[ntp:questions] Timing issue with Linux and kernel PPS?

David Taylor david-taylor at blueyonder.co.uk.invalid
Sat Nov 17 20:30:16 UTC 2012

I'm seeing behaviour on rebooting a Raspberry Pi Linux system which is 
stopping the PPS signal getting through to NTP.  It seems that the GPS 
device does not emit any pulses on the PPS line for some time after it 
has been powered up.  This I understand and it's the time taken for a 
good enough lock to be acquired.

However, it seems that if the PPS part of the kernel hasn't seen a 
transition, it does not create the pps device at least until the first 
transition is seen.  The delay seems to be some 133 seconds here:

$ dmesg | grep pps
[ 12.797224] pps_core: LinuxPPS API ver. 1 registered
[ 12.803850] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo 
tti <giometti at linux.it>
[ 12.824858] pps pps0: new PPS source pps-gpio.-1
[ 12.832182] pps pps0: Registered IRQ 194 as PPS source
[ 133.043038] pps_ldisc: PPS line discipline registered
[ 133.044841] pps pps1: new PPS source acm0
[ 133.044879] pps pps1: source "/dev/ttyACM0" added

This seems to affect ntpd by it never seeing a response from the ATOM 
driver (but I am not 100% sure about this).  A reboot (i.e. with the GPS 
delivering PPS signals immediately) fixes the problem.

Does this seem possible, and how might it be fixed?
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