Re: [Jack-Devel] Jack 0.120 - ALSA backend

On Sat, Apr 16, 2011 at 09:47:18AM +0000, Fons Adriaensen wrote:
> A second quick question: have there been changes to
> the ALSA backend recently ?

the diff is attached.
i dont really see anything that might cause it.

the most possible i am currently seeing is 
http://trac.jackaudio.org/changeset/4126

i dont have a loopback here.
but if the problem is easily reproducible, you could just bisect it.

http://eu.kernel.org/pub/software/scm/git/docs/git-bisect.html

i updated the master branch on
git://hochstrom.endofinternet.org/jack.git

git bisect start
git bisect bad
git bisect good 0.118.0 

should get you going.

> 
> 
> I can now trigger the problem reported before quite
> easily.
> 
> 1. Start jack using the ALSA backend and with -n 3.
> 
> 2. Start and wire up jack_iodelay. Make the required
>    external loopback. There is already a chance that
>    it will show the latency for -n 2 instead of the
>    expected one. In any case leave it running.
> 
> 3. Start a second jack client, one that will not 
>    exit cleanly but trigger a 'subgraph time out'
>    when termintated with ctl-C. Jack_iodelay itself
>    will do fine. No need to connect its ports.
> 
> 4. Terminate the second app. There is a good chance
>    that the measured delay will switch between the
>    expected values for -n 2 and -n 3.
> 
> 5. When the latency is the -n 2 value, check that
>    the signal is still passing via the external
>    loopback by disconnecting it for a second.
> 
> 6. Repeat 3,4,5 to taste.

So it randomly siwtches between 2 and 3 ?

> 
> 
> Conclusions:
> 
> * Measuring the latency for -n 2 while it should be
>   the one for -n 3, and the signal used *is* the one
>   via the external loopback is possible only if there
>   is some error in setting up the ALSA device in the
>   backend. There is no way the jack engine could 
>   'short circuit' the loop if signal being measured
>   is passing via the external loopback.
> 
> * This is confirmed by a 'subgraph timeout' triggering
>   a change in measured latency. IIRC this means a
>   restart of the ALSA device. 
> 
> So I'm now pretty sure there is some problem in the
> ALSA backend.

dont really have an idea on this one.

> 
> Ciao,
> 
> -- 
> FA
> 
> 
>    
> 
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-- 
torben Hohn

1302966386.30389_0.ltw:2,a <20110416150607.GE21881 at siel dot b>