dma-mapping: support for DMA_ATTR_NON_CONSISTENT DMA attribute

[Mike Looijmans]

On 29-06-15 07:24, Sylvain Munaut wrote:
>> ... and then using the *streaming* API dma_sync_* calls on it.  That
>> is invalid, and will lead to undefined results on implementations of
>> dma_alloc_coherent() which return remapped memory (as for older ARMs.)
>> This is an abuse of the API.
>
> Ok, fine, but then
>
> _How_ do I do it properly ?  From a driver module, how do I allocate a
> large chunk ( > 4M so no alloc_pages and no kmalloc ) of contiguous
> memory that's cacheable and that I can use with the streaming API ?

To the best of my knowledge, and after asking around here, the only answer 
I've so far been able to get is that alloc_pages (and hence, kmalloc) is the 
method to call.

It would be nice if there were a dma_alloc_streaming() interface, which might 
just translate to kalloc directly, but it would at least get rid of all the 
obscurities of allocating DMA memory.

I fell into the same pit as Sylvain here, initially I was also allocating with 
dma_alloc_...

I submitted a patch to amend the dma documentation a while ago, but never got 
a response, so I probably sent it to the wrong group.

> Or alternatively, what are the correct "sync points" that the doc refers to ?
>
> """ By using this API, you are guaranteeing to the platform that you
> have all the correct and necessary sync points for this memory in the
> driver should it choose to return non-consistent memory."""
>
> I had assumed it was the dma_sync_* calls, but apparently not.

I think the answer is "the dma_sync_* calls plus whatever it is your DMA 
controller hardware needs to do"

If that's the case, where should I send the documentation patch?



Kind regards,

Mike Looijmans
System Expert

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