From 6fd592daae2182adc47f405e20d07f34f52d07dd Mon Sep 17 00:00:00 2001 From: "Carlos R. Mafra" Date: Mon, 5 May 2008 20:11:22 -0300 Subject: 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 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: Carlos R. Mafra Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner --- arch/x86/kernel/hpet.c | 43 ++++++++++++++++++------------------------- 1 file changed, 18 insertions(+), 25 deletions(-) (limited to 'arch/x86/kernel/hpet.c') diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c index 9b5cfcdfc42..ea230ec6905 100644 --- a/arch/x86/kernel/hpet.c +++ b/arch/x86/kernel/hpet.c @@ -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); -- cgit v1.2.3