Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Robin,

On 2019-05-22 15:55, Robin Murphy wrote:
> On 22/05/2019 14:34, Christoph Hellwig wrote:
>> On Wed, May 22, 2019 at 02:25:38PM +0100, Robin Murphy wrote:
>>> Sure, but that should be irrelevant since the effective problem here 
>>> is in
>>> the sync_*_for_cpu direction, and it's the unmap which nobbles the 
>>> buffer.
>>> If the driver does this:
>>>
>>>     dma_map_single(whole buffer);
>>>     <device writes to part of buffer>
>>>     dma_unmap_single(whole buffer);
>>>     <contents of rest of buffer now undefined>
>>>
>>> then it could instead do this and be happy:
>>>
>>>     dma_map_single(whole buffer, SKIP_CPU_SYNC);
>>>     <device writes to part of buffer>
>>>     dma_sync_single_for_cpu(updated part of buffer);
>>>     dma_unmap_single(whole buffer, SKIP_CPU_SYNC);
>>>     <contents of rest of buffer still valid>
>>
>> Assuming the driver knows how much was actually DMAed this would
>> solve the issue.  Horia, does this work for you?
>
> Ohhh, and now I've just twigged what you were suggesting - your 
> DMA_ATTR_PARTIAL flag would mean "treat this as a read-modify-write of 
> the buffer because we *don't* know exactly which parts the device may 
> write to". So indeed if we did go down that route we wouldn't need any 
> of the sync stuff I was worrying about (but I might suggest naming it 
> DMA_ATTR_UPDATE instead). Apologies for being slow :)

Don't we have DMA_BIDIRECTIONAL for such case? Maybe we should update 
documentation a bit to point that DMA_FROM_DEVICE expects the whole 
buffer to be filled by the device?

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland