linux-kernel - Re: [PATCH v5 3/9] dma-mapping: add dma_{map,unmap}

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Date:	Fri, 11 Mar 2016 11:49:31 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Christoph Hellwig <hch@...radead.org>,
	linux-renesas-soc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	dmaengine@...r.kernel.org, iommu@...ts.linux-foundation.org,
	robin.murphy@....com, laurent.pinchart@...asonboard.com,
	geert+renesas@...der.be, linus.walleij@...aro.org,
	dan.j.williams@...el.com, arnd@...db.de, linux-arch@...r.kernel.org
Subject: Re: [PATCH v5 3/9] dma-mapping: add dma_{map,unmap}_resource

On Thu, Mar 10, 2016 at 05:05:23PM +0100, Niklas S??derlund wrote:
> Hi Christoph,
> 
> On 2016-03-07 23:38:47 -0800, Christoph Hellwig wrote:
> > Please add some documentation on where/how this should be used.  It's
> > not a very obvious interface.
> 
> Good idea, I have added the following to Documentation/DMA-API.txt and 
> folded it in to this patch. Do you feel it's adequate and do you know 
> anywhere else I should add documentation?
> 
> diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
> index 45ef3f2..248556a 100644
> --- a/Documentation/DMA-API.txt
> +++ b/Documentation/DMA-API.txt
> @@ -277,14 +277,29 @@ and <size> parameters are provided to do partial page mapping, it is
>  recommended that you never use these unless you really know what the
>  cache width is.
>  
> +dma_addr_t
> +dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
> +                enum dma_data_direction dir, struct dma_attrs *attrs)
> +
> +Maps a MMIO region so it can be accessed by the device and returns the
> +DMA address of the memory. API should only be used to map device MMIO,
> +mapping of RAM is not permitted.
> +
> +void
> +dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
> +                  enum dma_data_direction dir, struct dma_attrs *attrs)

Using function prototypes in documentation may not be a great idea as you
are explaining the role of this function and not arguments.

> +
> +Unmaps the IOMMU region previously mapped. All the parameters passed in
> +must be identical to those passed in (and returned) by the mapping API.
> +
>  int
>  dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
>  
> -In some circumstances dma_map_single() and dma_map_page() will fail to create
> -a mapping. A driver can check for these errors by testing the returned
> -DMA address with dma_mapping_error(). A non-zero return value means the mapping
> -could not be created and the driver should take appropriate action (e.g.
> -reduce current DMA mapping usage or delay and try again later).
> +In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
> +will fail to create a mapping. A driver can check for these errors by testing
> +the returned DMA address with dma_mapping_error(). A non-zero return value
> +means the mapping could not be created and the driver should take appropriate
> +action (e.g.  reduce current DMA mapping usage or delay and try again later).
> 
> -- 
> Regards,
> Niklas Söderlund

-- 
~Vinod