[systemd-devel] Support machines with multiple RTCs?

[Martin Langhoff]

On Nov 22, 2011 5:58 PM, "Kay Sievers" <kay.sievers at vrfy.org> wrote:
>
> On Tue, Nov 22, 2011 at 23:42, Martin Langhoff
> <martin.langhoff at gmail.com> wrote:
> > On Tue, Nov 22, 2011 at 3:21 PM, Kay Sievers <kay.sievers at vrfy.org>
wrote:
> >> Yeah, that's intentional. Udev on other platforms can't know which rtc
> >> should be the preferred one.
> >
> > Well, now you can: if it says hctosys == 1, it means that the kernel
> > config told it to pick that one, and that it's sync'd the system time
> > to it.
>
> You have rtc1 set in the kernel config for your box?

Correct. And rtc0 is only good for rtcwake -- won't hold "real" clock time,
because it isn't battery-backed.

"My box" is the XO-1.75 platform, of which we expect a few million will be
produced starting soon.

Future kernels may bring them up in different order, but that's a different
pile of pain.

> >> This link is mainly for backwards compat, because in earlier days it
> >> was a real kernel device.
> >
> > Well, the symlink has a very practical use! If you have several RTCs,
> > you can use udev to symlink rtc to it, and it gets htclock to DTRT.
> >
> > On our XO-1.75, the "real" rtc is rtc1.
> >
> > Next, we got to get src/utils.c to prefer "rtc" if it finds it ;-)
>
> I guess udev should claim /dev/rtc for the rtc device that has hctosys ==
1 set?

Yep. It's a trivial udev rule, perhaps belongs to Fedora's initscripts?

cheers,



m
{ Martin Langhoff - one laptop per child }
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