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Subject: Re: [PATCH] U300 sched_clock implementation
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Walleij <linus.ml.walleij@gmail.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu,
       linux-arm-kernel@lists.arm.linux.org.uk
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Date: Mon, 25 May 2009 15:01:50 +0200
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On Mon, 2009-05-25 at 14:13 +0200, Linus Walleij wrote:
> 2009/5/24 Peter Zijlstra <peterz@infradead.org>:
> 
> > On Sat, 2009-05-23 at 23:46 +0200, Linus Walleij wrote:
> >
> >> This overrides the global sched_clock() symbol in the Linux
> >> scheduler with a local implementation which takes advantage of
> >> the timesource in U300 giving a scheduling resolution of 1us. The
> >> solution is the same as found in the OMAP2 core code.
> >
> > We assume sched_clock() to return time in ns (e-9) resolution.
> 
> Yep okay and in this case:
> 
> >> +       ret = (unsigned long long) u300_get_cycles();
> >> +       ret = (ret * clocksource_u300_1mhz.mult_orig) >>
> >> +               clocksource_u300_1mhz.shift;
> >> +       return ret;
> 
> (mult_orig >> shift) == 1000

Ah, ok -- missed that little detail ;-)

> So for each cycle in cyclecount register we return 1000 * cycles
> i.e 1000ns.
> 
> If it looks nicer we can of course simply:
> return (unsigned long long) u300_get_cycles * 1000;
> 
> But the question here is whether this resolution is enough for
> sched_clock() or if it is irrelevant to override sched_clock()
> if it cannot schedule with better precision than 1000 ns.

No anything better than jiffies is good, 1us certainly is worth the
trouble.