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

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Date:	Thu, 10 Mar 2016 22:47:10 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@...radead.org>,
	Vinod Koul <vinod.koul@...el.com>,
	linux-renesas-soc@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	iommu@...ts.linux-foundation.org, robin.murphy@....com,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	geert+renesas@...der.be, Linus Walleij <linus.walleij@...aro.org>,
	Dan J Williams <dan.j.williams@...el.com>,
	Arnd Bergmann <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 8:05 AM, Niklas S??derlund
<niklas.soderlund@...natech.se> 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.
> +

I think it is confusing to use the dma_ prefix for this peer-to-peer
mmio functionality.  dma_addr_t is a device's view of host memory.
Something like bus_addr_t bus_map_resource().  Doesn't this routine
also need the source device in addition to the target device?  The
resource address is from the perspective of the host cpu, it may be a
different address space in the view of two devices relative to each
other.