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

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Date:	Fri, 3 Dec 2010 14:40:56 +0100
From:	Linus Walleij <linus.ml.walleij@...il.com>
To:	linux kernel <linux-kernel@...r.kernel.org>,
	David Brownell <david-b@...bell.net>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	per.forlin@...aro.org, Ulf Hansson <Ulf.Hansson@...ricsson.com>
Subject: dma_unmap_sg - what number of entries is to be passed as parameter really?

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.)

I will happily patch the culprit...

Yours,
Linus Walleij
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