Re: [RFC 0/2] fix dma_map_sg not to do barriers for each buffer
From: Abhijeet Dharmapurikar
Date: Wed Feb 10 2010 - 18:28:24 EST
Russell King - ARM Linux wrote:
On Wed, Feb 10, 2010 at 12:37:28PM -0800, adharmap@xxxxxxxxxxxxxx wrote:
From: Abhijeet Dharmapurikar <adharmap@xxxxxxxxxxx>
Please refer to the post here
http://lkml.org/lkml/2010/1/4/347
These changes are to introduce barrierless dma_map_area and dma_unmap_area and
use them to map the buffers in the scatterlist. For the last buffer, call
the normal dma_map_area(aka with barriers) effectively executing the barrier
at the end of the operation.
What if we make dma_map_area and dma_unmap_area both be barrier-less,
and instead have a separate dma_barrier method - eg, something like the
attached?
This might allow for better I-cache usage by not having to duplicate the
DMA cache coherence functions.
Agree, thanks for pointing this and for the patch.
@@ -369,6 +372,7 @@ static inline dma_addr_t dma_map_page(struct device *dev, struct page *page,
BUG_ON(!valid_dma_direction(dir));
__dma_page_cpu_to_dev(page, offset, size, dir);
+ __dma_barrier(dir);
return page_to_dma(dev, page) + offset;
}
dma_map_page is going to execute the barrier here.
/**
* dma_map_sg - map a set of SG buffers for streaming mode DMA
* @dev: valid struct device pointer, or NULL for ISA and EISA-like devices
@@ -537,6 +544,9 @@ int dma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
if (dma_mapping_error(dev, s->dma_address))
goto bad_mapping;
}
+
+ __dma_barrier(dir);
+
return nents;
This would call the barrier in addition to the ones executed by
dma_map_page.
We would need to call __dma_page_cpu_to_dev instead of dma_map_page and
do the barrier before returning.
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