[Bug 227687] PPS echo pulse on ppbus too short

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=227687

            Bug ID: 227687
           Summary: PPS echo pulse on ppbus too short
           Product: Base System
           Version: 11.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: lukas at fridolin.com

PPS echo signals are not detectable on the parallel port. The irq handler in
ppbus/pps.c creates the echo pulse by setting AUTOFEED, handling the pulse and
immediately unsetting AUTOFEED. The "handling the pulse" phase is not long
enough to create a usable echo pulse. Adding a DELAY of 100ms before disabling
the bit creates good pulses, however this is not a feasible solution in an
interrupt handler.

Fix:
Add a delay (maybe 100ms) before disabling the AUTOFEED bit.

How-To-Repeat:
Connect a PPS source (e.g. GPS receiver, signal generator) via the parallel
port (pin 10). Set PPS_ECHOASSERT (with pps-api utility in ntp). Try to read
the echo on pin 14 (e.g. with oscilloscope, LED, GPS receiver...).

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