[PATCH 1/2] linux/log2.h: Add round{up,down}_pow_of_two64()

[David Gow]

From: David Gow <david@xxxxxxxxxxxx>

The existing roundup_pow_of_two() and rounddown_pow_of_two() functions work
on values of type unsigned long, which is 32-bit on 32-bit systems. There
are cases (most notably in the GPU buddy allocator) which always operate on
64-bit values (as a 32-bit host can still use a GPU which internally has
64-bit addresses).

Add a version of these which always operate on a 64-bit value. These have
the (unimaginative) names roundup_pow_of_two64() and
rounddown_pow_of_two64(), and otherwise work identically to their unsigned
long counterparts. They are a bit ugly, but better than everyone hardcoding
calls to ilog2() -- which does handle 64-bit values -- directly.

Signed-off-by: David Gow <david@xxxxxxxxxxxx>
---

Hi all,

This series is basically a reworked and rebased version of
https://lore.kernel.org/all/20260326185413.1205870-40-jim.cromie@xxxxxxxxx/

Most importantly, the drm_test_buddy_alloc_exceeds_max_order KUnit test
was failing in linux-next on 32-bit systems, which this series fixes.

In rebasing it, I decided to add these helper functions, rather than
having an open-coded round{up,down} implementation in the buddy allocator,
but if no-one thinks it's worth having these for (at the moment) just one
user, I'm not too worried either way.

Cheers,
-- David

---
 include/linux/log2.h | 57 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index e17ceb32e0c9..67446d5ce9a3 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -195,6 +195,63 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 	__rounddown_pow_of_two(n)		\
  )
 
+/**
+ * __rounddown_pow_of_two64() - round a 64-bit value down to nearest power of two
+ * @n: value to round up
+ */
+static inline __attribute_const__
+u64 __rounddown_pow_of_two64(u64 n)
+{
+	return 1ULL << ilog2(n);
+}
+
+/**
+ * rounddown_pow_of_two64 - round a 64-bit value down to nearest power of two
+ * @n: parameter
+ *
+ * round the given value down to the nearest power of two
+ * - this always operates on 64-bit values, even on 32-bit systems
+ * - the result is undefined when n == 0
+ * - this can be used to initialise global variables from constant data
+ */
+#define rounddown_pow_of_two64(n)		\
+(						\
+	__builtin_constant_p(n) ? (		\
+		((n) == 1) ? 1ULL :		\
+		(1ULL << ilog2((n)))		\
+				   ) :		\
+	__rounddown_pow_of_two64(n)		\
+)
+
+
+/**
+ * __roundup_pow_of_two64() - round a 64-bit value up to nearest power of two
+ * @n: value to round up
+ */
+static inline __attribute_const__
+u64 __roundup_pow_of_two64(u64 n)
+{
+	return 1ULL << (ilog2(n - 1) + 1);
+}
+
+/**
+ * roundup_pow_of_two64 - round a 64-bit value up to nearest power of two
+ * @n: parameter
+ *
+ * round the given value up to the nearest power of two
+ * - this always operates on 64-bit values, even on 32-bit systems
+ * - the result is undefined when n == 0
+ * - this can be used to initialise global variables from constant data
+ */
+#define roundup_pow_of_two64(n)			\
+(						\
+	__builtin_constant_p(n) ? (		\
+		((n) == 1) ? 1ULL :		\
+		(1ULL << (ilog2((n) - 1) + 1))	\
+				   ) :		\
+	__roundup_pow_of_two64(n)		\
+)
+
 static inline __attribute_const__
 int __order_base_2(unsigned long n)
 {
-- 
2.55.0