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Date: Mon, 5 May 2008 20:11:22 -0300
From: "Carlos R. Mafra" <crmafra2@...il.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, venkatesh.pallipadi@...el.com
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 <crmafra@....unesp.br>
Date: Mon, 5 May 2008 19:38:53 -0300
Subject: [PATCH] x86: Clean up computation of HPET .mult variables
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 <crmafra@....unesp.br>
---
arch/x86/kernel/hpet.c | 43 ++++++++++++++++++-------------------------
1 files changed, 18 insertions(+), 25 deletions(-)
diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 9b5cfcd..ea230ec 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);
--
1.5.4.3
--
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