[time-nuts] Raspberry Pi NTP server

[Trent Piepho]

On Sun, Jul 12, 2020 at 11:51 AM jimlux <jimlux at earthlink.net> wrote:

>
> Is the PPS via USB CTS stamped at interrupt time? or is it stamped
> higher up in the stack?
>
> I started tracing this out, but then decided I'm not going to be writing
> Linux USB drivers any time soon, so gave it up.
>
> I could easily imagine that the interrupt comes in, marks a thread as
> "ready to run" and the "oh CTS has changed state" is detected at a
> higher level.

Best way to see this is to do live tracing.  Use perf to turn on event
captures with IRQs, context switches, PPS events, USB events.  Then in
something like Trace Compass you can see the timeline of what is
actually happening.  The hard IRQ context first, then maybe a context
switch into a kernel thread that was made runnable by an action of the
irq handler.