Re: [PATCH v7 01/10] ARM: davinci: move private EDMA API to arm/common

[Cyril Chemparathy]

On 02/05/2013 07:38 AM, Russell King - ARM Linux wrote:
> On Mon, Feb 04, 2013 at 09:47:38PM +0000, Arnd Bergmann wrote:
> > On Monday 04 February 2013, Linus Walleij wrote:
> > > So I think the above concerns are moot. The callback we can
> > > set on cookies is entirely optional, and it's even implemented by
> > > each DMA engine, and some may not even support it but require
> > > polling, and then it won't even be implemented by the driver.
> >
> > Just to ensure that everybody is talking about the same thing here:
> > Is it just the callback that is optional, or also the interrupt
> > coming from the hardware?
>
> If everyone implements stuff correctly, both.  The callback most certainly
> is optional as things stand.  The interrupt - that depends on the DMA
> engine.
>
> Some DMA engines you can't avoid it because you need to reprogram the
> hardware with the next+1 transfer upon completion of an existing transfer.
> Others may allow you to chain transfers in hardware.  That's all up to
> how the DMA engine driver is implemented and how the hardware behaves.
>
> Now, there's another problem here: that is, people abuse the API.  People
> don't pass DMA_CTRL_ACK | DMA_PREP_INTERRUPT into their operations by
> default.  People like typing '0'.
>
> The intention of the "DMA_PREP_INTERRUPT" is significant here: it means
> "ask the hardware to send an interrupt upon completion of this transfer".
>
> Because soo many people like to type '0' instead in their DMA engine
> clients, it means that this flag is utterly useless today - you have to
> ignore it.  So there's _no_ way for client drivers to actually tell the
> a DMA engine driver which _doesn't_ need to signal interrupts at the end
> of every transfer not to do so.
>
> So yes, the DMA engine API supports it.  Whether the _implementations_
> themselves do is very much hit and miss (and in reality is much more
> miss than hit.)

Don't these assume that the driver can determine the need for an 
interrupt upfront at prep/submit time?  AFAICT, this assumption doesn't 
hold true with NAPI.

Thanks
-- Cyril.
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