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From: Warner Losh <imp@bsdimp.com>
Date: Mon, 19 Aug 2019 08:32:15 -0600
Message-ID: <CANCZdfoVJ2ryUr+1uh9iCHKLWZO3MONAB9xv3MpWpzYqhycEyw@mail.gmail.com>
Subject: Re: Is it a good idea to use a usb-serial adapter for PPS input? Yes,
it is.
To: Ian Lepore <ian@freebsd.org>
Cc: "O'Connor, Daniel" <darius@dons.net.au>,
"usb@freebsd.org" <usb@freebsd.org>,
"freebsd-arm@FreeBSD.org" <freebsd-arm@freebsd.org>
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On Mon, Aug 19, 2019 at 8:26 AM Ian Lepore <ian@freebsd.org> wrote:
> On Mon, 2019-08-19 at 17:09 +0930, O'Connor, Daniel wrote:
> > > On 12 Aug 2019, at 09:09, O'Connor, Daniel <darius@dons.net.au>
> > > wrote:
> > > > always get lost on single-core processors which are in cpu_idle()
> > > > at
> > > > the time the hardclock interrupt happens. (But that's fixable by
> > > > just
> > > > increasing the number of timehands, I think at least 4 are
> > > > required.)
> > >
> > > OK, how do I increase the number of clock hands?
> >
> > I am going to try this diff but buildkernel is going to take a
> > while...
> >
> > diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
> > index 2656fb4d2..00440b6a2 100644
> > --- a/sys/kern/kern_tc.c
> > +++ b/sys/kern/kern_tc.c
> > @@ -83,8 +83,48 @@ struct timehands {
> > struct timehands *th_next;
> > };
> >
> > -static struct timehands th0;
> > +static struct timehands th2;
> > static struct timehands th1 = {
> > + .th_next = &th2
> > +};
> > +
> > +static struct timehands th3;
> > +static struct timehands th2 = {
> > + .th_next = &th3
> > +};
> > +
> > +static struct timehands th4;
> > +static struct timehands th3 = {
> > + .th_next = &th4
> > +};
> > +
> > +static struct timehands th5;
> > +static struct timehands th4 = {
> > + .th_next = &th5
> > +};
> > +
> > +static struct timehands th6;
> > +static struct timehands th5 = {
> > + .th_next = &th6
> > +};
> > +
> > +static struct timehands th7;
> > +static struct timehands th6 = {
> > + .th_next = &th7
> > +};
> > +
> > +static struct timehands th8;
> > +static struct timehands th7 = {
> > + .th_next = &th8
> > +};
> > +
> > +static struct timehands th9;
> > +static struct timehands th8 = {
> > + .th_next = &th9
> > +};
> > +
> > +static struct timehands th0;
> > +static struct timehands th9 = {
> > .th_next = &th0
> > };
> > static struct timehands th0 = {
> >
>
> Oh, I'm sorry, I forgot to respond about this. Yeah, that patch would
> do it. I think a minimum of 4 sets of timehands are needed for pps
> capture on a single-core system with a latching timecounter like
> dmtpps.
>
Do we have a good (or even a bad) writeup of how to know that 4 is good, or
when 5 or 6 might be needed?
Warner