By default DMA-mapping subsystem is allowed to assemble the buffer
allocated by dma_alloc_attrs() function from individual pages if it can
be mapped as contiguous chunk into device dma address space. By
-specifing this attribute the allocated buffer is forced to be contiguous
+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.
projects / firefly-linux-kernel-4.4.55.git / blobdiff