[alsa-devel] Alsa plugin unable to be loaded by Pulseaudio

[Ng Oon-Ee]

Hi, if top-posting is frowned upon here I apologize, please let me know
and I won't repeat it.

The "test" device is defined as follows (should have put it in
initially).

pcm.test {
 type plug
 slave { pcm "hw:0,0" }
}

It works with aplay. My issue is that the pulseaudio server cannot load
alsa plugins since 1.0.20 (as far as I can tell, ubuntuforums has a
thread which suggested as such). When I queried the pulse ML, Lennart
posted the below reply, which I've forwarded here to alsa-devel.

Basically, the issue as Lennart sees it is summarized by his last 3
paragraphs (bottom of this mail). He specifies the difference between
the way alsa is reacting to pulse and the way it 'should'.

On Wed, 2009-09-09 at 16:10 +0800, Raymond Yau wrote:
> what is the "test" device ?
> 
> The slave "plug" plugin usually is "hw" device
> 
> Do the following work
> 
> aplay -v -Dtest any.wav
> 
> aplay -v -M -Dtest any.wav
> 
> Why do you need the "plug" plugin ?
> do you mean that pulseaudio server cannot use the exact access , format ,
> rate , channels and route which required by the "test" device
> 
> aplay -v -Dplug:test any.wav
> 
> aplay -v -M -Dplug:test any.wav
> 
> 
> 2009/9/8 Ng Oon-Ee <ngoonee at gmail.com>
> 
> > Hello, please find below an issue I had with loading plugins into
> > pulseaudio. The pulse dev has sent this mail on his list concerning
> > alsa's interaction with pulse, indicating that there is something buggy
> > with how it works. Details below. I'd be willing to post something to
> > the bugtracker, if I only knew what the below means...
> >
> > -------- Forwarded Message --------
> > From: Lennart Poettering <lennart at poettering.net>
> > Reply-to: General PulseAudio Discussion
> > <pulseaudio-discuss at mail.0pointer.de>
> > To: pulseaudio-discuss at mail.0pointer.de
> > Subject: Re: [pulseaudio-discuss] Alsa output devices, able to use alsa
> > plugin?
> > Date: Fri, 4 Sep 2009 01:20:15 +0200
> >
> > On Wed, 26.08.09 12:27, Tanu Kaskinen (tanuk at iki.fi) wrote:
> >
> > > D: alsa-util.c: Trying test with SND_PCM_NO_AUTO_FORMAT ...
> > > D: alsa-util.c: Managed to open test
> > > D: alsa-util.c: Maximum hw buffer size is 371 ms
> > > I: alsa-util.c: snd_pcm_hw_params_set_periods_near() failed: Tiedostoa
> > tai hakemistoa ei ole
> > > I: alsa-util.c: snd_pcm_hw_params_set_buffer_size_near() failed:
> > Tiedostoa tai hakemistoa ei ole
> > > D: alsa-util.c: Trying test without SND_PCM_NO_AUTO_FORMAT ...
> > > D: alsa-util.c: Managed to open test
> > > D: alsa-util.c: Maximum hw buffer size is 371 ms
> > > I: alsa-util.c: snd_pcm_hw_params_set_periods_near() failed: Tiedostoa
> > tai hakemistoa ei ole
> > > I: alsa-util.c: snd_pcm_hw_params_set_buffer_size_near() failed:
> > Tiedostoa tai hakemistoa ei ole
> > > D: alsa-util.c: Trying plug:test with SND_PCM_NO_AUTO_FORMAT ...
> > > D: alsa-util.c: Managed to open plug:test
> > > D: alsa-util.c: Maximum hw buffer size is 371 ms
> > > I: alsa-util.c: snd_pcm_hw_params_set_periods_near() failed: Tiedostoa
> > tai hakemistoa ei ole
> > > I: alsa-util.c: snd_pcm_hw_params_set_buffer_size_near() failed:
> > Tiedostoa tai hakemistoa ei ole
> > > D: alsa-util.c: Trying plug:test without SND_PCM_NO_AUTO_FORMAT ...
> > > D: alsa-util.c: Managed to open plug:test
> > > D: alsa-util.c: Maximum hw buffer size is 371 ms
> > > I: alsa-util.c: snd_pcm_hw_params_set_periods_near() failed: Tiedostoa
> > tai hakemistoa ei ole
> > > I: alsa-util.c: snd_pcm_hw_params_set_buffer_size_near() failed:
> > Tiedostoa tai hakemistoa ei ole
> > > I: alsa-util.c: Failed to set hardware parameters on plug:test: Tiedostoa
> > tai hakemistoa ei ole
> > > E: module.c: Failed to load  module "module-alsa-sink" (argument:
> > "device=test"): initialization failed.
> > >
> > > That "Tiedostoa tai hakemistoa ei ole" error means "No such file or
> > > directory". I tried also with device=hw:0,0 and device=plughw:0,0 and
> > > the first worked while the latter gave the same errors. Since the failed
> > > function calls pertain to buffering, I also tried with tsched=0. It
> > > worked. So, maybe this is a bug in the "plug" alsa plugin, ie. it
> > > doesn't implement the *_near() functions properly.
> > >
> > > Lennart, any insights?
> >
> > I am pretty sure that ALSA is broken here. We nowadays first try to
> > set the number of periods, and then set the buffer size in the
> > hwparams struct. If this fails we print a warning message (the ones
> > you see above with all the vowels regarding
> > snd_pcm_hw_params_set_periods_near()
> > resp. snd_pcm_hw_params_set_buffer_size_near()), but we do not
> > consider this fatal.
> >
> > After doing this we call snd_pcm_hw_params() to activate our
> > settings -- and if this fails then we consider this a fatal problem.
> >
> > Now the brokeness in ALSA is that after refusing our parameters it
> > still fails to configure our settings, i.e. calling the two functions
> > that fail breaks the hwparams setup: if we wouldn't call them, no breakage
> > happens at all.
> >
> > The two functions in question should either succeed or fail, but if
> > they fail they should not modify hwparams in a way that it
> > subsequently becomes unusable for snd_pcm_hw_params().
> >
> > This issue needs to be fixed in ALSA.
> >
> > Lennart
> >
> >
> > _______________________________________________
> > Alsa-devel mailing list
> > Alsa-devel at alsa-project.org
> > http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
> >
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