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

[Russell King - ARM Linux]

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...

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.