Re: hw_pointer error message triggered without need

[Takashi Iwai <tiwai@suse.de>]

At Wed, 22 Oct 2003 12:58:53 +0200 (CEST),
Jaroslav wrote:
> 
> On Tue, 21 Oct 2003, Takashi Iwai wrote:
> 
> > > > well, it's not unnecessary but too strict.
> > > > especially, if the hardware generates too late (often seen on onboard
> > > > chips), and if only two periods are used, this situation can happen.
> > > >
> > > > i think delta < (runtime->period_size/2) would be enough to check the
> > > > negative (invalid) hwptr rather than delta < (runtime->buffer_size/2)
> > > > as it is.
> > >
> > > Nope. This check will be wrong because we will fall to the wrap point and
> > > increase hw_ptr_base with buffer_size.
> >
> > the problem is that this check is too strict when only two periods are
> > used.  since buffer_size/2 == period_size, if the update of hwptr is
> > delayed even for one sample, it won't pass.
> >
> > there is a little difference between snd_pcm_update_hw_ptr() and
> > snd_pcm_update_hw_ptr_interrupt(), and this problem doesn't happen in
> > the former but only in the latter function, which is invoked by
> > snd_pcm_lib_write1().
> >
> > in the former function, delta is the difference between the expected
> > pointer (lastptr + period_size) and the current pointer.  meanwhile,
> > in the latter function, delta is the difference between the last
> > pointer and the new pointer.
> > hence, the calculation of delta in the latter function can be easily
> > less than buffer_size/2 when nperiods = 2.
> >
> > > Anyway, I think that we should leave the check as is for debugging
> > > purposes (we should know that something is failing).
> >
> > sure, it's harmless message (as long as it comes from
> > snd_pcm_update_hw_ptr()) but too annoying.  there are too many
> > sloppy hardwares...
> 
> I've tried to reduce the debugging output for some conditions where
> periods is a small value. In this case, it's really difficult to determine
> which pointer is bad.

thanks, the code looks better than my hack.

yes, it's difficult to guess the wrap in the case nperiods <= 2.
we must trust that interrupts are enough fast not to bring the
critical delays.


Takashi


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