[PATCH] Fix get_order() [try #2]

[David Howells]

From: David Howells <dhowells@xxxxxxxxxx>

Provide an ilog2_up() that rounds its result up (ilog2() rounds down).

Fix get_order() to use ilog2_up() not ilog2() to get the correct rounding.

Adjust the documentation surrounding ilog2() and co. to indicate what rounding
they perform.

Fix roundup_pow_of_two(1) to give 1, not 0.

Signed-Off-By: David Howells <dhowells@xxxxxxxxxx>
---

 include/asm-generic/page.h |   14 +++++++++-----
 include/linux/log2.h       |   20 ++++++++++++++++++--
 2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/include/asm-generic/page.h b/include/asm-generic/page.h
index b55052c..c79dc23 100644
--- a/include/asm-generic/page.h
+++ b/include/asm-generic/page.h
@@ -17,11 +17,15 @@ 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;
+	if (size <= (1UL << page_shift))
+		return 0;
+	else
+		return __ilog2_u32(size - 1) + 1 - page_shift;
 #elif BITS_PER_LONG == 64 && defined(ARCH_HAS_ILOG2_U64)
-	int order = __ilog2_u64(size) - page_shift;
-	return order >= 0 ? order : 0;
+	if (size <= (1UL << page_shift))
+		return 0;
+	else
+		return __ilog2_u64(size - 1) + 1 - page_shift;
 #else
 	int order;
 
@@ -46,7 +50,7 @@ int __get_order(unsigned long size, int page_shift)
 #define get_order(n)							\
 (									\
 	__builtin_constant_p(n) ?					\
-	((n < (1UL << PAGE_SHIFT)) ? 0 : ilog2(n) - PAGE_SHIFT) :	\
+	((n < (1UL << PAGE_SHIFT)) ? 0 : ilog2_up(n) - PAGE_SHIFT) :	\
 	__get_order(n, PAGE_SHIFT)					\
  )
 
diff --git a/include/linux/log2.h b/include/linux/log2.h
index 57e641e..bc3a0eb 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -70,6 +70,8 @@ unsigned long __roundup_pow_of_two(unsigned long n)
  * constant-capable log of base 2 calculation
  * - this can be used to initialise global variables from constant data, hence
  *   the massive ternary operator construction
+ * - the result is rounded down
+ * - the result is undefined when n < 1
  *
  * selects the appropriately-sized optimised version depending on sizeof(n)
  */
@@ -149,6 +151,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
  )
 
 /**
+ * ilog2_up - rounded up log of base 2 of 32-bit or a 64-bit unsigned value
+ * @n - parameter
+ *
+ * constant-capable log of base 2 calculation
+ * - this can be used to initialise global variables from constant data, hence
+ *   the massive ternary operator construction
+ * - the result is rounded up
+ * - the result is undefined when n < 1
+ *
+ * selects the appropriately-sized optimised version depending on sizeof(n)
+ */
+#define ilog2_up(n) ((n) == 1 ? 0 : ilog2((n) - 1) + 1)
+
+/**
  * roundup_pow_of_two - round the given value up to nearest power of two
  * @n - parameter
  *
@@ -159,8 +175,8 @@ unsigned long __roundup_pow_of_two(unsigned long n)
 #define roundup_pow_of_two(n)			\
 (						\
 	__builtin_constant_p(n) ? (		\
-		(n == 1) ? 0 :			\
-		(1UL << (ilog2((n) - 1) + 1))	\
+		(n == 1) ? 1 :			\
+		(1UL << ilog2_up(n))		\
 				   ) :		\
 	__roundup_pow_of_two(n)			\
  )

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