Subject: Re: [RFC] RTC subsystem

From: Alessandro Zummo <[email-masked]>
Date: Thu, 22 Dec 2005 07:48:50 +0100

On Thu, 22 Dec 2005 00:18:33 +0100
Simon Richter <[email-masked]> wrote:

> A good ntpd will adjust the speed rather than write to the clock; the
> ntpd shipped by most distributions can already handle multiple time sources.

 Yes, but there's the kernel who writes to the clock,
 for example http://lxr.linux.no/source/arch/arm/kernel/time.c?a=arm#L103 .

> >  later. The /dev/rtc interface only supports one clock.
> >  It can either be extended to have /dev/rtcX or we
> >  can extend the sysfs one to allow clock updating.
> 
> /dev is the way to go IMO. As far as I've understood sysfs, it carries
> meta information about devices and drivers only, the actual
> communication then happens through device nodes still.

 Ok. We can use dynamic device numbers and go for the /dev
 interface. 

> 
> >  NTP mode could then be adjusted to update one or more
> >  of the rtcs. Maybe each RTC could have an attribute
> >  (let's say /sys/class/rtc/rtcX/ntp) which tells the
> >  kernel whether to update it or not.
> 
> That's entirely a userspace thing. What the userspace needs to know from
> the kernel is whether the clock is writable and whether its speed can be
> adjusted.

 agreed. there are are also some variables of interest in
 http://lxr.linux.no/source/include/linux/timex.h?a=arm#L188
 some of them may be usefully exported in sysfs.

-- 

 Best regards,

 Alessandro Zummo,
  Tower Technologies - Turin, Italy

  http://www.towertech.it