linux-kernel - [PATCH 3/4] LOG2: Alter get_order() so that it can make use of ilog2() on a constant [try #4]

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Message-Id: <20061006133420.9972.82238.stgit@warthog.cambridge.redhat.com>
Date:	Fri, 06 Oct 2006 14:34:20 +0100
From:	David Howells <dhowells@...hat.com>
To:	torvalds@...l.org, akpm@...l.org, sfr@...b.auug.org.au
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	dhowells@...hat.com
Subject: [PATCH 3/4] LOG2: Alter get_order() so that it can make use of ilog2() on a constant [try #4]

From: David Howells <dhowells@...hat.com>

Alter get_order() so that it can make use of ilog2() on a constant to produce a
constant value, retaining the ability for an arch to override it in the
non-const case.

Signed-Off-By: David Howells <dhowells@...hat.com>
---

 include/asm-generic/page.h |   38 ++++++++++++++++++++++++++++++++++----
 1 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/include/asm-generic/page.h b/include/asm-generic/page.h
index a96b5d9..b55052c 100644
--- a/include/asm-generic/page.h
+++ b/include/asm-generic/page.h
@@ -4,21 +4,51 @@ #define _ASM_GENERIC_PAGE_H
 #ifdef __KERNEL__
 #ifndef __ASSEMBLY__
 
-#include <linux/compiler.h>
+#include <linux/log2.h>
 
-/* Pure 2^n version of get_order */
-static __inline__ __attribute_const__ int get_order(unsigned long size)
+/*
+ * non-const pure 2^n version of get_order
+ * - the arch may override these in asm/bitops.h if they can be implemented
+ *   more efficiently than using the arch log2 routines
+ * - we use the non-const log2() instead if the arch has defined one suitable
+ */
+#ifndef ARCH_HAS_GET_ORDER
+static inline __attribute__((const))
+int __get_order(unsigned long size, int page_shift)
 {
+#if BITS_PER_LONG == 32 && defined(ARCH_HAS_ILOG2_U32)
+	int order = __ilog2_u32(size) - page_shift;
+	return order >= 0 ? order : 0;
+#elif BITS_PER_LONG == 64 && defined(ARCH_HAS_ILOG2_U64)
+	int order = __ilog2_u64(size) - page_shift;
+	return order >= 0 ? order : 0;
+#else
 	int order;
 
-	size = (size - 1) >> (PAGE_SHIFT - 1);
+	size = (size - 1) >> (page_shift - 1);
 	order = -1;
 	do {
 		size >>= 1;
 		order++;
 	} while (size);
 	return order;
+#endif
 }
+#endif
+
+/**
+ * get_order - calculate log2(pages) to hold a block of the specified size
+ * @n - size
+ *
+ * calculate allocation order based on the current page size
+ * - this can be used to initialise global variables from constant data
+ */
+#define get_order(n)							\
+(									\
+	__builtin_constant_p(n) ?					\
+	((n < (1UL << PAGE_SHIFT)) ? 0 : ilog2(n) - PAGE_SHIFT) :	\
+	__get_order(n, PAGE_SHIFT)					\
+ )
 
 #endif	/* __ASSEMBLY__ */
 #endif	/* __KERNEL__ */
-
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