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Diffstat (limited to 'Documentation/DMA-attributes.txt')
| -rw-r--r-- | Documentation/DMA-attributes.txt | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt index 18dc52c4f2a0..e8cf9cf873b3 100644 --- a/Documentation/DMA-attributes.txt +++ b/Documentation/DMA-attributes.txt @@ -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. |