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

From: Måns Rullgård
Date: Fri Nov 25 2016 - 09:03:36 EST

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.

Måns Rullgård