kern/52459: audio_fill_silence() stops playback.

[isaki%NetBSD.org@localhost]

>Number:         52459
>Category:       kern
>Synopsis:       audio_fill_silence() stops playback.
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Aug 03 04:05:00 +0000 2017
>Originator:     Tetsuya Isaki
>Release:        NetBSD-current
>Organization:
NetBSD
>Environment:
NetBSD 8.99.1 x68k
>Description:
If the audio play chain enters audio_fill_silence() in
audio_pint() even once, playback stops.

The audio play chain is:
- audiostartp() calls mix_write().
- mix_write() calls hw_if->start_output().
- start_output() calls audio_pint() if DMA transfer is done.

and then current audio_pint() does:
  :
  if (available datasize < blksize) {
    audio_fill_silence()
  } else {
    mix_write()
  }
  :

So if the available data is not enough even once,
mix_write() is not called anymore and audio playback stops.
I think that it should call mix_write() even if it calls
audio_fill_silence().

Index: sys/dev/audio.c
===================================================================
RCS file: /cvsroot/src/sys/dev/audio.c,v
retrieving revision 1.384
diff -u -r1.384 audio.c
--- sys/dev/audio.c	30 Jul 2017 02:41:58 -0000	1.384
+++ sys/dev/audio.c	3 Aug 2017 03:32:41 -0000
@@ -3691,12 +3691,10 @@
 			    vc->sc_mpr.s.inp, cc);
 			used -= cc;
 		}
-		goto wake_mix;
 	}
 
 	mix_write(sc);
 
-wake_mix:
 	cv_broadcast(&sc->sc_condvar);
 }
 

>How-To-Repeat:
I don't know how to repeat on your environment.
>Fix:
See above patch.