[PATCH] LOG2: Alter get_order() so that it can make use of ilog2() on a constant

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@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/asm-generic/page.h

@@ -4,21 +4,51 @@
 #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__ */