[LinuxPPS] multiple access to /dev/ppsN entries

[Ben Gardiner]

On Wed, Mar 17, 2010 at 3:55 PM, Bernhard Schiffner
<bernhard at schiffner-limbach.de> wrote:
>
> On Wednesday, 17. March 2010 20:16:52 Miroslav Lichvar wrote:
> > On Wed, Mar 17, 2010 at 12:15:59PM -0600, clemens at dwf.com wrote:
> > > > Unless they set the PPS parameters differently, the first started
> > > > application could be receiving something else than it's expecting.
> > >
> > > How so?
> > > Seems that they should both be reading the same register.
> >
> > I tried opening the same PPS device twice, one for assert events and
> > one for clear events, and only the later worked, so I'm assuming the
> > setting is per device instead of per open. If both capture same
> > events, it works fine.
> >
> > On Wed, Mar 17, 2010 at 07:21:09PM +0100, Bernhard Schiffner wrote:
> > > because I'am a little bit out of business here: does cat /dev/pps* return
> > > nowadays something usefull?
> > > IIR it is intended as a container for functioncalls (RCF2783) and does
> > > provide -EINVAL to a normal read(2).
> >
> > # cat /dev/pps0
> > cat: /dev/pps0: Invalid argument
> Ok. Nothing changed.
>
> > > I used a patch from George Spelvin and was _very_ happy to have a (multi)
> > > readable  /dev/pps*.
> > >
> > > It starts with
> > > http://ml.enneenne.com/pipermail/linuxpps/2009-February/002831.html
> > >
> > > and George sent me (later, PM) his patch-queue against 2.6.32-rc6. If
> > > there is interest in, I'll ask him to post this to the list.
> >
> > Not sure if this is good enough for your need, but it's possible to
> > read the timestamps also directly from /sys/class/pps/pps0/{assert,clear}.
>  Yes, I know this.
>
> Again:
> 1.)  An "Unixer" simply expects something useable from reading a character
> device.
> 2.) The RFC2783 does neither prohibit nor force a human readable readout of
> /dev/pps.
> 3.) Mar 2009 were some related discussions about (IIRC) /dev/time(?) | Plan9 |
> BSD etc. There was no conclusion about a standardized format of timestamps. I
> don't know the state of the onion today.
> 4.) Mar 2009 were other priorities to work on first (inclusion into mainline,
> hick hack about ldisk etc.)
> 5.) I think it's a good time to discuss this topic again.
> 6.) This thread is a base to do so.
> 7.) Last and very least George and me want this feature in one or an other
> form implemented.  A patch exists.
>
> What's your POW?

Thank you Reg, Miroslav and Bernhard for your rapid reply with expert
opinions. We're happy to hear that multiple access to a single
/dev/ppsN entry is possible with the caveat that both processes be
listening for the same events.

I will bow out of the oncoming debate on the output of 'cat /dev/pps*'
due to lack of qualifications. :)

Thank you again,

Ben Gardiner