[mythtv] Maybe it *is* interrupt latency

Kyle Rose krose+mythtv at krose.org
Mon Jan 10 15:14:01 EST 2005


So I think I had it backwards: the pcHDTV driver seems to report
buffer overrun when the device read buffer fills up because the
application isn't reading fast enough.  This is probably therefore not
the issue.

I did a test today outside of myth with dtvstream and dd.  I set up
dd's from /dev/video1 and /dev/video3 (the two PVR cards) to some file
on the NFS recordings partition, and then started a dtvstream to a
file on the same partition, which somewhat emulates the behavior of
mythtv but allows me finer-grained control over what happens when.

Well, even when running dtvstream with fifo priority, I still get
sync-loss messages intermittently:

Lost sync at input offset 8467669
Achieved sync after 188 bytes.
Lost sync at input offset 12109605
Achieved sync after 188 bytes.
Lost sync at input offset 13320325
Achieved sync after 72 bytes.
Lost sync at input offset 13369465
Achieved sync after 116 bytes.
Lost sync at input offset 15749473
Achieved sync after 188 bytes.
Lost sync at input offset 16711484
Achieved sync after 188 bytes.
Lost sync at input offset 26417172
Achieved sync after 188 bytes.
Lost sync at input offset 54481624
Achieved sync after 564 bytes.

which suggests the driver is losing part of a packet or several
packets because the first byte it encounters isn't the SYNC_BYTE.

Anyone seen this before?  Not clear whether anything short of (a) a
faster process or (b) IRQ threads will solve this problem, and I don't
know if (b) is stable enough for me to want to try again.

Cheers,
Kyle


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