[alsa-devel] Alsa-OSS Duplex bug (revisited)

[Takashi Iwai]

At Sun, 3 Jun 2007 20:55:27 +0100,
Alan Horstmann wrote:
> 
> On Friday 01 June 2007 10:56, you wrote:
> > At Thu, 31 May 2007 22:00:25 +0100,
> >
> > Alan Horstmann wrote:
> > > On Wednesday 30 May 2007 07:47, you wrote:
> > > > Alan Horstmann wrote:
> 
> << Edited snippets >>
> 
> > > > fd_playback = open("/dev/dsp", O_WRONLY);
> > > > fd_capture = open("/dev/dsp", O_RDONLY);
> > > >
> > > > > > - change portaudio to open each direction separately, O_RDONLY and
> > > > > >   O_WRONLY at first, then use O_RDWR as fallback
> > > >
> > > > This would be the preferred way.  After all, this is the only way that
> > > > is possible with the existing OSS API when you want to use different
> > > > sample formats, and it is recommended in all cases (see
> > > > <http://manuals.opensound.com/developer/full_duplex.html>).
> > >
> > > My reading of the 4Front info is different.  I read that they recommend
> > > best opening SEPARATE devices, one O_RDONLY and the other O_WRONLY, in
> > > their scheme called 'twodevice'.  It also states in fulldup.c:-
> > >
> > > "... The one device full duplex scheme requires that the device file is
> > > opened with O_RDWR."
> > >
> 
> > Well, I'd say that the current ALSA-OSS implementation _is_ compatible
> > to OSS API.  What they do is also a pure hack, as you know, just a
> > workaround because the API doesn't provide the functionality
> > properly for full-duplex with multi-channels.  The solution might be
> > different, but it's not the question about "compatibility".
> >
> >
> > Following this thread, I come to believe that the problem with
> > audacity is actually the application problem.  It tries to open a
> > full-duplex device with different number of channels for both
> > directions.  It cannot work because it's not defined in API.  OTOH, if
> > the number of channels are same for both directions, it should work
> > with O_RDWR.
> >
> 
> Thanks for sticking with it so far -hopefully we are moving towards a 
> conclusion so that the result is a definitive recommendation for how to solve 
> the problem of the OSS API limitation.
> 
> So that there is no misunderstanding, are you willing to confirm that, as far 
> as Alsa-OSS goes, opening both
> 	input_handle = open("/dev/dsp", O_RDONLY)
> and
> 	output_handle = open("/dev/dsp", O_WRONLY)
> at the same time is a reliable way to open the device bi-directional, enabling 
> different sample formats and/or numbers of channels to be set?  (Actually, I 
> am curious whether different sample rates would also work?)

Yes, although I didn't confirm it.  If it doesn't work, we should
really fix it.  The dirrerent sample rates should work, too, because
they are different streams.

> Provided this is so, the recommendation then for duplex as I understand it 
> is:-
> 	a) where the format and no.channels is the same in each direction, 
> 	    use 
> 		input_handle = output_handle = open("device", O_RDWR)
> 
> 	b) where the 2 directions have different format or no.channels use
> 		input_handle = open("device", O_RDONLY)
> 		output_handle = open("device", O_WRONLY)
> 
> since a) may improve compatibility with other OSS's but b) is necessary due to 
> the limitations of the OSS API, whether or not it works with other OSS's.

Exactly.  But, remember that the solution for this kind of problems
must be practical - if the theory above isn't true for the practice,
it's of course useless :)


Takashi