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Date: Thu, 04 Jan 2007 19:27:28 +0100
From: Markus Franke <Markus.Franke@domain.hid>
MIME-Version: 1.0
Subject: Re: [Xenomai-help] parallelport module for
	measuring	external	interrupt latency
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To: Anders Blomdell <anders.blomdell@domain.hid>
Cc: xenomai@xenomai.org

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Anders Blomdell wrote:
> The code looks OK, but if you hit some unsigned - signed - unsigned
> conversion during wraparounds, the latencies experienced suggest that
> you run a 1.5 GHz computer (or possibly a 3GHz, never get the 2^32 vs
> 2^31 stuff right):
> 
>   2^31/1.5 -> 1.4317e+09 Hz
> 
> BTW: Why not use the 64 bit value from rdtsc (but storing the diff in a
> 32 bit quantity to get the divide right in kernel space)?.

This was a rather good idea. I changed it in a way you suggested.
Another problem was that I wrote something like:

outb(...)
rdtsc(start_time)

Sometimes the ISR set up end_time before the start_time could be set.
This was another problem. I just swapped the two lines above and reduced
 the measured time by the overhead which is incured by rdtsc(). The
results look quite reasonable.

Regards,
Markus

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