[alsa-devel] [PATCH 1/2] ALSA: pcm - Ensure wakeup at each periodic interrupt

[Takashi Iwai]

At Thu, 17 Dec 2009 16:31:08 +0900,
jassi brar wrote:
> 
> On Thu, Dec 17, 2009 at 4:02 PM, Takashi Iwai <tiwai at suse.de> wrote:
> > At Thu, 17 Dec 2009 15:00:02 +0900,
> > jassisinghbrar at gmail.com wrote:
> >>
> >> From: Jassi Brar <jassi.brar at samsung.com>
> >>
> >> The check for at least 'avail_min' available data before calling wake_up
> >> doesn't always hold good as it does not guarantee callbacks at each periodic
> >> interrupt.
> >
> > Well, avail_min can be greater than period_size.  And, avail_min won't be
> > less than period size.
> >
> > For example, when avail_min = 2.5 x period_size, the driver wakes up
> > in periods like 3, 2, 3, 2, ...
> correct, but if we ensure wake_up's after each period and let the 'sleepers'
> track if the data available is enough or not, we will have more fine grained
> control.
> The point is:- Waking up _after_ avail_min is working, but does waking up before
> avail_min(but at period boundary) break the system?

PulseAudio may complain :)

> >> An example of such situation is snd_pcm_lib_read1/write1 consuming some space
> >> of the period and going to sleep from wait_for_avail_min upon syncing with
> >> the DMA pointer. Clearly just the remainder of period size is needed, but
> >> wake_up is called only after _two_ periodic interrupts from that point.
> >
> > In that case, the original behavior is correct.
> going by current implementation, that is correct, but is that desirable?

Correct = working as designed.


Takashi