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Message-ID: <20060928130847.GD1120@skybase>
Date: Thu, 28 Sep 2006 15:08:47 +0200
From: Martin Schwidefsky <schwidefsky@...ibm.com>
To: linux-kernel@...r.kernel.org
Subject: [S390] __div64_32 for 31 bit.
From: Martin Schwidefsky <schwidefsky@...ibm.com>
[S390] __div64_32 for 31 bit.
The clocksource infrastructure introduced with commit
ad596171ed635c51a9eef829187af100cbf8dcf7 broke 31 bit s390.
The reason is that the do_div() primitive for 31 bit always
had a restriction: it could only divide an unsigned 64 bit
integer by an unsigned 31 bit integer. The clocksource code
now uses do_div() with a base value that has the most
significant bit set. The result is that clock->cycle_interval
has a funny value which causes the linux time to jump around
like mad.
The solution is "obvious": implement a proper __div64_32
function for 31 bit s390.
Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
---
arch/s390/Kconfig | 4 +
arch/s390/lib/Makefile | 1
arch/s390/lib/div64.c | 151 +++++++++++++++++++++++++++++++++++++++++++++++
include/asm-s390/div64.h | 48 --------------
4 files changed, 156 insertions(+), 48 deletions(-)
diff -urpN linux-2.6/arch/s390/Kconfig linux-2.6-patched/arch/s390/Kconfig
--- linux-2.6/arch/s390/Kconfig 2006-09-28 14:58:39.000000000 +0200
+++ linux-2.6-patched/arch/s390/Kconfig 2006-09-28 14:58:54.000000000 +0200
@@ -51,6 +51,10 @@ config 64BIT
Select this option if you have a 64 bit IBM zSeries machine
and want to use the 64 bit addressing mode.
+config 32BIT
+ bool
+ default y if !64BIT
+
config SMP
bool "Symmetric multi-processing support"
---help---
diff -urpN linux-2.6/arch/s390/lib/div64.c linux-2.6-patched/arch/s390/lib/div64.c
--- linux-2.6/arch/s390/lib/div64.c 1970-01-01 01:00:00.000000000 +0100
+++ linux-2.6-patched/arch/s390/lib/div64.c 2006-09-28 14:58:54.000000000 +0200
@@ -0,0 +1,151 @@
+/*
+ * arch/s390/lib/div64.c
+ *
+ * __div64_32 implementation for 31 bit.
+ *
+ * Copyright (C) IBM Corp. 2006
+ * Author(s): Martin Schwidefsky (schwidefsky@...ibm.com),
+ */
+
+#include <linux/types.h>
+#include <linux/module.h>
+
+#ifdef CONFIG_MARCH_G5
+
+/*
+ * Function to divide an unsigned 64 bit integer by an unsigned
+ * 31 bit integer using signed 64/32 bit division.
+ */
+static uint32_t __div64_31(uint64_t *n, uint32_t base)
+{
+ register uint32_t reg2 asm("2");
+ register uint32_t reg3 asm("3");
+ uint32_t *words = (uint32_t *) n;
+ uint32_t tmp;
+
+ /* Special case base==1, remainder = 0, quotient = n */
+ if (base == 1)
+ return 0;
+ /*
+ * Special case base==0 will cause a fixed point divide exception
+ * on the dr instruction and may not happen anyway. For the
+ * following calculation we can assume base > 1. The first
+ * signed 64 / 32 bit division with an upper half of 0 will
+ * give the correct upper half of the 64 bit quotient.
+ */
+ reg2 = 0UL;
+ reg3 = words[0];
+ asm volatile(
+ " dr %0,%2\n"
+ : "+d" (reg2), "+d" (reg3) : "d" (base) : "cc" );
+ words[0] = reg3;
+ reg3 = words[1];
+ /*
+ * To get the lower half of the 64 bit quotient and the 32 bit
+ * remainder we have to use a little trick. Since we only have
+ * a signed division the quotient can get too big. To avoid this
+ * the 64 bit dividend is halved, then the signed division will
+ * work. Afterwards the quotient and the remainder are doubled.
+ * If the last bit of the dividend has been one the remainder
+ * is increased by one then checked against the base. If the
+ * remainder has overflown subtract base and increase the
+ * quotient. Simple, no ?
+ */
+ asm volatile(
+ " nr %2,%1\n"
+ " srdl %0,1\n"
+ " dr %0,%3\n"
+ " alr %0,%0\n"
+ " alr %1,%1\n"
+ " alr %0,%2\n"
+ " clr %0,%3\n"
+ " jl 0f\n"
+ " slr %0,%3\n"
+ " alr %1,%2\n"
+ "0:\n"
+ : "+d" (reg2), "+d" (reg3), "=d" (tmp)
+ : "d" (base), "2" (1UL) : "cc" );
+ words[1] = reg3;
+ return reg2;
+}
+
+/*
+ * Function to divide an unsigned 64 bit integer by an unsigned
+ * 32 bit integer using the unsigned 64/31 bit division.
