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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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