dma-buf for the common case,
so:

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

One question is whether we should go right ahead and add kmap support for
this, too (with a default implementation that simply returns a pointer to
the coherent&contigous dma mem), but I guess that can wait until a
use-case pops up.
-Daniel

> ---
> Documentation/DMA-attributes.txt | 18 ++++++++++++++++++
> include/linux/dma-attrs.h | 1 +
> 2 files changed, 19 insertions(+), 0 deletions(-)
>
> diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
> index 5c72eed..725580d 100644
> --- a/Documentation/DMA-attributes.txt
> +++ b/Documentation/DMA-attributes.txt
> @@ -49,3 +49,21 @@ DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
> consistent or non-consistent memory as it sees fit. By using this API,
> you are guaranteeing to the platform that you have all the correct and
> necessary sync points for this memory in the driver.
> +
> +DMA_ATTR_NO_KERNEL_MAPPING
> +--------------------------
> +
> +DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
> +virtual mapping for the allocated buffer. On some architectures creating
> +such mapping is non-trivial task and consumes very limited resources
> +(like kernel virtual address space or dma consistent address space).
> +Buffers allocated with this attribute can be only passed to user space
> +by calling dma_mmap_attrs(). By using this API, you are guaranteeing
> +that you won't dereference the pointer returned by dma_alloc_attr(). You
> +can threat it as a cookie that must be passed to dma_mmap_attrs() and
> +dma_free_attrs(). Make sure that both of these also get this attribute
> +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.
> diff --git a/include/linux/dma-attrs.h b/include/linux/dma-attrs.h
> index 547ab56..a37c10c 100644
> --- a/include/linux/dma-attrs.h
> +++ b/include/linux/dma-attrs.h
> @@ -15,6 +15,7 @@ enum dma_attr {
> DMA_ATTR_WEAK_ORDERING,
> DMA_ATTR_WRITE_COMBINE,
> DMA_ATTR_NON_CONSISTENT,
> + DMA_ATTR_NO_KERNEL_MAPPING,
> 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