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

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Date:	Wed, 14 Jan 2009 14:29:20 -0800
From:	"Vineet Gupta" <vineetg76@...il.com>
To:	"Stefan Richter" <stefanr@...6.in-berlin.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Query about DMA map API implementation in non-coherent archs

On Sat, Jan 10, 2009 at 1:30 AM, Stefan Richter
<stefanr@...6.in-berlin.de> wrote:
> 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.

I feel that DMA-api.txt should explicitly say that in part Id, just
before detailing each of the enumerator DMA_TO_DEVICE,
DMA_FROM_DEVICE...

shall I prepare a patch...


> --
> Stefan Richter
> -=====-==--= ---= -=-=-
> http://arcgraph.de/sr/
>
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