[mythtv-users] Firewire on a combo backend/frontend

[Steven Adeff]

On 7/31/06, John P Poet <jppoet at gmail.com> wrote:
> Since capturing firewire video data on my backend machine resulted in
> a constant stream of discontinuity errors, I turned my frontend
> machine into a combo backend/frontend, and hooked the firewire up
> there.
>
> On this frontend machine, I am able to capture pretty clean video via
> firewire.  However, if I try to watch anything while it is recording
> via firewire, the video is very jumpy, and I get a lot of "video is 30
> frames behind audio" type messages.  This is on a Athlon X2 4200,
> runing 2.6.17.7.
>
> I figured that it had been long enough since Jim Westfall submitted
> his ieee1394 dma patch that it would be a part of 2.6.17, but I
> discovered it was not.  After applying his patch:
>
> http://www.gossamer-threads.com/lists/mythtv/dev/187783#187783
>
> and building a new kernel, the video playback while recording problem
> is now solved.  I did have to "hand" apply the patch because the
> formatting of the dma.c file in 2.6.17 has changed slightly.  Applying
> this patch caused the CPU usage of mythbackend to drop from 10-11% to
> 3-4% while recording via firewire.
>
> Unfortunately, I now had an audio problem.  The sound would drop-out
> for a fraction of a second, every couple of seconds.  This only
> happened if I was recording via firewire at the same time.  I figured
> my firewire port and my sound card must be sharing an interrupt, but a
> quick look at the motherboard manual disproved that theory.  I then
> check the latencies of those two devices (lspci) and discovered that
> both were at 32.  After adjusting both up to 64 (setpci), my audio
> problem is also solved.
>
> I still have a little problem with channel changes.  The channel
> change program I am using works 100% of the time, when I am testing it
> via the command line.  With Myth in control it only seems to be 75%
> reliable.  This program changes channels on my SA3250HD via firewire.
>
> I figured that to get 100% reliable channel changes, I would switch to
> lirc.  I have an ir-blaster and decided to hook it up along side my
> ir-reciever.  Unfortunately, both are serial and therefore use the
> lirc_serial driver.  It turns out the lirc_serial driver cannot handle
> two serial ports at the same time.  Furthermore, you cannot run two
> lirc_serial drivers at the same time.  To get around this, some people
> have hacked the lirc source tree to create a lirc_serial1.  This is
> actually much harder than it sounds, and is generally a royal PITA.  I
> may try a little harder to get channel changes to work via firewire...

Did you try Jim's patch on your dedicated backend to see if that
solved the dropout issue? I'm still hesitant to roll my own kernel
just for this patch, though now I wonder why its been so long with the
linux1394 people ignoring it.

-- 
Steve
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