[alsa-devel] Invalid PCM pointer with CMI8738 and pcm.c buffer underuns

[Raymond Yau]

>
> Continued from here:
http://article.gmane.org/gmane.linux.alsa.devel/121575
>
> > http://www.alsa-project.org/main/index.php/XRUN_Debug
> >
> > # Enable basic debugging and dump stack, check hardware pointer on the
> > period update
> > # Usefull to just see, if PCM stream is stopped for a reason (usually
wrong
> > audio process timing from scheduler)
> > # And to check the values from driver
> >
> > echo 11 > /proc/asound/card0/pcm0p/xrun_debug
>
> Okay, I not compiled my own kernel with CONFIG_SND_PCM_XRUN_DEBUG,
> CONFIG_SND_VERBOSE_PROCFS, CONFIG_SND_DEBUG all enabled in the kernel
> configuration, I have an xrun_debug file, but it doesn't seem to list
> anything, then I also tried with cat instead of echo, and just a 0 showed
up:
>
> ___________________________________________________________________
> $ echo 11 > /proc/asound/card0/pcm0p/xrun_debug
> bash: /proc/asound/card0/pcm0p/xrun_debug: Permission denied
>
> # echo 11 > /proc/asound/card0/pcm0p/xrun_debug
> bash: /proc/asound/card0/pcm0p/xrun_debug: Permission denied
>
> $ cat /proc/asound/card0/pcm0p/xrun_debug
> 0

You need root privilege to debug

sudo echo 11 > /proc/asound/card0/pcm0p/xrun_debug

Check the system log , you should find the hwptr change at 10ms interval if
vlc use period time of 10ms

Try different period time/size to verify that the hardware really support
any  arbitrary period time/size requested by the application ?