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ARM: 8506/1: common: DMA-mapping: add DMA_ATTR_ALLOC_SINGLE_PAGES attribute

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This patch adds the DMA_ATTR_ALLOC_SINGLE_PAGES attribute to the
DMA-mapping subsystem.

This attribute can be used as a hint to the DMA-mapping subsystem that
it's likely not worth it to try to allocate large pages behind the
scenes.  Large pages are likely to make an IOMMU TLB work more
efficiently but may not be worth it.  See the Documentation contained in
this patch for more details about this attribute and when to use it.

Note that the name of the hint (DMA_ATTR_ALLOC_SINGLE_PAGES) is loosely
based on the name MADV_NOHUGEPAGE.  Just as there is MADV_NOHUGEPAGE
vs. MADV_HUGEPAGE we could also add an "opposite" attribute to
DMA_ATTR_ALLOC_SINGLE_PAGES.  Without having the "opposite" attribute
the lack of DMA_ATTR_ALLOC_SINGLE_PAGES means "use your best judgement
about whether to use small pages or large pages".

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Doug Anderson 2016-01-29 23:07:26 +01:00 committed by Russell King
parent 33298ef6d8
commit df05c6f6e0
2 changed files with 27 additions and 0 deletions
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26
Documentation/DMA-attributes.txt
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@ -100,3 +100,29 @@ allocated by dma_alloc_attrs() function from individual pages if it can
be mapped as contiguous chunk into device dma address space. By
specifying this attribute the allocated buffer is forced to be contiguous
also in physical memory.
DMA_ATTR_ALLOC_SINGLE_PAGES
---------------------------
This is a hint to the DMA-mapping subsystem that it's probably not worth
the time to try to allocate memory to in a way that gives better TLB
efficiency (AKA it's not worth trying to build the mapping out of larger
pages). You might want to specify this if:
- You know that the accesses to this memory won't thrash the TLB.
You might know that the accesses are likely to be sequential or
that they aren't sequential but it's unlikely you'll ping-pong
between many addresses that are likely to be in different physical
pages.
- You know that the penalty of TLB misses while accessing the
memory will be small enough to be inconsequential. If you are
doing a heavy operation like decryption or decompression this
might be the case.
- You know that the DMA mapping is fairly transitory. If you expect
the mapping to have a short lifetime then it may be worth it to
optimize allocation (avoid coming up with large pages) instead of
getting the slight performance win of larger pages.
Setting this hint doesn't guarantee that you won't get huge pages, but it
means that we won't try quite as hard to get them.
NOTE: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM,
though ARM64 patches will likely be posted soon.

1
include/linux/dma-attrs.h
View File

@ -18,6 +18,7 @@ enum dma_attr {
DMA_ATTR_NO_KERNEL_MAPPING,
DMA_ATTR_SKIP_CPU_SYNC,
DMA_ATTR_FORCE_CONTIGUOUS,
DMA_ATTR_ALLOC_SINGLE_PAGES,
DMA_ATTR_MAX,
};