On 09.04.2012, at 06:33, Bharat Bhushan wrote:
> Time for which the hrtimer is started for decrementer emulation is calculated
> using tb_ticks_per_usec. While hrtimer uses the clockevent for DEC
> reprogramming (if needed) and which calculate timebase ticks using the
> multiplier and shifter mechanism implemented within clockevent layer. It was
> observed that this conversion (timebase->time->timebase) are not correct
> because the mechanism are not consistent. In our setup it adds 2% jitter.
>
> With this patch clockevent multiplier and shifter mechanism are used when
> starting hrtimer for decrementer emulation. Now the jitter is < 0.5%.
>
> Signed-off-by: Bharat Bhushan <bharat.bhus...@freescale.com>
> ---
> v2:
> - decrementer_clockevent is made non-static rather than a seprate API to get
> mult/shift
>
> arch/powerpc/include/asm/time.h | 1 +
> arch/powerpc/kernel/time.c | 2 +-
> arch/powerpc/kvm/emulate.c | 5 +++--
> 3 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
> index 7eb10fb..b3c7959 100644
> --- a/arch/powerpc/include/asm/time.h
> +++ b/arch/powerpc/include/asm/time.h
> @@ -28,6 +28,7 @@
> extern unsigned long tb_ticks_per_jiffy;
> extern unsigned long tb_ticks_per_usec;
> extern unsigned long tb_ticks_per_sec;
> +extern struct clock_event_device decrementer_clockevent;
>
> struct rtc_time;
> extern void to_tm(int tim, struct rtc_time * tm);
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 567dd7c..e237225 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -105,7 +105,7 @@ static int decrementer_set_next_event(unsigned long evt,
> static void decrementer_set_mode(enum clock_event_mode mode,
> struct clock_event_device *dev);
>
> -static struct clock_event_device decrementer_clockevent = {
> +struct clock_event_device decrementer_clockevent = {
This one also needs a EXPORT_SYMBOL ...
> .name = "decrementer",
> .rating = 200,
> .irq = 0,
> diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
> index afc9154..c8b5206 100644
> --- a/arch/powerpc/kvm/emulate.c
> +++ b/arch/powerpc/kvm/emulate.c
> @@ -23,6 +23,7 @@
> #include <linux/types.h>
> #include <linux/string.h>
> #include <linux/kvm_host.h>
> +#include <linux/clockchips.h>
>
> #include <asm/reg.h>
> #include <asm/time.h>
> @@ -104,8 +105,8 @@ void kvmppc_emulate_dec(struct kvm_vcpu *vcpu)
> */
>
> dec_time = vcpu->arch.dec;
> - dec_time *= 1000;
> - do_div(dec_time, tb_ticks_per_usec);
> + dec_time = dec_time << decrementer_clockevent.shift;
> + do_div(dec_time, decrementer_clockevent.mult);
... because emulate.c can be built as a module.
Alex
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