Re: IA64 implementation of timesource for new time of day subsystem

[john stultz <johnstul@us.ibm.com>]

On Mon, 2005-05-16 at 13:27 -0700, Christoph Lameter wrote:
> On Mon, 16 May 2005, john stultz wrote:
> 
> > 
> > No. I intend to preserve the existing functionality (and performance) of
> > the current code. The current timeofday core should allow for this (as I
> > described in my last mail), so really its just a matter of either me or
> > someone else getting around to properly converting that arch with the
> > help of the arch maintainer. Until the arch is really ready to use the
> > new timeofday core, no changes are necessary.
> 
> Its not an arch specific issue. The time sources need to have a field that 
> specifies that jitter protection is needed and there needs to be some 
> logic to implement it. Otherwise we have to develop special functions for 
> each timesource that deal with jitter protection. 

You've only pointed out two timesources that could want this (ITC and
TSC), so I think its reasonable to do your jitter handling in the
timesource driver. If there are other arches that have non hardware
synced per-cpu counters, then it would be something to consider.

> Function will make a 
> fastcall for the clocks that use jitter protection not possible and thus 
> timer access will slow down.


I disagree. I already explained how this can be done via the
arch_update_vsyscall_gtod() interface by special casing for this
specific well known time source.


> > What I'm trying to shake out, with Christoph's help, is any major
> > limitations in the core timeofday code that would keep an arch from
> > being able to use it.  I feel Christoph's concerns have been addressed,
> > but please let me know if you disagree.
> 
> Please add jitter protection to the arch independent code.

If more timesources need that functionality, then I'll be happy to.
Until then it should stay in the ia64 specific itc driver and fastcall
code.

thanks
-john