Fix default disk_io_chunk_frames for diskstream so that it

isn't larger than the default playback buffer size when
starting Ardour without a previous ~/.config; having
disk_io_chunk_frames larger violates basic design
assumptions and causes phantom underruns in playback
(#4308).


git-svn-id: svn://localhost/ardour2/branches/3.0@10079 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/ardour/audio_diskstream.cc

@@ -1021,6 +1021,10 @@ AudioDiskstream::_do_refill (Sample* mixdown_buffer, float* gain_buffer)
 
 	   at higher speeds, just do it because the sync between butler
 	   and audio thread may not be good enough.
+
+	   Note: it is a design assumption that disk_io_chunk_frames is smaller
+	   than the playback buffer size, so this check should never trip when
+	   the playback buffer is empty.
 	*/
 
 	if ((total_space < disk_io_chunk_frames) && fabs (_actual_speed) < 2.0f) {

libs/ardour/diskstream.cc

@@ -70,7 +70,7 @@ using namespace PBD;
  * default from configuration_vars.h).  0 is not a good value for
  * allocating buffer sizes..
  */
-ARDOUR::framecnt_t Diskstream::disk_io_chunk_frames = 1024 * 256;
+ARDOUR::framecnt_t Diskstream::disk_io_chunk_frames = 1024 * 256 / sizeof (Sample);
 
 PBD::Signal0<void>                Diskstream::DiskOverrun;
 PBD::Signal0<void>                Diskstream::DiskUnderrun;