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

["Martijn Sipkema" <martijn@entmoot.nl>]

> > > If the device uses a timer to generate interrupts at a fixed frequency
> > > then you have to manually compare the frames processed to the period
> > > size and call snd_pcm_period_elapsed if the period has indeed elapsed.
> >
> > Does that mean that ALSA will always have a large (relative to the interrupt
> > frequency) scheduling jitter for cards that interrupt at a fixed frequency?
> 
> Yes, if the hardware periods don't exaclty match ALSA periods.
> (Using e.g. snd-ymfpci at 48 kHz with periods of 256 samples shouldn't
> introduce any additional jitter.)
> 
> > Or is it possible to have ALSA return samples immediately after they become
> > available?
> 
> The ALSA API assumes constant-sized periods.  (Jack's Tascam USB
> driver bypasses the ALSA API.)

Ah, I'll have a look at that; I was able to buy a Swissonic USB studio D for
a very good price and now I want low latency with this "bad" hardware. :)
I would be nice to have ALSA support non-constant periods (transfer is a
better term in that case I think).


--ms



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