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

[David Brownell <david-b@pacbell.net>]

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.

- Dave