Re: [PATCH] clocksource: shift helper

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 01 May 2008 10:31:23 -0700
Daniel Walker <dwalker@mvista.com> wrote:

> 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.
> 
> ...
>
> ===================================================================
> --- 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;
> +}

If we expect this to have more than one callsite then it would be best to
uninline it.

Unless we always expect it to be called from __init code, in which case
it's best to inline it ;)