linux-kernel - Re: dma_unmap_sg - what number of entries is to be passed as parameter really?

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Message-ID: <20101203153645.GB30898@n2100.arm.linux.org.uk>
Date:	Fri, 3 Dec 2010 15:36:45 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Linus Walleij <linus.ml.walleij@...il.com>
Cc:	linux kernel <linux-kernel@...r.kernel.org>,
	David Brownell <david-b@...bell.net>, per.forlin@...aro.org,
	Ulf Hansson <Ulf.Hansson@...ricsson.com>
Subject: Re: dma_unmap_sg - what number of entries is to be passed as
	parameter really?

On Fri, Dec 03, 2010 at 02:40:56PM +0100, Linus Walleij wrote:
> Now we have a contradiction between two pieces of documentation,
> in Documentation/DMA-API.txt
> 
> 
>         void
>         dma_unmap_sg(struct device *dev, struct scatterlist *sg,
>                 int nhwentries, enum dma_data_direction direction)
> 
>   Unmap the previously mapped scatter/gather list.  All the parameters
>   must be the same as those and passed in to the scatter/gather mapping
>   API.
> 
>   Note: <nents> must be the number you passed in, *not* the number of
>   physical entries returned.
> 
> 
> Note the last paragraph! But in arch/arm/mm/dma-mapping.c;
> 
> 
> /**
>  * dma_unmap_sg - unmap a set of SG buffers mapped by dma_map_sg
>  * @dev: valid struct device pointer, or NULL for ISA and EISA-like devices
>  * @sg: list of buffers
>  * @nents: number of buffers to unmap (returned from dma_map_sg)
>    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>  * @dir: DMA transfer direction (same as was passed to dma_map_sg)
>  *
>  * Unmap a set of streaming mode DMA translations.  Again, CPU access
>  * rules concerning calls here are the same as for dma_unmap_single().
>  */
> 
> So the documentation in Documentation/ says one thing, whereas
> the ARM implementation documentation says something else.
> 
> Which one is it? (Looks like the first one from Documentation/ to me.)

The only from Documentation/.  Luckily though, it doesn't make any
odds to the implementation on ARM as we don't have IOMMUs involved
in the DMA layer where we could coalesce entries.
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