diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/DMA-API.txt | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt index 364a6cb444a5..29a48fbae779 100644 --- a/Documentation/DMA-API.txt +++ b/Documentation/DMA-API.txt @@ -472,6 +472,64 @@ void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr, .... +Part Ie - Optimizing Unmap State Space Consumption +-------------------------------- + +On some platforms, dma_unmap_{single,page}() is simply a nop. +Therefore, keeping track of the mapping address and length is a waste +of space. Instead of filling your drivers up with ifdefs and the like +to "work around" this (which would defeat the whole purpose of a +portable API) the following facilities are provided. + +Actually, instead of describing the macros one by one, we'll +transform some example code. + +1) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures. + Example, before: + + struct ring_state { + struct sk_buff *skb; + dma_addr_t mapping; + __u32 len; + }; + + after: + + struct ring_state { + struct sk_buff *skb; + DEFINE_DMA_UNMAP_ADDR(mapping); + DEFINE_DMA_UNMAP_LEN(len); + }; + +2) Use dma_unmap_{addr,len}_set to set these values. + Example, before: + + ringp->mapping = FOO; + ringp->len = BAR; + + after: + + dma_unmap_addr_set(ringp, mapping, FOO); + dma_unmap_len_set(ringp, len, BAR); + +3) Use dma_unmap_{addr,len} to access these values. + Example, before: + + dma_unmap_single(dev, ringp->mapping, ringp->len, + DMA_FROM_DEVICE); + + after: + + dma_unmap_single(dev, + dma_unmap_addr(ringp, mapping), + dma_unmap_len(ringp, len), + DMA_FROM_DEVICE); + +It really should be self-explanatory. We treat the ADDR and LEN +separately, because it is possible for an implementation to only +need the address in order to perform the unmap operation. + + Part II - Advanced dma_ usage ----------------------------- |