Re: [PATCH] U300 sched_clock implementation

Peter Zijlstra
2009-05-24T07:57:34Z

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.

> Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
> ---
> arch/arm/mach-u300/timer.c | 16 ++++++++++++++++
> 1 files changed, 16 insertions(+), 0 deletions(-)
>
> diff --git a/arch/arm/mach-u300/timer.c b/arch/arm/mach-u300/timer.c
> index 4bf82f8..222de18 100644
> --- a/arch/arm/mach-u300/timer.c
> +++ b/arch/arm/mach-u300/timer.c
> @@ -346,6 +346,22 @@ static struct clocksource clocksource_u300_1mhz = {
> .flags = CLOCK_SOURCE_IS_CONTINUOUS,
> };
>
> +/*
> + * Override the global weak sched_clock symbol with this
> + * local implementation which uses the clocksource to get some
> + * better resolution when scheduling the kernel. We accept that
> + * this wraps around for now, since it is just a relative time
> + * stamp. (Inspired by OMAP implementation.)
> + */
> +unsigned long long sched_clock(void)
> +{
> + unsigned long long ret;
> +
> + ret = (unsigned long long) u300_get_cycles();
> + ret = (ret * clocksource_u300_1mhz.mult_orig) >>
> + clocksource_u300_1mhz.shift;
> + return ret;
> +}
>
> /*
> * This sets up the system timers, clock source and clock event.
> --
> 1.6.2.1