Re: Query about DMA map API implementation in non-coherent archs

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

Vineet Gupta wrote:
...
> As per dma-mapping and dma-api docs, for streaming DMA mappings arch
> needs to provide 2 seperate APIs
> 
> 1. map API to convert cpu address into DMA address
> 2. sync API to flush/invalidate the dcaches for non-coherent archs
> (essentially when changing the ownership of buffer between cpu and
> device).
> 
> However from what I understood, the map API need not take care of
> proper cache sync.However both ARM and MIPS do the cache sync
> operations in map API as well (this is done standalone in sync APIs
> too). Is there a specific reason for this.
> 
> Drivers such as libata don't even call any sync API. For e.g. libata
> only calls dma_map_sg ( ) while setting up the DMA.
...

dma_map_*() transfers ownership of a buffer from CPUs to the device too.
 In case of DMA_TO_DEVICE or DMA_BIDIRECTIONAL, these calls therefore
have to make sure that the device sees current buffer contents.
-- 
Stefan Richter
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