Re: [patch] Generic time trailing clean-ups

[Jun Sun <jsun@mvista.com>]

On Tue, Aug 12, 2003 at 03:34:25PM +0200, Maciej W. Rozycki wrote:
> On Mon, 11 Aug 2003, Jun Sun wrote:
> 
> > >  Here is hopefully the final part (for now) of the generic time changes.
> > > It addresses the following problems:
> > > 
> > > -	 */
> > > -	if (!jiffies) {
> > > -		timerhi = timerlo = 0;
> > > -		mips_hpt_init(count);
> > > +	 *
> > > +	 * The first timer interrupt comes late as interrupts are
> > > +	 * enabled long after timers are initialized.  Therefore the
> > > +	 * high precision timer is fast, leading to wrong gettimeoffset()
> > > +	 * calculations.  We deal with it by setting it based on the
> > > +	 * number of its ticks between the second and the third interrupt.
> > > +	 * That is still somewhat imprecise, but it's a good estimate.
> > > +	 * --macro
> > > +	 */
> > > +	j = jiffies;
> > > +	if (j < 4) {
> > > +		static unsigned int prev_count;
> > > +		static int hpt_initialized;
> > > +
> > > +		switch (j) {
> > > +		case 0:
> > > +			timerhi = timerlo = 0;
> > > +			mips_hpt_init(count);
> > > +			break;
> > > +		case 2:
> > > +			prev_count = count;
> > > +			break;
> > > +		case 3:
> > > +			if (!hpt_initialized) {
> > > +				unsigned int c3 = 3 * (count - prev_count);
> > > +
> > > +				timerhi = 0;
> > > +				timerlo = c3;
> > > +				mips_hpt_init(count - c3);
> > > +				hpt_initialized = 1;
> > > +			}
> > > +			break;
> > > +		default:
> > > +			break;
> > > +		}
> > >  	}
> > > 
> > 
> > The first gettimeoffset() call is way after many jiffies (~50 normally?).  Such
> > an estimate is not necessary.
> 
>  As a number of interrupts is lost (at least half a second worth of; it
> depends on how long console_init() executes), it takes a few minutes for
> gettimeoffset() to recover from the error -- r0 (which is the number of
> HPT ticks in a jiffy) is too high.  As a result, offsets within jiffies as
> calculated by gettimeoffset() are distributed unevenly.  You may not care,
> but I use NTP on my systems and I do care.  With the above initialization,
> r0 is almost correct from the beginning and after a few minutes of uptime
> the error is no higher than one tick. 
> 
>  The fixed_rate_gettimeoffset() backend doesn't care but the calibrate_*()
> ones do.

Perhaps we should always calibrate the counter frequency and forget about
all those calibrate_*() routines.  This will allow us to get rid of a few
funtions.  Plus knowing the frequency is generally a good thing (for some
performance measurement, for example).

I have a patch floating around just doing that, and in MontaVista tree
we have already done for a long time.

The patch is at 

http://linux.junsun.net/patches/oss.sgi.com/experimental/011128.calibrate_mips_counter.patch

(Wow!  I can't believe it was done almost two years ago.)

Jun