Commit 78c47830 authored by Robin Murphy's avatar Robin Murphy Committed by Christoph Hellwig
Browse files

dma-debug: check scatterlist segments 



Drivers/subsystems creating scatterlists for DMA should be taking care
to respect the scatter-gather limitations of the appropriate device, as
described by dma_parms. A DMA API implementation cannot feasibly split
a scatterlist into *more* entries than originally passed, so it is not
well defined what they should do when given a segment larger than the
limit they are also required to respect.

Conversely, devices which are less limited than the rather conservative
defaults, or indeed have no limitations at all (e.g. GPUs with their own
internal MMU), should be encouraged to set appropriate dma_parms, as
they may get more efficient DMA mapping performance out of it.

Signed-off-by: default avatarRobin Murphy <robin.murphy@arm.com>
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
parent 7f5c1ea3

lib/Kconfig.debug

+17 −0
Original line number Diff line number Diff line
@@ -1651,6 +1651,23 @@ config DMA_API_DEBUG 


	  If unsure, say N.



config DMA_API_DEBUG_SG

	bool "Debug DMA scatter-gather usage"

	default y

	depends on DMA_API_DEBUG

	help

	  Perform extra checking that callers of dma_map_sg() have respected the

	  appropriate segment length/boundary limits for the given device when

	  preparing DMA scatterlists.



	  This is particularly likely to have been overlooked in cases where the

	  dma_map_sg() API is used for general bulk mapping of pages rather than

	  preparing literal scatter-gather descriptors, where there is a risk of

	  unexpected behaviour from DMA API implementations if the scatterlist

	  is technically out-of-spec.



	  If unsure, say N.



menuconfig RUNTIME_TESTING_MENU

	bool "Runtime Testing"

	def_bool y

lib/dma-debug.c

+28 −0
Original line number Diff line number Diff line
@@ -1286,6 +1286,32 @@ out: 
	put_hash_bucket(bucket, &flags);

}



static void check_sg_segment(struct device *dev, struct scatterlist *sg)

{

#ifdef CONFIG_DMA_API_DEBUG_SG

	unsigned int max_seg = dma_get_max_seg_size(dev);

	u64 start, end, boundary = dma_get_seg_boundary(dev);



	/*

	 * Either the driver forgot to set dma_parms appropriately, or

	 * whoever generated the list forgot to check them.

	 */

	if (sg->length > max_seg)

		err_printk(dev, NULL, "DMA-API: mapping sg segment longer than device claims to support [len=%u] [max=%u]\n",

			   sg->length, max_seg);

	/*

	 * In some cases this could potentially be the DMA API

	 * implementation's fault, but it would usually imply that

	 * the scatterlist was built inappropriately to begin with.

	 */

	start = sg_dma_address(sg);

	end = start + sg_dma_len(sg) - 1;

	if ((start ^ end) & ~boundary)

		err_printk(dev, NULL, "DMA-API: mapping sg segment across boundary [start=0x%016llx] [end=0x%016llx] [boundary=0x%016llx]\n",

			   start, end, boundary);

#endif

}



void debug_dma_map_page(struct device *dev, struct page *page, size_t offset,

			size_t size, int direction, dma_addr_t dma_addr,

			bool map_single)
@@ -1416,6 +1442,8 @@ void debug_dma_map_sg(struct device *dev, struct scatterlist *sg, 
			check_for_illegal_area(dev, sg_virt(s), sg_dma_len(s));

		}



		check_sg_segment(dev, s);



		add_dma_entry(entry);

	}

}

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