[PATCH 5/6] Alter get_order() so that it can make use of long_log2() on a constant

[David Howells]

From: David Howells <dhowells@xxxxxxxxxx>

Alter get_order() so that it can make use of long_log2() 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@xxxxxxxxxx>
---

 include/asm-generic/page.h |   14 -------------
 include/linux/log2.h       |   46 ++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 44 insertions(+), 16 deletions(-)

diff --git a/include/asm-generic/page.h b/include/asm-generic/page.h
index a96b5d9..a7e374e 100644
--- a/include/asm-generic/page.h
+++ b/include/asm-generic/page.h
@@ -6,20 +6,6 @@ #ifndef __ASSEMBLY__
 
 #include <linux/compiler.h>
 
-/* Pure 2^n version of get_order */
-static __inline__ __attribute_const__ int get_order(unsigned long size)
-{
-	int order;
-
-	size = (size - 1) >> (PAGE_SHIFT - 1);
-	order = -1;
-	do {
-		size >>= 1;
-		order++;
-	} while (size);
-	return order;
-}
-
 #endif	/* __ASSEMBLY__ */
 #endif	/* __KERNEL__ */
 
diff --git a/include/linux/log2.h b/include/linux/log2.h
index 9bef055..a2f6858 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -1,4 +1,4 @@
-/* Log base 2 calculation
+/* Base 2 logarithm calculation stuff
  *
  * Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
  * Written by David Howells (dhowells@xxxxxxxxxx)
@@ -25,6 +25,7 @@ int ____log2_NaN(void);
  * non-constant log of base 2 calculation
  * - the arch may override these in asm/bitops.h if they can be implemented
  *   more efficiently than using fls() and fls64()
+ * - the arch is not required to handle n==0 if implementing the fallback
  */
 #ifndef ARCH_HAS_LOG2
 static inline __attribute__((const))
@@ -43,6 +44,36 @@ int __ll_log2(u64 n)
 #endif
 
 /*
+ * 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_LOG2)
+	int order = __log2(size) - page_shift;
+	return order >= 0 ? order : 0;
+#elif BITS_PER_LONG == 64 && defined(ARCH_HAS_LL_LOG2)
+	int order = __ll_log2(size) - page_shift;
+	return order >= 0 ? order : 0;
+#else
+	int order;
+
+	size = (size - 1) >> (page_shift - 1);
+	order = -1;
+	do {
+		size >>= 1;
+		order++;
+	} while (size);
+	return order;
+#endif
+}
+#endif
+
+/*
  * constant-capable 32-bit log of base 2 calculation
  * - this can be used to initialise global variables from constant data, hence
  *   the massive ternary operator construction
@@ -167,7 +198,7 @@ (						\
  )
 
 /*
- * constant-capable unsigned long log of base 2 calculation
+ * constant-capable log of base 2 calculation of unsigned long
  * - this can be used to initialise global variables from constant data
  */
 #if BITS_PER_LONG == 32
@@ -176,4 +207,15 @@ #else
 #define long_log2(n) ll_log2(n)
 #endif
 
+/*
+ * 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 : long_log2(n) - PAGE_SHIFT) :	\
+	__get_order(n, PAGE_SHIFT)					\
+ )
+
 #endif /* _LINUX_LOG2_H */
-
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