Driver recovers from underrun in plain ALSA mode, but not in OSS emulation

Driver recovers from underrun in plain ALSA mode, but not in OSS emulation

[Timur Tabi]

I'm stress-testing my sound driver using mplayer to see how it handles underrun
during jumps in CPU loads.  That is, I start playing a movie, and then I cause
the CPU to jump to 100%, and then I drop the CPU back to normal.  During the
times when the CPU is at 100%, I get underruns and other problems.  That's normal.

When I tell mplayer to use the ALSA API, my driver recovers nicely from an
underrun situation when the CPU load drops back to normal.  However, if I tell
mplayer to use the OSS API (i.e. ALSA's OSS emulation), it does not recover from
an underrun situation.  The audio and video playback remains distorted.  I need
to tell mplayer to pause and resume to get it to recover.

Is this normal for ALSA drivers?  Before I try to debug this problem the hard
way, I want to know if this could be the result of a common programming mistake
in my driver or something like similar.

(FYI, the source code for my driver is in sound/soc/fsl/)

-- 
Timur Tabi
Linux kernel developer at Freescale

Re: Driver recovers from underrun in plain ALSA mode, but not in OSS emulation

[Takashi Iwai]

At Fri, 29 Feb 2008 17:56:43 -0600,
Timur Tabi wrote:
> 
> I'm stress-testing my sound driver using mplayer to see how it handles underrun
> during jumps in CPU loads.  That is, I start playing a movie, and then I cause
> the CPU to jump to 100%, and then I drop the CPU back to normal.  During the
> times when the CPU is at 100%, I get underruns and other problems.  That's normal.
> 
> When I tell mplayer to use the ALSA API, my driver recovers nicely from an
> underrun situation when the CPU load drops back to normal.  However, if I tell
> mplayer to use the OSS API (i.e. ALSA's OSS emulation), it does not recover from
> an underrun situation.  The audio and video playback remains distorted.  I need
> to tell mplayer to pause and resume to get it to recover.
> 
> Is this normal for ALSA drivers?  Before I try to debug this problem the hard
> way, I want to know if this could be the result of a common programming mistake
> in my driver or something like similar.

No, it's not normal.

The question is whether OSS emulation really notices the underrun.
More likely, it's caused by the difference of parameters.


Takashi

Re: Driver recovers from underrun in plain ALSA mode, but not in OSS emulation

[Timur Tabi]

Takashi Iwai wrote:

> The question is whether OSS emulation really notices the underrun.
> More likely, it's caused by the difference of parameters.

Can you explain, please?  Which parameters could be different?

-- 
Timur Tabi
Linux kernel developer at Freescale

Re: Driver recovers from underrun in plain ALSA mode, but not in OSS emulation

[Takashi Iwai]

At Tue, 04 Mar 2008 09:20:36 -0600,
Timur Tabi wrote:
> 
> Takashi Iwai wrote:
> 
> > The question is whether OSS emulation really notices the underrun.
> > More likely, it's caused by the difference of parameters.
> 
> Can you explain, please?  Which parameters could be different?

In many cases, some of hw_params or sw_params values are different
even on the same app when different APIs are used.  You can check
pcm*/sub*/* proc files to compare. 
Usually it's because of different constraints, e.g. OSS API requires
power-of-two bytes alignment for buffer controls.


Takashi