linux-kernel - Re: [PATCH RFCv2 1/2] dmaengine: add support for scatterlist to scatterlist transfers

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Date:	Mon, 27 Sep 2010 10:35:09 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Ira W. Snyder" <iws@...o.caltech.edu>
Cc:	Linus Walleij <linus.ml.walleij@...il.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Per Fridén <per.friden@...ricsson.com>
Subject: Re: [PATCH RFCv2 1/2] dmaengine: add support for scatterlist to
 scatterlist transfers

On Mon, Sep 27, 2010 at 10:23 AM, Ira W. Snyder <iws@...o.caltech.edu> wrote:
> On Mon, Sep 27, 2010 at 05:23:34PM +0200, Linus Walleij wrote:
>> 2010/9/25 Ira W. Snyder <iws@...o.caltech.edu>:
>>
>> > This adds support for scatterlist to scatterlist DMA transfers.
>>
>> This is a good idea, we have a local function to do this in DMA40 already,
>> stedma40_memcpy_sg().
>>
>
> I think that having two devices that want to implement this
> functionality as part of the DMAEngine API is a good argument for making
> it available as part of the core API. I think it would be good to add
> this to struct dma_device, and add a capability (DMA_SG?) for it as
> well.
>
> I have looked at the stedma40_memcpy_sg() function, and I think we would
> want to extend it slightly for the generic API. Is there any good reason
> to prohibit scatterlists with different numbers of elements?
>
> For example:
> src scatterlist: 10 elements, each with 4K length (40K total)
> dst scatterlist: 40 elements, each with 1K length (40K total)
>
> The total length of both scatterlists is equal, but the number of
> scatterlist entries is different. The freescale DMA controller can
> handle this just fine.
>
> I'm proposing this function signature:
> struct dma_async_tx_descriptor *
> dma_memcpy_sg(struct dma_chan *chan,
>              struct scatterlist *dst_sg, unsigned int dst_nents,
>              struct scatterlist *src_sg, unsigned int src_nents,
>              unsigned long flags);
>
>> > This is
>> > currently hidden behind a configuration option, which will allow drivers
>> > which need this functionality to select it individually.
>>
>> Why? Isn't it better to add this as a new capability flag
>> if you don't want to announce it? Or is the intent to save
>> memory footprint?
>>
>
> Dan wanted this, probably for memory footprint. If >1 driver is using
> it,

Yes, I did not see a reason to increment the size of dmaengine.o for
everyone if only one out-of-tree user of the function existed.

> I would rather have it as part of struct dma_device along with a
> capability.

I think having this as a dma_device method makes sense now that more
than one driver would implement it, and let's drivers see the entirety
of the transaction in one call.

--
Dan
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