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

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Message-ID: <20160310160522.GB1111@bigcity.dyn.berto.se>
Date:	Thu, 10 Mar 2016 17:05:23 +0100
From:	"Niklas S??derlund" <niklas.soderlund@...natech.se>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	vinod.koul@...el.com, 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

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)
+
+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