[linux-dvb] Re: Latency ?

[Rene Bartsch <ml@bartschnet.de>]

Am Die, 2003-07-15 um 20.57 schrieb Lauri Tischler:
> I tweaked the saa7146_core.c to set latency to 128,
> mainly to see if I could get my NOVA-T (the DVB 3) to work
> properly, without artefacts, (I have this crappy VIA based
> uATX Motherboard)
> Picture breaks and glitches whenewer there is a write to disk
> and NOVA-T is in use.
> 
> Board 1 is NEXUS-T, Board 2 is NEXUS-S and Board 3 is NOVA-T
> 
> Could somebody explain to me what does the 'Max.Lat =38' mean,
> does that mean that even if I set the latency to 128,
> it will not take, the list below is from 'cat /proc/pci'
> 
>    Bus  0, device   8, function  0:
>      Multimedia controller: Philips Semiconductors SAA7146 (rev 1).
>        IRQ 5.
>        Master Capable.  Latency=128.  Min Gnt=15.Max Lat=38.
>        Non-prefetchable 32 bit memory at 0xe8003000 [0xe80031ff].
>    Bus  0, device   9, function  0:
>      Multimedia controller: Philips Semiconductors SAA7146 (#2) (rev 1).
>        IRQ 10.
>        Master Capable.  Latency=128.  Min Gnt=15.Max Lat=38.
>        Non-prefetchable 32 bit memory at 0xe8000000 [0xe80001ff].
>    Bus  0, device  10, function  0:
>      Multimedia controller: Philips Semiconductors SAA7146 (#3) (rev 1).
>        IRQ 11.
>        Master Capable.  Latency=128.  Min Gnt=15.Max Lat=38.
>        Non-prefetchable 32 bit memory at 0xe8001000 [0xe80011ff].
> 

Use "lspci -vvv" to get human readable values.

The output for a SAA7146 should look like this:

"Latency: 128 (3750ns min, 9500ns max)"

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.

So the overall latency of all devices on an IRQ mustn't exceed the max
latency of the device with the lowest max latency.

To calculate latency:

latency = (min/max latency in ns)/(PCI-bus frequency in MHz); latency
has to be divisible by 16 - so round it up.

-- 
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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