linux-kernel - [PATCH] timekeeping: improve arithmetic division in scale64_check

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Message-Id: <20200120100523.45656-1-wenyang@linux.alibaba.com>
Date:   Mon, 20 Jan 2020 18:05:23 +0800
From:   Wen Yang <wenyang@...ux.alibaba.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Wen Yang <wenyang@...ux.alibaba.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] timekeeping: improve arithmetic division in scale64_check_overflow()

do_div() does a 64-by-32 division. Use div64_u64() instead of it
if the divisor is u64, to avoid truncation to 32-bit.
And as a nice side effect also cleans up the function a bit.

Signed-off-by: Wen Yang <wenyang@...ux.alibaba.com>
Cc: John Stultz <john.stultz@...aro.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Stephen Boyd <sboyd@...nel.org>
Cc: linux-kernel@...r.kernel.org
---
 kernel/time/timekeeping.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ca69290bee2a..4fc2af4367a7 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1005,9 +1005,8 @@ static int scale64_check_overflow(u64 mult, u64 div, u64 *base)
 	    ((int)sizeof(u64)*8 - fls64(mult) < fls64(rem)))
 		return -EOVERFLOW;
 	tmp *= mult;
-	rem *= mult;
 
-	do_div(rem, div);
+	rem = div64_u64(rem * mult, div);
 	*base = tmp + rem;
 	return 0;
 }
-- 
2.23.0