Re: dmaengine/Query: What about scatter/gather for mem to mem transfers.

[Pratyush Anand]

On 12/15/2011 10:54 AM, Pratyush Anand wrote:
> On 12/15/2011 10:36 AM, Koul, Vinod wrote:
> > > Hello Vinod/Dan,
> > >
> > > Please write your opinion.
> > I am on vacation, but will reply briefly here, more next week
>
> Thanks for your quick reply.
>
> > > Regards
> > > Pratyush
> > >
> > > On 12/14/2011 12:17 PM, Pratyush Anand wrote:
> > > > Hello Vinod/Dan,
> > > >
> > > > In continuation to the scatter/gather requirement:
> > > > We might need some generic transfer where source and destination
> > > > address may be overlapped and also gap between two chunk of source and
> > > > destination might not be same.
> > > >
> > > > For examaple,
> > > > Transfer size is - -0x4000
> > > > Our Src is something like this:
> > > > 0x1000 -- 0x2000
> > > > 0x3000 -- 0x5000
> > > > 0x6000 -- 0x7000
> > > >
> > > > and dst is something like this:
> > > > 0x6000 -- 0x8000
> > > > 0x9000 -- 0xB000
> > So why can't it be split like:
> > 0x1000--0x2000 => 0x6000 --0x7000
> > 0x3000 -- 0x4000 => 0x7000 - 0x8000
> > 0x4000 -- 0x5000 => 0x9000 - 0xA000
> > 0x6000 -- 0x7000 => 0xA000 - 0xB000
> >
> > That way existing mechanism would work well for you.
> > You need to split the chunks properly, which is what dma would do anyway
>
> Yes, they can be split like this, but then splitting onus will go on dma
> user driver, and so there would be replication of similar logic at
> several places. Therefore, I was thinking to make device_prep_dma_sg as
> generic by adding these flags.

I see one more issue in using device_prep_interleaved_dma.

Src and Dst address has been allocated in user space.
Now a kernel module extracts physical addresses from these pages and
prepares a sg list, which it submits to DMA.
These addresses would be virtually contiguous and incrementing. But, I 
am not sure if they are always physically incrementing too. If they are 
not guaranteed to be incrementing, then I see issue.

Otherwise also, a situation can arise when scattered memory is not 
always incrementing or decrementing in the same sg list.

> Regards
> Pratyush
>
> > --
> > ~Vinod

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