Subject: Re: [RFC PATCH] rtc: add rtc_systohc for ntp use

From: Gabriel Paubert <[email-masked]>
Date: Tue, 11 Nov 2008 18:17:34 +0100

On Tue, Nov 11, 2008 at 02:10:39PM +0000, David Woodhouse wrote:
> On Mon, 2008-11-10 at 16:40 +0100, Alessandro Zummo wrote:
> > Adds in-kernel hctosys functionality that can
> > be used by ntp sync code.
> > 
> > This is an RFC and has not been tested, I just want
> > to check if something similar could solve the problems
> > of those who want the NTP sync mode.
> 
> You might do better to open the device once and keep it open, rather
> than taking the mutex and opening it again _during_ each call? You're
> going to be perturbing the timing by doing that.
> 
> I believe you were also concerned that some device wouldn't want the
> behaviour given by the existing sync_cmos_clock() function and workqueue
> stuff in kernel/ntp.c, where we update the clock precisely half-way
> through the second?

FYI, the RTC that are in my PPC machines definitely want an update
on the whole second, within a few hundred microseconds that is, since
it seems that not all LSB bits are reset by a write of the time: the 
jitter is much higher than the 31µs peak-peak you'd expect from a 32768Hz
crystal but I can't remember how much I found.

	Regards,
	Gabriel