Re: [RFC] [PATCH] ARM: OMAP1: Add clocksource driver for OMAP1

[tony@atomide.com]

* David Brownell <david-b@pacbell.net> [061130 13:30]:
> On Thursday 30 November 2006 11:57 am, Woodruff, Richard wrote:
> 
> > That code uses two timers to account for isr latency and to provide
> > accurate timestamps.  One timer is periodic and the other is free
> > running. The 2nd is used as a relative reference.  Along with being used
> > as a reference the 2nd free runner uses its current value and its roll
> > over interrupt to sum/calculate the number of ns the system has been up.
> > This enables sched_clock to be enabled with accuracy (vs. just
> > multiplying times jiffies).
> > 
> > The single timer method does not do either of these.  The code was
> > reasonably clear, I also sent in a path long ago to enable this.  Is
> > there some aspect of this which I mis-understood?
> 
> If you did, I had the same mis-understanding.  Ergo the patch(es) I've
> sent.
> 
> I don't think the roll-over IRQ is significant in the new clock framework,
> but I do think that the second MPU timer is important if there's a chance
> of lost timer IRQs ... probably negligible at HZ=100, less so when the option
> of HZ=1000 is configured.
> 
> Plus, ISTR that the upcoming generic dyntick support will need to reprogram
> the periodic IRQ, just like the current OMAP-specific code using timer32k.

Yeah, with the upcoming timer code time is updated from a freely running
timer, so time should be always as accurate as the hardware timer.

Regards,

Tony