[linux-dvb] Re: Removing PCI-latency settings from CVS

[Rene Bartsch <ml@bartschnet.de>]

Am Fre, 2003-08-01 um 14.22 schrieb Dr. Werner Fink:
> On Fri, Aug 01, 2003 at 01:20:21PM +0200, Oliver Endriss wrote:
> > >
> > > Wouldn't it be better to do the calculating instead of breaking other
> > > systems which do it better _with_ a latency of 64?
> > 

What systems should that be? The SAA7146 needs at least 3750 ns access
to the PCI-bus in one turn to deliver it's data. This is a latency of
128 on 33 MHz and 64 on 66 MHz systems. So a latency of 64 makes only
sense for 66 MHz PCI-bus. Otherwise you will loose data when setting
latency to 64 on a 33 MHz system.

> > Sure, if someone really *knows* how to do that correctly. ;-)
> > (I don't know enough about the PCI bus specs.)
> > 
> > I considered the following:
> > - DVB kernel does not set this value.
> > - If required you can simply put setpci into your local boot script.
> > - Many bioses allow to specify the default latency value.
> >   (64 seems to be a common default value.)

That's for all devices on the board. But this may exceed the max.
latency of a device.

> > - The value 64 produces artefacts on some systems.

I'm maintaining four different systems and all have artefacts with a
latency of 64 but work flawless with a latency of 128 - see above for
reason.

> > - A built-in value makes it harder to use setpci to specify the correct

And overwrites setpci in case of a driver-reload. Quite annoying if
you're not in and all recordings get messed up.

> >   value. You have to call setpci each time after the driver has been 
> >   loaded.
> > - I'm not sure whether the DVB driver ist the correct place to do it.

You're right, it's the kernel to handle the PCI-bus.

> > 
> > Anyway, if you prefer I could make it a module parameter.
> > (default is 0 --> no change, otherwise set the specified value).
> 
> Yep ... would be perfect for me.  This will help to do recording and
> replaying on the same card with(out|less) glitches at least on my
> 33MHz PCI system ;^) ... OK beside this a setpci command would
> also help but requires lspci for getting the correct device/vendor.

Put "post-install dvb-ttpci /usr/bin/setpci -d 1131:7146
latency_timer=80" into modules.conf (80 = latency of 128, 40 = 64).

Maybe your problem is vice versa. If your overall-latency exceeds the
max. latenty of a card sharing with your DVB-card, this may make the
PCI-bus swallow and break time-shift, as data doesn't get delivered fast
enough. So you don't have the problem that 128 is to much for your
DVB-card, but for the overall-latency of one of your cards.

You can only solve such a problem by reconfiguring your cards in the
slots - but you'll need lspci for checking, of course.


-- 
Rene Bartsch
Faculties MNI
Computer Science 8th Semester
FH Giessen/Friedberg, Germany

Facsimile/Phone: +49 7 00/72 27 87 24
Mail:  rene@bartschnet.de



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