Re: [PATCH 2/2] MN10300: Make sched_clock() report time since boot

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2008-09-24 at 17:48 +0100, David Howells wrote:
> Make sched_clock() report time since boot rather than time since last timer
> interrupt.
> 
> Make sched_clock() expand and scale the 32-bit TSC value running at IOCLK
> speed (~33MHz) to a 64-bit nanosecond counter, using cnt32_to_63() acquired
> from the ARM arch and without using slow DIVU instructions every call.

Right, so since you have to up-scale the 63 bit counter obtained using
the smarty pants cnt32_to_63 code, you actually end up with a genuine
64bit counter.. sweet.

I'm not going to pretend to understand the NM10300 details, but the
generic idea makes sense.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
> 
>  arch/mn10300/kernel/time.c |   52 +++++++++++++++++++++++++++++++++++---------
>  1 files changed, 41 insertions(+), 11 deletions(-)
> 
> 
> diff --git a/arch/mn10300/kernel/time.c b/arch/mn10300/kernel/time.c
> index babb7c2..e460658 100644
> --- a/arch/mn10300/kernel/time.c
> +++ b/arch/mn10300/kernel/time.c
> @@ -1,6 +1,6 @@
>  /* MN10300 Low level time management
>   *
> - * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
> + * Copyright (C) 2007-2008 Red Hat, Inc. All Rights Reserved.
>   * Written by David Howells (dhowells@redhat.com)
>   * - Derived from arch/i386/kernel/time.c
>   *
> @@ -16,6 +16,7 @@
>  #include <linux/init.h>
>  #include <linux/smp.h>
>  #include <linux/profile.h>
> +#include <linux/cnt32_to_63.h>
>  #include <asm/irq.h>
>  #include <asm/div64.h>
>  #include <asm/processor.h>
> @@ -40,27 +41,54 @@ static struct irqaction timer_irq = {
>  	.name		= "timer",
>  };
>  
> +static unsigned long sched_clock_multiplier;
> +
>  /*
>   * scheduler clock - returns current time in nanosec units.
>   */
>  unsigned long long sched_clock(void)
>  {
>  	union {
> -		unsigned long long l;
> -		u32 w[2];
> -	} quot;
> +		unsigned long long ll;
> +		unsigned l[2];
> +	} tsc64, result;
> +	unsigned long tsc, tmp;
> +	unsigned product[3]; /* 96-bit intermediate value */
> +
> +	/* read the TSC value
> +	 */
> +	tsc = 0 - get_cycles(); /* get_cycles() counts down */
>  
> -	quot.w[0] = mn10300_last_tsc - get_cycles();
> -	quot.w[1] = 1000000000;
> +	/* expand to 64-bits.
> +	 * - sched_clock() must be called once a minute or better or the
> +	 *   following will go horribly wrong - see cnt32_to_63()
> +	 */
> +	tsc64.ll = cnt32_to_63(tsc) & 0x7fffffffffffffffULL;
>  
> -	asm("mulu %2,%3,%0,%1"
> -	    : "=r"(quot.w[1]), "=r"(quot.w[0])
> -	    : "0"(quot.w[1]), "1"(quot.w[0])
> +	/* scale the 64-bit TSC value to a nanosecond value via a 96-bit
> +	 * intermediate
> +	 */
> +	asm("mulu	%2,%0,%3,%0	\n"	/* LSW * mult ->  0:%3:%0 */
> +	    "mulu	%2,%1,%2,%1	\n"	/* MSW * mult -> %2:%1:0 */
> +	    "add	%3,%1		\n"
> +	    "addc	0,%2		\n"	/* result in %2:%1:%0 */
> +	    : "=r"(product[0]), "=r"(product[1]), "=r"(product[2]), "=r"(tmp)
> +	    :  "0"(tsc64.l[0]),  "1"(tsc64.l[1]),  "2"(sched_clock_multiplier)
>  	    : "cc");
>  
> -	do_div(quot.l, MN10300_TSCCLK);
> +	result.l[0] = product[1] << 16 | product[0] >> 16;
> +	result.l[1] = product[2] << 16 | product[1] >> 16;
>  
> -	return quot.l;
> +	return result.ll;
> +}
> +
> +/*
> + * initialise the scheduler clock
> + */
> +static void __init mn10300_sched_clock_init(void)
> +{
> +	sched_clock_multiplier =
> +		__muldiv64u(NSEC_PER_SEC, 1 << 16, MN10300_TSCCLK);
>  }
>  
>  /*
> @@ -128,4 +156,6 @@ void __init time_init(void)
>  	/* start the watchdog timer */
>  	watchdog_go();
>  #endif
> +
> +	mn10300_sched_clock_init();
>  }
>