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Diffstat (limited to 'Documentation/DMA-attributes.txt')
| -rw-r--r-- | Documentation/DMA-attributes.txt | 62 |
1 files changed, 59 insertions, 3 deletions
diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt index 18dc52c4f2a0..8f8d97f65d73 100644 --- a/Documentation/DMA-attributes.txt +++ b/Documentation/DMA-attributes.txt @@ -1,8 +1,9 @@ - DMA attributes - ============== +============== +DMA attributes +============== This document describes the semantics of the DMA attributes that are -defined in linux/dma-attrs.h. +defined in linux/dma-mapping.h. DMA_ATTR_WRITE_BARRIER ---------------------- @@ -100,3 +101,58 @@ 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. + +DMA_ATTR_NO_WARN +---------------- + +This tells the DMA-mapping subsystem to suppress allocation failure reports +(similarly to __GFP_NOWARN). + +On some architectures allocation failures are reported with error messages +to the system logs. Although this can help to identify and debug problems, +drivers which handle failures (eg, retry later) have no problems with them, +and can actually flood the system logs with error messages that aren't any +problem at all, depending on the implementation of the retry mechanism. + +So, this provides a way for drivers to avoid those error messages on calls +where allocation failures are not a problem, and shouldn't bother the logs. + +.. note:: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC. + +DMA_ATTR_PRIVILEGED +------------------- + +Some advanced peripherals such as remote processors and GPUs perform +accesses to DMA buffers in both privileged "supervisor" and unprivileged +"user" modes. This attribute is used to indicate to the DMA-mapping +subsystem that the buffer is fully accessible at the elevated privilege +level (and ideally inaccessible or at least read-only at the +lesser-privileged levels). |