[ntp:questions] latencies if going through Linux parallel or serial port drivers?

Unruh unruh-spam at physics.ubc.ca
Tue Sep 23 14:45:49 UTC 2008

I am wondering if anyone has investigated what kind and magnitude of
latencies one gets by going through the parallel and serial port drivers on
Linux? I have a module, gps, which attaches to the parallel port interrupts
to deliver time to one of my systems. I seem to be getting latencies around
the 1usec region (bot from teh jitter on the timing and from the direct
measurement of the time between asserting pin 9 on the port and the receipt
of the interupt-- ie use do_gettimofday just before writing to the parallel
port register and getting the interrupt. 
I have not tried using the standard driver modules on the system
(parport_pc or serial modules) to see what kind of latency they introduce
and was wondering if anyone else has. 
One reason for asking is that if the parport_pc is loaded, when I unload it
to install my module, that unloading leaves the parallel port switched off
and useless, and a reboot is in order. Now there may be a way of switching
on that port again, but I have not been able to delve into the intricacies
of pci programming enough to know how. 

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