linux-kernel - [PATCH v16 1/2] math64.h: provide rounddown

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Message-ID: <20250625000059.20040-1-ansuelsmth@gmail.com>
Date: Wed, 25 Jun 2025 02:00:38 +0200
From: Christian Marangi <ansuelsmth@...il.com>
To: Uwe Kleine-König <ukleinek@...nel.org>,
	Christian Marangi <ansuelsmth@...il.com>,
	linux-kernel@...r.kernel.org,
	linux-pwm@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...el.com>
Subject: [PATCH v16 1/2] math64.h: provide rounddown_u64 variant for rounddown macro

There is currently a problem with the usage of rounddown() macro with
u64 dividends. This causes compilation error on specific arch where
64-bit division is done on 32-bit system.

To be more specific GCC try to optimize the function and replace it
with __umoddi3() but this is actually not compiled in the kernel.

Example:
pwm-airoha.c:(.text+0x8f8): undefined reference to `__umoddi3'

To better handle this, introduce a variant of rounddown() macro,
rounddown_u64() that can be used exactly for this scenario.

The new rounddown_u64() in math64.h uses do_div() to do the heavy work
of handling internally all the magic for the 64-bit division on 32-bit
(and indirectly fix the compilation error).

Signed-off-by: Christian Marangi <ansuelsmth@...il.com>
---
Changes v16:
- Move to math64.h
- Fix spelling mistake
- Use inline function (to follow roundup_u64 pattern)
Changes v15:
- Add this patch

 include/linux/math64.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 6aaccc1626ab..60cf79d0d7d6 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -370,4 +370,20 @@ static inline u64 roundup_u64(u64 x, u32 y)
 {
 	return DIV_U64_ROUND_UP(x, y) * y;
 }
+
+/**
+ * rounddown_u64 - Round down a 64bit value to the next specified 32bit multiple
+ * @x: the value to round
+ * @y: 32bit multiple to round down to
+ *
+ * Rounds @x down to the next multiple of @y. For 32bit @x values, see rounddown
+ * and the faster round_down() for powers of 2.
+ *
+ * Return: rounded up value.
+ */
+static inline u64 rounddown_u64(u64 x, u32 y)
+{
+	u64 tmp = x;
+	return x - do_div(tmp, y);
+}
 #endif /* _LINUX_MATH64_H */
-- 
2.48.1