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    SubjectRe: [PATCH] U300 sched_clock implementation
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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.


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