Re: [RFC][PATCH] new timeofday core subsystem (v. A2)

[Ulrich Windl]

On 24 Jan 2005 at 17:54, Christoph Lameter wrote:

> On Mon, 24 Jan 2005, john stultz wrote:
> 
> > We talked about this last time. I do intend to re-work ntp_scale() so
> > its not a function call, much as you describe above.
> >
> > hopelessly endeavoring,
> 
> hehe.... But seriously: The easiest approach may be to modify the time
> sources to allow a fine tuning of the scaling factor. That way ntp_scale
> may be moved into tick processing where it would adjust the scaling of the
> time sources up or downward. Thus no ntp_scale in the monotonic clock
> processing anymore.

It depends what you want to have between ticks: If your ticks are too wide, the 
clock will do a little jump forward at the start of a new tick; if they are too 
narrow, the clock will jump back a bit at the start of a new tick (assuming tick 
interpolation and tick generation are correlated. (The old kernel code uses a 
constant to scale the timer's register to a tick. However if the time is too fast 
or slow, the interpolation will also be). Those being blessed with a GPS or better 
clock will be able to demonstrate the quality of the code as well as the tuning 
possibilities against frequency errors.

> 
> Monotonic clocks could be calculated
> 
> monotime = ns_at_last_tick + (time_source_cycles_since_tick *
> current_scaling_factor) >> shift_factor.
> 
> This would also be easy to implement in asm if necessary.
> 
> tick processing could then increment or decrement the current scaling
> factor to minimize the error between ticks. It could also add
> nanoseconds to ns_at_last_tick to correct the clock forward.

Is that what corresponds to "adjust_nanoscale()" in my PPSkit?

> 
> With the appropiate shift_factor one should be able to fine tune time much
> more accurately than ntp_scale would do. Over time the necessary
> corrections could be minimized to just adding a few ns once in a while.
> 

Regards,
Ulrich

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