Re: [PATCH 1/7 v2] dmaengine: add a simple dma library
From: Vinod Koul
Date: Wed Feb 01 2012 - 00:37:51 EST
On Tue, 2012-01-31 at 09:59 +0100, Guennadi Liakhovetski wrote:
> Hi Vinod
> Thanks for your reply.
> On Tue, 31 Jan 2012, Vinod Koul wrote:
> > 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?
> If I was sure about that, I'd call the library shmobile-dma or something
> > Btw does you hardware only support single transfers and no sg support,
> Well, the controller does have some repeat- and reload-modes, but they are
> currently not supported by the driver and from the documentation I cannot
> easily understand, how useful those modes could be to implement SG.
> > would this remain the same in future?
> Who knows?...
> > > (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
> > simple.
> > >
> > > 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
> > > hardware.
> > I think it would be good to have both, this can be used by folks who
> > don't have sg support available.
> Ok, so, should I just rename the driver to sgsoft? Is this the only
> change, that you'd like to see?
That would be one :)
Also, I would review the other patch patches by today/tomorrow, you can
add any changes in next version as well
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