[linux-dvb] Re: Latency ?

[Rene Bartsch <ml@bartschnet.de>]

Am Die, 2003-07-15 um 23.13 schrieb Robert Schlabbach:
> From: "Rene Bartsch" <ml@bartschnet.de>
> > Use "lspci -vvv" to get human readable values.
> >
> > The output for a SAA7146 should look like this:
> > "Latency: 128 (3750ns min, 9500ns max)"
> 
> Hmm, how do they come up with these numbers? I thought these latencies were
> in PCI clock cycles. Since the PCI clock is 33MHz, one cycle is 30ns. Thus,
> 128 cycles would be 3840ns, and 38 cycles would be 1140ns...?

I don't know in which unit it is in /proc, but "lspci -vvv" is in
nanoseconds.

> 
> > Min Latency is the minimum latency necessary for the SAA7146 to send all
> > data off from his queue and max latency is the maximum latency until to
> > be in turn again before the queue of the SAA7146 gets an overflow.
> 
> I don't understand :( What is the "minimum latency"? The "Latency: 128"
> number above? And what does the "Minimum Grant" number mean?
> 

The 3750ns is the minimum latency. That means the SAA7146 needs at least
3750 ns in one piece to clear it's queue into the PCI-bus. 9500 ns is
the maximum latency. That means 9500 ns after the SAA7146 has cleared
it's queue the clear is filled up to the brim and data is lost if the
SAA7146 can't clear it's queue for new data by sending to the PCI-bus.

For calculation:

3750 ns : 33 MHz = 100
 450 ns : 33 MHz =  13
  31 ns : 33 Mhz =   0,xxx
----------------------------
                   113 	-> next value being a multiple of 16 is 128
			-> 128 in HEX is 80.

> more confused than enlightened,

Some spotlights in sight? ;o)


-- 
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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