SD card -image- for the beaglebone

[Ian Lepore]

On Wed, 2013-02-06 at 08:16 +0000, Iain Young wrote:
> On 06/02/13 04:18, Ian Lepore wrote:
> 
> > Check out what I taught my beaglebone to do today...
> >
> > root at bb:/ # ntpq -p
> >       remote           refid  st t when poll reach   delay   offset  jitter
> > ==========================================================================
> > oPPS(0)          .PPS.        0 l    4   16  377    0.000    0.399   0.008
> > +dvb.hippie.lan  .GPS.        1 u   55   64  377    1.411    0.666   0.126
> > +tflex.hippie.la .GPS.        1 u   15   64  377    0.901    1.867   0.904
> > +paranoia.hippie .PPS.        1 u   61   64  377    0.396    0.426   0.025
> > xutcnist2.colora .ACTS.       1 u   71   64  376   34.136  -11.277   4.656
> > xnist1.symmetric .ACTS.       1 u   63   64  377   59.880   12.327   2.113
> > -ntp.your.org    .CDMA.       1 u    4   64  377   51.185   -4.212   4.215
> > xntp1.conectiv.c .IRIG.       1 u   51   64  377   99.067   17.784   3.445
> >
> > It turns out the chip has nice timer hardware that can latch the
> > freeruning timercounter in response to the PPS signal.  That allows for
> > a PPS driver that doesn't use interrupts at all.  The timecounter code
> > has a specific interface for such hardware, but there isn't much example
> > code in the base for how to use it.  Soon there'll be more.  :)
>  >
> > I'll attach a patch in case anyone else wants to play with this.  To use
> > it, apply the patch, add "options PPS_SYNC" to the kernel config, and
> > choose which timer pin you want to put the pps on.  The attached patch
> > uses the timer4 pin, which is exposed on the P8 connector as pin #7.
> > You can use any of the timer4-timer7 pins, just set the one you want to
> > use to "input" in the dts and the driver will use it.
> 
> Thanks for that, looks great, and very interesting. I'll do more than
> play with it (well when the 8 Gig card arrives anyway!)
> 
> First thought, extending for multiple PPS :) [Because I can :P]
> 

This kind of no-interrupt PPS driver that's tied closely to the system
timecounter can only handle a single PPS signal.  (There's only one
system timer, and it has just one trigger pin.)  Connecting more than
one PPS is still going to require an interrupt-driven gpio solution.

Even though there are four timers on the chip that have an external
trigger input wired to a pin, it's basically impossible with the current
freebsd timing code to combine triggered-capture of the pps pulse time
with traditional interrupt handling to harvest the captured counter.
When you try to do this, you occasionally end up with a sequence where
the hardware latches the counter, then the timekeeping code switches to
a new set of timehands before the interrupt handler is invoked, then you
end up grabbing a latched counter value that matches up with the wrong
set of timehands, and you get a measurement jump equal to the timer tick
period.  (I know most people don't care about these details, but it's
actually nice to get them in writing somewhere searchable for future
reference.)

I've never configured a system for multiple PPS inputs, that'll be an
interesting thing to play with.  I need to learn more about our gpio
framework (such as it is) and see if it's possible to put in a nice
generic PPS driver that uses it.

-- Ian