linux-kernel - [PATCH] x86/tsc: improve arithmetic division

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Date:   Thu, 30 Jan 2020 21:08:38 +0800
From:   Wen Yang <wenyang@...ux.alibaba.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, hpa@...or.com,
        Ingo Molnar <mingo@...hat.com>
Cc:     Wen Yang <wenyang@...ux.alibaba.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] x86/tsc: improve arithmetic division

do_div() does a 64-by-32 division. Use div64_ul64() or div64_ul()
instead of it if the divisor is 'ul64' or 'unsigned long', to avoid
truncation to lower 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: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
---
 arch/x86/kernel/tsc.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 7e322e2daaf5..4c0320e68699 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -357,9 +357,7 @@ static unsigned long calc_pmtimer_ref(u64 deltatsc, u64 pm1, u64 pm2)
 	pm2 -= pm1;
 	tmp = pm2 * 1000000000LL;
 	do_div(tmp, PMTMR_TICKS_PER_SEC);
-	do_div(deltatsc, tmp);
-
-	return (unsigned long) deltatsc;
+	return (unsigned long) div64_u64(deltatsc, tmp);
 }
 
 #define CAL_MS		10
@@ -778,8 +776,7 @@ static unsigned long pit_hpet_ptimer_calibrate_cpu(void)
 		tsc_ref_min = min(tsc_ref_min, (unsigned long) tsc2);
 
 		/* Check the reference deviation */
-		delta = ((u64) tsc_pit_min) * 100;
-		do_div(delta, tsc_ref_min);
+		delta = div64_ul(((u64) tsc_pit_min) * 100, tsc_ref_min);
 
 		/*
 		 * If both calibration results are inside a 10% window
-- 
2.23.0