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    SubjectRe: [PATCH 04/10] tile: convert to use clocksource_register_hz
    On 11/1/2010 4:12 PM, John Stultz wrote:
    > Convert tile to use clocksource_register_hz.
    >
    > Untested. Help from maintainers would be appreciated.
    >
    > CC: Chris Metcalf <cmetcalf@tilera.com>
    > CC: Thomas Gleixner <tglx@linutronix.de>
    > Signed-off-by: John Stultz <johnstul@us.ibm.com>
    > ---
    > arch/tile/kernel/time.c | 5 +----
    > 1 files changed, 1 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c
    > index 6bed820..8c06cb2 100644
    > --- a/arch/tile/kernel/time.c
    > +++ b/arch/tile/kernel/time.c
    > @@ -76,7 +76,6 @@ static struct clocksource cycle_counter_cs = {
    > .rating = 300,
    > .read = clocksource_get_cycles,
    > .mask = CLOCKSOURCE_MASK(64),
    > - .shift = 22, /* typical value, e.g. x86 tsc uses this */
    > .flags = CLOCK_SOURCE_IS_CONTINUOUS,
    > };

    We were using clocksource_calc_mult_shift() for a while to compute both
    this value and our sched_clock() value, and, like
    clocksource_register_hz(), it suggested that a shift value of 31 was best.

    Perhaps unsurprisingly, we saw wraparound with sched_clock(), which is why
    we use a fixed shift value of "10" to avoid problems. This is clearly
    required since sched_clock() values are fairly long-lived.

    I admit that I don't really understand what the wraparound implications of
    using a shift of "31" for the clocksource is, but since it will cause the
    clocksource to wrap around fairly quickly, I wanted to make sure this shift
    value was OK for whatever uses that clocksource is put to. (The underlying
    clock in question is a 64-bit cycle counter, so with "<< 31" at e.g. 1 GHz
    it will wrap negative after about four seconds.)

    > @@ -89,8 +88,6 @@ void __init setup_clock(void)
    > cycles_per_sec = hv_sysconf(HV_SYSCONF_CPU_SPEED);
    > sched_clock_mult =
    > clocksource_hz2mult(cycles_per_sec, SCHED_CLOCK_SHIFT);
    > - cycle_counter_cs.mult =
    > - clocksource_hz2mult(cycles_per_sec, cycle_counter_cs.shift);
    > }
    >
    > void __init calibrate_delay(void)
    > @@ -105,7 +102,7 @@ void __init calibrate_delay(void)
    > void __init time_init(void)
    > {
    > /* Initialize and register the clock source. */
    > - clocksource_register(&cycle_counter_cs);
    > + clocksource_register_hz(&cycle_counter_cs, cycles_per_sec);
    >
    > /* Start up the tile-timer interrupt source on the boot cpu. */
    > setup_tile_timer();
    --
    Chris Metcalf, Tilera Corp.
    http://www.tilera.com



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