Invalid PCM pointer with CMI8738 and pcm.c buffer underuns

[andoru <andoru.blah@gmail.com>]

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:

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$ 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
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