[alsa-devel] [PATCH 1/7 v2] dmaengine: add a simple dma library
vinod.koul at intel.com
Tue Jan 31 07:41:30 CET 2012
On Mon, 2012-01-30 at 10:34 +0100, Guennadi Liakhovetski wrote:
> > I don't still comprehend the need for a library on top of dmaengine
> > which gain is just a library between clients and dmacs. Surely we don't
> > want to write another abstraction on top of one provided?
> > If the question is to handle scatter-gather even if the hardware doesn't
> > have the capability, then why don't add that in dmaengine itself rather
> > than one more layer?
> Well, yes, adding new abstraction layers is always a decision, that has to
> be well justified. In this case it does at least make the life easier for
> two sh-mobile drivers: shdma and the new SUDMAC driver.
> However, I did name the library in a generic way without reference to sh,
> assuming, that it might with time become useful for other architectures
> too. The reasons why I prefered to keep it as an optional addition to
> dmaengine core, instead of tightly integrating it with it are, that (1) I
> did not want to add useless code to drivers, that do not need it,
So are we sure that only sh-mobile drivers need this capablity?
Btw does you hardware only support single transfers and no sg support,
would this remain the same in future?
> (2) I am
> not sure if and when this library will become useful for other drivers:
> apart from sh I am only familiar with one more dmaengine driver:
> ipu/ipu_idmac.c, and that one supports scatter-gather lists in a limited
> way and has some further peculiarities, that would likely make it a bad
> match for the simple DMA library,
typically the dmacs will support this in some form or other, so your
point is valid :)
> (3) keeping it separate makes its
> further development easier.
> OTOH, I'm certainly fine with a tighter library integration with the
> dmaengine core. I think, it still would be better to keep it in a separate
> file and only build it if needed, right? This woult also simplify code
> debugging and further development. I can remove the "simple" notation,
> which does make it look like an additional abstraction layer, and replace
> it with, say, sgsoft (scatter-gather software implementation)?
that would be more apt :)
> A more
> interesting question is what to do with struct dma_simple_dev, struct
> dma_simple_chan, struct dma_simple_desc, that embed struct dma_device,
> struct dma_chan and struct dma_async_tx_descriptor respectively. I don't
> think we want to merge all the additions from those wrapping structs back
> into their dmaengine counterparts?
Sure they should be kept separate. I like the wrapping, this keeps it
> How would you like to do this? Don't you think, it would be good to allow
> both: either implement a dmaengine driver directly, exactly as all drivers
> are doing now, or use the additional helper library for suitable (simple)
> hardware types? I see it similar to I2C, where you either implement an I2C
> driver directly, or you use the bitbanging abstraction for simpler
I think it would be good to have both, this can be used by folks who
don't have sg support available.
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