[mythtv] missing subtitle packets in embedded vbi

[Martin Barnasconi]

I noticed some strange differences between the embedded vbi data while doing 
video capture in MythTV (using mpegrecorder) and plain recordings from the 
prompt.
Both capture the MPEG PS from /dev/video. MythTV just reads a block of data 
and dumps this into a ringbuffer file, so nothing fancy. Same happens when 
doing it from the prompt: 'cat /dev/video > test.mpg' 
but....
The MythTV ringbuffer file (still an MPEG PS) does misses some vbi packets 
(not all - irregular missed packets), while the file created with 'cat' 
contains all packets!
The most annoying thing is that the missed packets contain subtitle 
information...

Both techniques use identical VBI settings (vbi service set, vbi packet size, 
vbi io size) and codec parameters (bitrate, framerate, etc) so this is not 
the reason.

Has anyone an idea what causes this? Saw some issues on the -dev lists related 
to DMA and memcopy differences, but my knowledge on this is limited.
Please advice.

Martin