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

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Message-ID: <20120613185202.GM4829@phenom.ffwll.local>
Date:	Wed, 13 Jun 2012 20:52:02 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Abhinav Kochhar <abhinav.k@...sung.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Subash Patel <subash.ramaswamy@...aro.org>
Subject: Re: [Linaro-mm-sig] [PATCHv2 1/6] common: DMA-mapping: add
 DMA_ATTR_NO_KERNEL_MAPPING attribute

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@...sung.com>
> Reviewed-by: Kyungmin Park <kyungmin.park@...sung.com>

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

Reviewed-by: Daniel Vetter <daniel.vetter@...ll.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
> 
> 
> _______________________________________________
> Linaro-mm-sig mailing list
> Linaro-mm-sig@...ts.linaro.org
> http://lists.linaro.org/mailman/listinfo/linaro-mm-sig

-- 
Daniel Vetter
Mail: daniel@...ll.ch
Mobile: +41 (0)79 365 57 48
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