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

From: Måns Rullgård
Date: Fri Nov 25 2016 - 09:40:44 EST

Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> writes:

> 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...

So what do you suggest? Stick our heads in the sand and pretend
everything is perfect?

Måns Rullgård