+ */
+uint32_t __div64_32(uint64_t *n, uint32_t base)
+{
+ uint32_t r;
+
+ /*
+ * If the most significant bit of base is set, divide n by
+ * (base/2). That allows to use 64/31 bit division and gives a
+ * good approximation of the result: n = (base/2)*q + r. The
+ * result needs to be corrected with two simple transformations.
+ * If base is already < 2^31-1 __div64_31 can be used directly.
+ */
+ r = __div64_31(n, ((signed) base < 0) ? (base/2) : base);
+ if ((signed) base < 0) {
+ uint64_t q = *n;
+ /*
+ * First transformation:
+ * n = (base/2)*q + r
+ * = ((base/2)*2)*(q/2) + ((q&1) ? (base/2) : 0) + r
+ * Since r < (base/2), r + (base/2) < base.
+ * With q1 = (q/2) and r1 = r + ((q&1) ? (base/2) : 0)
+ * n = ((base/2)*2)*q1 + r1 with r1 < base.
+ */
+ if (q & 1)
+ r += base/2;
+ q >>= 1;
+ /*
+ * Second transformation. ((base/2)*2) could have lost the
+ * last bit.
+ * n = ((base/2)*2)*q1 + r1
+ * = base*q1 - ((base&1) ? q1 : 0) + r1
+ */
+ if (base & 1) {
+ int64_t rx = r - q;
+ /*
+ * base is >= 2^31. The worst case for the while
+ * loop is n=2^64-1 base=2^31+1. That gives a
+ * maximum for q=(2^64-1)/2^31 = 0x1ffffffff. Since
+ * base >= 2^31 the loop is finished after a maximum
+ * of three iterations.
+ */
+ while (rx < 0) {
+ rx += base;
+ q--;
+ }
+ r = rx;
+ }
+ *n = q;
+ }
+ return r;
+}
+
+#else /* MARCH_G5 */
+
+uint32_t __div64_32(uint64_t *n, uint32_t base)
+{
+ register uint32_t reg2 asm("2");
+ register uint32_t reg3 asm("3");
+ uint32_t *words = (uint32_t *) n;
+
+ reg2 = 0UL;
+ reg3 = words[0];
+ asm volatile(
+ " dlr %0,%2\n"
+ : "+d" (reg2), "+d" (reg3) : "d" (base) : "cc" );
+ words[0] = reg3;
+ reg3 = words[1];
+ asm volatile(
+ " dlr %0,%2\n"
+ : "+d" (reg2), "+d" (reg3) : "d" (base) : "cc" );
+ words[1] = reg3;
+ return reg2;
+}
+
+#endif /* MARCH_G5 */
+
+EXPORT_SYMBOL(__div64_32);
diff -urpN linux-2.6/arch/s390/lib/Makefile linux-2.6-patched/arch/s390/lib/Makefile
--- linux-2.6/arch/s390/lib/Makefile 2006-09-28 14:58:39.000000000 +0200
+++ linux-2.6-patched/arch/s390/lib/Makefile 2006-09-28 14:58:54.000000000 +0200
@@ -5,5 +5,6 @@
EXTRA_AFLAGS := -traditional
lib-y += delay.o string.o uaccess_std.o
+lib-$(CONFIG_32BIT) += div64.o
lib-$(CONFIG_64BIT) += uaccess_mvcos.o
lib-$(CONFIG_SMP) += spinlock.o
diff -urpN linux-2.6/include/asm-s390/div64.h linux-2.6-patched/include/asm-s390/div64.h
--- linux-2.6/include/asm-s390/div64.h 2006-09-20 05:42:06.000000000 +0200
+++ linux-2.6-patched/include/asm-s390/div64.h 2006-09-28 14:58:54.000000000 +0200
@@ -1,49 +1 @@
-#ifndef __S390_DIV64
-#define __S390_DIV64
-
-#ifndef __s390x__
-
-/* for do_div "base" needs to be smaller than 2^31-1 */
-#define do_div(n, base) ({ \
- unsigned long long __n = (n); \
- unsigned long __r; \
- \
- asm (" slr 0,0\n" \
- " l 1,%1\n" \
- " srdl 0,1\n" \
- " dr 0,%2\n" \
- " alr 1,1\n" \
- " alr 0,0\n" \
- " lhi 2,1\n" \
- " n 2,%1\n" \
- " alr 0,2\n" \
- " clr 0,%2\n" \
- " jl 0f\n" \
- " slr 0,%2\n" \
- " ahi 1,1\n" \
- "0: st 1,%1\n" \
- " l 1,4+%1\n" \
- " srdl 0,1\n" \
- " dr 0,%2\n" \
- " alr 1,1\n" \
- " alr 0,0\n" \
- " lhi 2,1\n" \
- " n 2,4+%1\n" \
- " alr 0,2\n" \
- " clr 0,%2\n" \
- " jl 1f\n" \
- " slr 0,%2\n" \
- " ahi 1,1\n" \
- "1: st 1,4+%1\n" \
- " lr %0,0" \
- : "=d" (__r), "=m" (__n) \
- : "d" (base), "m" (__n) : "0", "1", "2", "cc" ); \
- (n) = (__n); \
- __r; \
-})
-
-#else /* __s390x__ */
#include <asm-generic/div64.h>
-#endif /* __s390x__ */
-
-#endif
-
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