Re: hw_pointer error message triggered without need

[Takashi Iwai <tiwai@suse.de>]

At Tue, 21 Oct 2003 22:00:33 -0400,
Manuel Jander wrote:
> 
> Hallo,
> 
> There really something wrong. 
> 
> 718, max jitter = 8192): wrong interrupt acknowledge?
> ALSA sound/core/pcm_lib.c:157: Unexpected hw_pointer value (stream = 0,
> delta: -297, max jitter = 1024): wrong interrupt acknowledge?
> ALSA sound/core/pcm_lib.c:157: Unexpected hw_pointer value (stream = 0,
> delta: -722, max jitter = 8192): wrong interrupt acknowledge?
> ALSA sound/core/pcm_lib.c:157: Unexpected hw_pointer value (stream = 0,
> delta: -301, max jitter = 1024): wrong interrupt acknowledge?
> ALSA sound/core/pcm_lib.c:157: Unexpected hw_pointer value (stream = 0,
> delta: -726, max jitter = 8192): wrong interrupt acknowledge?
> ALSA sound/core/pcm_lib.c:157: Unexpected hw_pointer value (stream = 0,
> delta: -305, max jitter = 1024): wrong interrupt acknowledge?
> ALSA sound/core/pcm_lib.c:157: Unexpected hw_pointer value (stream = 0,
> delta: -730, max jitter = 8192): wrong interrupt acknowledge?
> 
> How do you explain this ???

the hwptr is not updated properly.
> 
> At first i would vote for eliminating that "-" since it leads one to
> think into negative deltas, that really dones make any sense.
 
yes, it's a bit confusing, but minus makes sense because the delta
value (= new - old) is in fact negative.  in the calculation,
(old - new) is used to avoid the sign conversion, though.

> And why is the "max jitter" changing all the time ?

do they come from the same stream?

anyway, your code above is too old now.  the line doesn't correspond
to the latest version.  so i cannot say exactly.

> And the delta is
> smaller than the "max jitter", so why should it complain ??

it's not the comparison like delta > max_jitter.
it's the check for the overlap of the ring buffer.
the pcm routine tries to detect whether the hwptr moved across the
ring buffer boundary.  and for that, delta > -max_jitter is
performed.  since delta in the code is (old - new), the comparison is
reversed.

> I just want to know if me soundboard is smoking crack or what ?

if this comes from snd_pcm_update_hw_ptr_interrupt(), then it's a
serious problem.  if it comes from snd_pcm_update_hw_ptr(), it's
mostly harmless, as i wrote in the previous post.

the alsa pcm tries to update hwptr often, not only when the interrupts
are issued and snd_pcm_period_elapsed() is called.
(e.g. at each read or write call, or when RESET, FORWARD, REWIND,
HWSYNC or DELAY ioctl is used.)   this problem happens likely in such
a case, especially when interrupt handling is sloppy, or pointer
callback is not reliable.


Takashi


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