linux-kernel - [RFC PATCH] math.h: Account for 64-bit division on i386

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Message-ID: <20250419115157.567249-1-cem@kernel.org>
Date: Sat, 19 Apr 2025 13:51:46 +0200
From: cem@...nel.org
To: linux-kernel@...r.kernel.org
Cc: Hans.Holmberg@....com,
	oe-kbuild-all@...ts.linux.dev,
	hch@....de,
	lukas@...ner.de,
	angelogioacchino.delregno@...labora.com,
	Jonathan.Cameron@...wei.com
Subject: [RFC PATCH] math.h: Account for 64-bit division on i386

From: Carlos Maiolino <cem@...nel.org>

Building linux on i386 might fail if a 64bit type is passed to
mult_fract(). To prevent the failure, use do_div() for the division
calculation instead of hardcoding a / b.

Signed-off-by: Carlos Maiolino <cmaiolino@...hat.com>
Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504181233.F7D9Atra-lkp@intel.com/
---

I'm sending it as a RFC because I didn't to extensive testing on this
patch, also I'm not sure if mult_frac() was intended to work on 32-bit
only types. If that's the case, perhaps, a new mult_frac64() might be a
better idea?!

 include/linux/math.h | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/include/linux/math.h b/include/linux/math.h
index 0198c92cbe3e..05ea853b75b4 100644
--- a/include/linux/math.h
+++ b/include/linux/math.h
@@ -133,15 +133,16 @@ __STRUCT_FRACT(u32)
 #undef __STRUCT_FRACT
 
 /* Calculate "x * n / d" without unnecessary overflow or loss of precision. */
-#define mult_frac(x, n, d)	\
-({				\
-	typeof(x) x_ = (x);	\
-	typeof(n) n_ = (n);	\
-	typeof(d) d_ = (d);	\
-				\
-	typeof(x_) q = x_ / d_;	\
-	typeof(x_) r = x_ % d_;	\
-	q * n_ + r * n_ / d_;	\
+#define mult_frac(x, n, d)		\
+({					\
+	typeof(x) x_ = (x);		\
+	typeof(n) n_ = (n);		\
+	typeof(d) d_ = (d);		\
+					\
+	typeof(x_) r = do_div(x_, d_);	\
+	r *= n_;			\
+	do_div(r, d_);			\
+	x_ * n_ + r;			\
 })
 
 #define sector_div(a, b) do_div(a, b)
-- 
2.49.0