linux-kernel - Re: [PATCH] scatterlist: use sg_dma_len() in sg_set

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Date:	Mon, 11 Apr 2016 11:31:04 +0100
From:	Robin Murphy <robin.murphy@....com>
To:	Shawn Guo <shawnguo@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] scatterlist: use sg_dma_len() in sg_set_page()

Hi Shawn,

On 11/04/16 03:47, Shawn Guo wrote:
> The macro sg_dma_len(sg) is commonly used to retrieve length of sg,
> which could be 'dma_length' or 'length' field, depending on whether
> NEED_SG_DMA_LENGTH is enabled or not.  On the other hand, many driver
> code use helper function sg_set_page() to set an sg entry pointing at
> a page, with offset and length set up in one call.  But sg_set_page()
> does not consider NEED_SG_DMA_LENGTH case and only set up 'length'
> field.  This causes problem on platforms like ARM64, where
> NEED_SG_DMA_LENGTH is enabled by default, i.e. sg_set_page() sets up
> 'length' while sg_dma_len(sg) returns 'dma_length' field.
>
> The patch changes sg_set_page() to use sg_dma_len() for sg length setup
> as well, so that NEED_SG_DMA_LENGTH case can be handled.
>
> Signed-off-by: Shawn Guo <shawnguo@...nel.org>
> ---
>   include/linux/scatterlist.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 556ec1ea2574..b0e32ea594c3 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -114,7 +114,7 @@ static inline void sg_set_page(struct scatterlist *sg, struct page *page,
>   {
>   	sg_assign_page(sg, page);
>   	sg->offset = offset;
> -	sg->length = len;
> +	sg_dma_len(sg) = len;

This looks wrong. If a driver is building a scatterlist, then it needs 
to fill in the page, offset and length fields to describe the physical 
layout - leaving sg->length uninitialised would be a recipe for disaster 
- then pass it to the DMA API. Only the DMA API implementation should be 
setting dma_addr and dma_len, and they may not correspond to the 
physical layout at all (e.g. an IOMMU could concatenate the entire list 
into a single much longer segment at whatever arbitrary DMA address it 
chooses).

Is there some particular driver hiccup behind this?

Robin.

>   }
>
>   static inline struct page *sg_page(struct scatterlist *sg)
>