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Date: Mon, 25 May 2009 14:13:41 +0200
Message-ID: <63386a3d0905250513q6ca56eeepcf7bebe46c447fb4@mail.gmail.com>
Subject: Re: [PATCH] U300 sched_clock implementation
From: Linus Walleij <linus.ml.walleij@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu,
       linux-arm-kernel@lists.arm.linux.org.uk
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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

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.

Yours,
Linus Walleij