[ntp:questions] Attn Linux distributors - pse include PPS

William Unruh unruh at invalid.ca
Sun Apr 27 18:07:20 UTC 2014

On 2014-04-27, Rob <nomail at example.com> wrote:
> William Unruh <unruh at invalid.ca> wrote:
>> But those modules give timing to one a few (5-10) usec. because of
>> interrupt handling issues. Your shm solution would seem to me to be more
>> than adequate for any timing requirements if they can be solved with an
>> interrupt driven pps. 
> Well, the kernel PPS API does the timestamping in the interrupt handler
> while my gpsd/SHM solution does timestamping in a user process that is
> marked to be ready to run in an interrupt handler.

?? I do not think so-- the kernel has interrupt handlers which stamp the
PPS input via interrupt routines. They hand the result to gpsd which
puts it into an shm, if I understand correctly. Thus both the kernel and
gpsd use the same interrupt timestamping. 

> Especially on a loaded system it can be less accurate due to scheduling.
> I experimented with marking the process for realtime scheduling, but
> the disadvantage is that it hangs the entire system when it goes in
> a loop due to some bug.
> On my system the offset and jitter for kernel PPS from a professional
> GPS receiver (Datum) are usually 1 or 2 microseconds in the ntpq display,
> but always less than 5.

Yes, that is consistant with what I found with my own interrupt handler.
I got the feeling but have not done any real tests, that the serial port
pps (eg
routine was a bit more ragged.) Or is this module what you mean by
kernel PPS? 

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