Re: Re: [Jackit-devel] irq handler top half timestamps

[Lee Revell <rlrevell@joe-job.com>]

On Fri, 2004-12-10 at 15:51 -0500, Paul Davis wrote:
> >Maybe I am not understanding the ALSA code correctly, but I was under
> >the impression that ALSA interrupt handlers do not really have a bottom
> >half as such, in that they do not schedule any work for later via
> >raising a softirq or scheduling a tasklet.  They just do it all in the
> >top half.  It's possible for ALSA to do everything in the top half
> >because all you do in an ALSA interrupt handler is call
> >snd_pcm_period_elapsed on one or more substreams.  For this to work it
> >seem like snd_pcm_period_elapsed has to be O(N) and therefore realtime
> >safe.
> 
> with ingo's RP patches, every IRQ has a top half and bottom half. the
> question is whether the top half calls the bottom half directly or
> arranges for it to run in a thread "later".

Yeah, exactly.  I was speaking in terms of the traditional bottom
half/top half model.

With Ingo's patches we actually have 3 levels of IRQ context, they
effectively add a level above the traditional "top half" which is just a
tiny asm routine to mark the IRQ thread runnable (if threaded) or
execute the "real" top half directly if nonthreaded.

I don't really consider this new level very significant in practice
because it's fast and exactly the same for all devices.  IOW the
snd_pcm_period_elapsed timestamp should be good.  I don't think we need
to worry about the threaded ALSA IRQ case.

Lee



-------------------------------------------------------
SF email is sponsored by - The IT Product Guide
Read honest & candid reviews on hundreds of IT Products from real users.
Discover which products truly live up to the hype. Start reading now. 
http://productguide.itmanagersjournal.com/