Re: Tearing down DMA transfer setup after DMA client has finished

From: Russell King - ARM Linux
Date: Fri Nov 25 2016 - 09:28:49 EST

On Fri, Nov 25, 2016 at 02:03:20PM +0000, Måns Rullgård wrote:
> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> writes:
> > On Fri, Nov 25, 2016 at 01:50:35PM +0000, Måns Rullgård wrote:
> >> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> writes:
> >> > It would be unfair to augment the API and add the burden on everyone
> >> > for the new API when 99.999% of the world doesn't require it.
> >>
> >> I don't think making this particular dma driver wait for the descriptor
> >> callback to return before reusing a channel quite amounts to a horrid
> >> hack. It certainly wouldn't burden anyone other than the poor drivers
> >> for devices connected to it, all of which are specific to Sigma AFAIK.
> >
> > Except when you stop to think that delaying in a tasklet is exactly
> > the same as randomly delaying in an interrupt handler - the tasklet
> > runs on the return path back to the parent context of an interrupt
> > handler. Even if you sleep in the tasklet, you're sleeping on behalf
> > of the currently executing thread - if it's a RT thread, you effectively
> > destroy the RT-ness of the thread. Let's hope no one cares about RT
> > performance on that hardware...
> That's why I suggested to do this only if the needed delay is known to
> be no more than a few bus cycles. The completion callback is currently
> the only post-transfer interaction we have between the dma and device
> drivers. To handle an arbitrarily long delay, some new interface will
> be required.

And now we're back at the point I made a few emails ago about undue
burden which is just about quoted above...

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