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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