Debugging a strange issue with multi card on JACK2

[Stéphane Letz]

Hi all,

I'm currently trying to debug what seems to be a long standing bug in  
ALSA backend behaviour under JACK2 when used with multi cards (but  
that maybe happens in other situations as well...). Rick Wright (CC)  
is using 2 M-Audio 1010LT' card and is reporting problems under JACK2  
(and not in JACK1) . The command is the following :

jackd -v -S -R -P75 -dalsa -r48000 -p1024 -n2 -D -Cmulti_capture -
Pmulti_playback

and  ALSA backend fails in first "alsa_driver_wait" (that report a  
nframes = 0), then the following code is taken:

  if (nframes == 0) {
         /* we detected an xrun and restarted: notify
          * clients about the delay.
          */
         jack_log("ALSA XRun wait_status = %d", wait_status);
         NotifyXRun(fBeginDateUst, fDelayedUsecs);
         return -1;
     }

the cycle is "skipped" but ALSA backend never recovers, and continue  
to fails forever...

If we activate DEBUG_WAKEUP flags to see a bit more what happens, then  
things start to work.. and we have the following kind of trace:

Jack: JackThreadedDriver::Init IsRealTime
678676670960: checked 2 fds, 21266 usecs since poll entered
678676670960 playback stream ready
678676670960 capture stream ready
wakeup complete, avail = 1024, pavail = 1025 cavail = 1024
Jack: fPollTable i = 1 fd = 5
Jack: JackRequest::Notification
Jack: JackEngine::NotifyClients: no callback for event = 4
Jack: JackEngine::NotifyClients: no callback for event = 4
Jack: JackEngine::NotifyClients: no callback for event = 4
678676692285: checked 2 fds, 21117 usecs since poll entered
678676692285 playback stream ready
678676692285 capture stream ready
wakeup complete, avail = 1024, pavail = 1025 cavail = 1024
678676713617: checked 2 fds, 21234 usecs since poll entered
678676713617 playback stream ready
678676713617 capture stream ready
wakeup complete, avail = 1024, pavail = 1025 cavail = 1024
678676734946: checked 2 fds, 21245 usecs since poll entered
678676734946 playback stream ready
678676734946 capture stream ready
wakeup complete, avail = 1024, pavail = 1025 cavail = 1024
678676756281: checked 2 fds, 21249 usecs since poll entered
678676756281 playback stream ready

So basically it seems that a subtle timing difference, that cause the  
first cycle to fail (or not when DEBUG_WAKEUP is on), and recovery  
code does not work as expected.

Anybody has a brilliant idea of what could be the issue? Or any advice  
how to debug that?

Thanks

Stephane


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[John Rigg-3]

I haven't had a chance to try Jack2 recently with my multi Delta 1010 system,
but Jack1 reports a lot of xruns which don't occur with a single Delta 1010.
These don't have any audible effect so I've been ignoring them. The alsa driver
isn't reporting xruns, just jackd.

I've tried looking at sample values where xruns been indicated by Ardour, but
couldn't find any discontinuities in the recorded audio. Is it possible that
jackd's xrun detection is a little paranoid in this situation? (The other
possibility is that my ears aren't as good as I think they are and my method
of dumping sample values into a text file for inspection is flawed, but I'm
hoping that isn't the case).

John
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