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    Subject[PATCH] clocksource: shift helper
    This is a little helper I pulled from the mips tree. I've seen a couple 
    of bogus shift values, and this helper calculates the shift. This
    should move the shift selection away from the user, and systematically
    pick the value base on the hz.

    I also modified the acpi_pm timer to use this new interface. The original
    shift was 22 , and after this patch it's increased to 23. Which should make
    the clocksource slightly more accurate, but shouldn't have much of a visible
    effect.

    Signed-off-by: Daniel Walker <dwalker@mvista.com>

    ---
    drivers/clocksource/acpi_pm.c | 5 ++++-
    include/linux/acpi_pmtmr.h | 5 +++--
    include/linux/clocksource.h | 21 +++++++++++++++++++++
    3 files changed, 28 insertions(+), 3 deletions(-)

    Index: linux-2.6.25/drivers/clocksource/acpi_pm.c
    ===================================================================
    --- linux-2.6.25.orig/drivers/clocksource/acpi_pm.c
    +++ linux-2.6.25/drivers/clocksource/acpi_pm.c
    @@ -72,7 +72,7 @@ static struct clocksource clocksource_ac
    .read = acpi_pm_read,
    .mask = (cycle_t)ACPI_PM_MASK,
    .mult = 0, /*to be calculated*/
    - .shift = 22,
    + .shift = 0,
    .flags = CLOCK_SOURCE_IS_CONTINUOUS,

    };
    @@ -183,6 +183,9 @@ static int __init init_acpi_pm_clocksour
    if (!pmtmr_ioport)
    return -ENODEV;

    + clocksource_acpi_pm.shift =
    + clocksource_hz2shift(ACPI_PM_BITS, PMTMR_TICKS_PER_SEC);
    +
    clocksource_acpi_pm.mult = clocksource_hz2mult(PMTMR_TICKS_PER_SEC,
    clocksource_acpi_pm.shift);

    Index: linux-2.6.25/include/linux/acpi_pmtmr.h
    ===================================================================
    --- linux-2.6.25.orig/include/linux/acpi_pmtmr.h
    +++ linux-2.6.25/include/linux/acpi_pmtmr.h
    @@ -7,10 +7,11 @@
    #define PMTMR_TICKS_PER_SEC 3579545

    /* limit it to 24 bits */
    -#define ACPI_PM_MASK CLOCKSOURCE_MASK(24)
    +#define ACPI_PM_BITS 24
    +#define ACPI_PM_MASK CLOCKSOURCE_MASK(ACPI_PM_BITS)

    /* Overrun value */
    -#define ACPI_PM_OVRRUN (1<<24)
    +#define ACPI_PM_OVRRUN (1 << ACPI_PM_BITS)

    #ifdef CONFIG_X86_PM_TIMER

    Index: linux-2.6.25/include/linux/clocksource.h
    ===================================================================
    --- linux-2.6.25.orig/include/linux/clocksource.h
    +++ linux-2.6.25/include/linux/clocksource.h
    @@ -157,6 +157,27 @@ static inline u32 clocksource_hz2mult(u3
    }

    /**
    + * clocksource_hz2shift - calculates shift from hz and # of bits
    + * @bits: Number of bits this clocksource uses
    + * @hz: Clocksource frequency in Hz
    + *
    + * Helper functions that calculates the best shift value
    + * based on the hz and # of bits of any given clock.
    + */
    +static inline u32 clocksource_hz2shift(u32 bits, u32 hz)
    +{
    + u64 temp;
    +
    + for (; bits > 0; bits--) {
    + temp = (u64) NSEC_PER_SEC << bits;
    + do_div(temp, hz);
    + if ((temp >> 32) == 0)
    + break;
    + }
    + return bits;
    +}
    +
    +/**
    * clocksource_read: - Access the clocksource's current cycle value
    * @cs: pointer to clocksource being read
    *
    --
    --


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