[ntp:questions] Building FreeBSD V8.0 kernel for PPS

unruh unruh at wormhole.physics.ubc.ca
Wed Apr 14 21:15:56 UTC 2010


On 2010-04-14, Chuck Swiger <cswiger at mac.com> wrote:
> Hi--
>
> On Apr 14, 2010, at 9:08 AM, David J Taylor wrote:
>> I think you are correct in your final statement, but my understanding (albeit limited) is that kernel PPS provides better performance, and is used for a very limited set of operations (possibly just timestamping the PPS signal).  All the filtering /is/ done in  user-land.  It isn't a pain to do, once you have found the appropriate two or three command-lines for your system.
>
> The main point seems to be that a PPS signal should have much more reliable latency and less jitter, so when you examine the current clock and any adjustment, you can use the PPS signal offset instead of the offset being provided to adjtime() and get better results, assuming the clock is already relatively close to accurate time. [1]

??? Interrupt processing on say a serial or parallel interrups is in the
1usec regime. HOw much better are you claiming that the kernel PPS is?
Seems to me it uses exactly the same serial or parallel interrupts. 


>
> Search for PPS_SYNC in:
>
>   http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/kern/kern_ntptime.c?rev=1.64.2.1.6.1
>
> ...especially hardpps().
>

It looks like he tries to impliment the FLL or PLL loops in kernel
space, which is really not clear to me that it belongs there. Once you
have the timestamped PPS input, there is no need to do the rest of the
calculation in kernel space. 




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