[LinuxPPS] PPS ghost source? (fwd)

[Hal V. Engel]

On Sunday 12 July 2009 06:46:12 pm K. Connolly wrote:
> Dear All,
>
> Pardon the repeat message but I was not subscribed to the mailing list
> when I sent my last e-mail (below). I'm afraid I may have missed responses
> (if any). I am subscribed now and look forward to any helpful
> recommendations.
>
> Cheers,
>
> -Kevin
>
> ---------- Forwarded message ----------
> Date: Wed, 8 Jul 2009 19:49:49 -0700 (PDT)
> To: linuxpps at ml.enneenne.com
> Subject: PPS ghost source?
>
> Dear all,
>
> First of all, thank you for the active development and thorough
> documentation -- it has been very useful as I am very new to NTP and
> LinuxPPS.
>
> My current state: I followed the installation wiki and made it through with
> no errors and everything has gone smoothly. However, I am slightly
> perplexed because there seems to be a 1PPS signal whether or not I have my
> GPS device connected to the serial port or not. That is, with nothing
> connected to my computer I can run "./ppstest /dev/pps0" and get:
>
> trying PPS source "/dev/pps0"
> found PPS source "/dev/pps0"
> ok, found 1 source(s), now start fetching data...
> source 0 - assert 1247105841.815051558, sequence: 3323 - clear 0.000000000,
> sequence: 0
> ...etc
>
> I look at /sys/call/pps/pps0/assert and it continues to increment. This is
> concerning, as I don't know whether or not the results I'm getting when I
> do attach my GPS device are coming from the GPS device or this mysterious
> ghost source.

This appears to be from PPS_CLIENT_KTIMER being enabled in the kernel.  
PPS_CLIENT_KTIMER is a fake PPS for testing.  The main clues are that it is on 
even with no device and there are no clear interrupts.  Disable it in the 
kernel and this should go away.

>
> I am very new to this, so I do not quite know what all information would be
> useful to provide. I used git to get the latest LinuxPPS source
> (2.6.28-rc6-20450-gace6faa). I defaultly had /dev/ttySX (X=0,3), and
> /dev/pps0 present (I did not have multiple pps devices). The
> /sys/clas/pps/pps0 tree was also present. The only parts of the wiki
> install I did NOT follow was the issuing of "sudo modprobe 8250" command
> and the editing of the udev conf file. (By the way, it may be just because
> I'm a newb, but the ldattach command is a bit cryptic... What is the "18"
> signify?)
>
> Thanks in advance for any advice and pointers, and my apologies if I've
> forgotten to include some crucial system information. Let me know and I'll
> try and respond with any necessary bits.
>
> Cheers,
>
> -Kevin
>
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