[LinuxPPS] LinuxPPS GPIO Generator

[Tom Kurmann]

Hello,

For a project I need to to synchronize a handful of Cortex A8 based devices
to a master clock so I can time stamp data which is coming from an ADC. The
master clock should be synchronized to GPS and the slaves will be locally
synchronized with PTP. The ADCs have a SYNC input which I would like to sync
with a GPIO generating a PPS. Since I have been away from the Linux Kernel
for a couple of years these questions may sound dumb, but a lot has changed
since I last worked on the kernel...I need a synchronization of around 50 us
in the network.

- Is it correct that the IRQ time stamping for say a GPS PPS happens before
the ISR gets called? Since the interrupt latency on the cortex a8 is max.
around 120 us and the jitter is quit high i would assume that the time
stamping has to be done before the ISR gets called.

- I see that there are no drivers yet to generate a PPS on a GPIO, which
would mean I could try to contribute something here :) Since hrtimers has
been implemented I think it should be possible to generate an interrupt
every full second for a gpio (the full second is to ensure that the clocks
are synced), but again the problem with the interrupt latency comes up. If I
have say an average of 100 us latency and a jitter of 50 us I would be at
the limits of the specifications. Or is the hrtimer interrupt handled
differently from external interrupts?

Thanks
Tom
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