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Commit 6fd592da authored by Carlos R. Mafra's avatar Carlos R. Mafra Committed by Ingo Molnar
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x86: clean up computation of HPET .mult variables 



While reading through the HPET code I realized that the
computation of .mult variables could be done with less
lines of code, resulting in a 1.6% text size saving
for hpet.o

So I propose the following patch, which applies against
today's Linus -git tree.

>From 0c6507e400e9ca5f7f14331e18f8c12baf75a9d3 Mon Sep 17 00:00:00 2001
From: Carlos R. Mafra <crmafra@ift.unesp.br>
Date: Mon, 5 May 2008 19:38:53 -0300

The computation of clocksource_hpet.mult

       tmp = (u64)hpet_period << HPET_SHIFT;
       do_div(tmp, FSEC_PER_NSEC);
       clocksource_hpet.mult = (u32)tmp;

can be streamlined if we note that it is equal to

       clocksource_hpet.mult = div_sc(hpet_period, FSEC_PER_NSEC, HPET_SHIFT);

Furthermore, the computation of hpet_clockevent.mult

       uint64_t hpet_freq;

       hpet_freq = 1000000000000000ULL;
       do_div(hpet_freq, hpet_period);
       hpet_clockevent.mult = div_sc((unsigned long) hpet_freq,
                                     NSEC_PER_SEC, hpet_clockevent.shift);

can also be streamlined with the observation that hpet_period and hpet_freq are
inverse to each other (in proper units).

So instead of computing hpet_freq and using (schematically)
div_sc(hpet_freq, 10^9, shift) we use the trick of calling with the
arguments in reverse order, div_sc(10^6, hpet_period, shift).

The different power of ten is due to frequency being in Hertz (1/sec)
and the period being in units of femtosecond. Explicitly,

mult = (hpet_freq * 2^shift)/10^9    (before)
mult = (10^6 * 2^shift)/hpet_period  (after)

because hpet_freq = 10^15/hpet_period.

The comments in the code are also updated to reflect the changes.

As a result,

   text    data     bss     dec     hex filename
   2957     425      92    3474     d92 arch/x86/kernel/hpet.o
   3006     425      92    3523     dc3 arch/x86/kernel/hpet.o.old

a 1.6% reduction in text size.

Signed-off-by: default avatarCarlos R. Mafra <crmafra@ift.unesp.br>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 492c2e47

arch/x86/kernel/hpet.c

+18 −25
Original line number Diff line number Diff line
@@ -17,7 +17,7 @@ 


/* FSEC = 10^-15

   NSEC = 10^-9 */

#define FSEC_PER_NSEC	1000000

#define FSEC_PER_NSEC	1000000L



/*

 * HPET address is set in acpi/boot.c, when an ACPI entry exists
@@ -206,20 +206,19 @@ static void hpet_enable_legacy_int(void) 


static void hpet_legacy_clockevent_register(void)

{

	uint64_t hpet_freq;



	/* Start HPET legacy interrupts */

	hpet_enable_legacy_int();



	/*

	 * The period is a femto seconds value. We need to calculate the

	 * scaled math multiplication factor for nanosecond to hpet tick

	 * conversion.

	 * The mult factor is defined as (include/linux/clockchips.h)

	 *  mult/2^shift = cyc/ns (in contrast to ns/cyc in clocksource.h)

	 * hpet_period is in units of femtoseconds (per cycle), so

	 *  mult/2^shift = cyc/ns = 10^6/hpet_period

	 *  mult = (10^6 * 2^shift)/hpet_period

	 *  mult = (FSEC_PER_NSEC << hpet_clockevent.shift)/hpet_period

	 */

	hpet_freq = 1000000000000000ULL;

	do_div(hpet_freq, hpet_period);

	hpet_clockevent.mult = div_sc((unsigned long) hpet_freq,

				      NSEC_PER_SEC, hpet_clockevent.shift);

	hpet_clockevent.mult = div_sc((unsigned long) FSEC_PER_NSEC,

				      hpet_period, hpet_clockevent.shift);

	/* Calculate the min / max delta */

	hpet_clockevent.max_delta_ns = clockevent_delta2ns(0x7FFFFFFF,

							   &hpet_clockevent);
@@ -324,7 +323,7 @@ static struct clocksource clocksource_hpet = { 


static int hpet_clocksource_register(void)

{

	u64 tmp, start, now;

	u64 start, now;

	cycle_t t1;



	/* Start the counter */
@@ -351,21 +350,15 @@ static int hpet_clocksource_register(void) 
		return -ENODEV;

	}



	/* Initialize and register HPET clocksource

	 *

	 * hpet period is in femto seconds per cycle

	 * so we need to convert this to ns/cyc units

	 * approximated by mult/2^shift

	 *

	 *  fsec/cyc * 1nsec/1000000fsec = nsec/cyc = mult/2^shift

	 *  fsec/cyc * 1ns/1000000fsec * 2^shift = mult

	 *  fsec/cyc * 2^shift * 1nsec/1000000fsec = mult

	 *  (fsec/cyc << shift)/1000000 = mult

	 *  (hpet_period << shift)/FSEC_PER_NSEC = mult

	/*

	 * The definition of mult is (include/linux/clocksource.h)

	 * mult/2^shift = ns/cyc and hpet_period is in units of fsec/cyc

	 * so we first need to convert hpet_period to ns/cyc units:

	 *  mult/2^shift = ns/cyc = hpet_period/10^6

	 *  mult = (hpet_period * 2^shift)/10^6

	 *  mult = (hpet_period << shift)/FSEC_PER_NSEC

	 */

	tmp = (u64)hpet_period << HPET_SHIFT;

	do_div(tmp, FSEC_PER_NSEC);

	clocksource_hpet.mult = (u32)tmp;

	clocksource_hpet.mult = div_sc(hpet_period, FSEC_PER_NSEC, HPET_SHIFT);



	clocksource_register(&clocksource_hpet);