Received-SPF: pass (google.com: domain of linux-kernel+bounces-161550-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Date: Sun, 28 Apr 2024 12:13:11 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
To: Jules Irenge <jbi.octave@gmail.com>, mingo@redhat.com
CC: tglx@linutronix.de, bp@alien8.de, x86@kernel.org, peterz@infradead.org,
        rdunlap@infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] x86/tsc: Use div64_ul instead of do_div()
User-Agent: K-9 Mail for Android
In-Reply-To: <Zi6H5P_mc6DPR_v3@octinomon.home>
References: <Zi6H5P_mc6DPR_v3@octinomon.home>
Message-ID: <7EE9AB38-3103-4091-A558-BF0DEAF61E1D@zytor.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain;
 charset=utf-8
Content-Transfer-Encoding: quoted-printable

On April 28, 2024 10:31:16 AM PDT, Jules Irenge <jbi=2Eoctave@gmail=2Ecom> =
wrote:
>do_div() truncates a u64 divisor to 32 bit=2E
>This can lead to non-zero being truncated to zero for division=2E
>
>Fix coccinelle warning
>WARNING: do_div() does a 64-by-32 division, please consider using div64_u=
l instead
>
>Signed-off-by: Jules Irenge <jbi=2Eoctave@gmail=2Ecom>
>---
> arch/x86/kernel/tsc=2Ec | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/kernel/tsc=2Ec b/arch/x86/kernel/tsc=2Ec
>index 5a69a49acc96=2E=2E2da37f33dd17 100644
>--- a/arch/x86/kernel/tsc=2Ec
>+++ b/arch/x86/kernel/tsc=2Ec
>@@ -484,7 +484,7 @@ static unsigned long pit_calibrate_tsc(u32 latch, uns=
igned long ms, int loopmin)
>=20
> 	/* Calculate the PIT value */
> 	delta =3D t2 - t1;
>-	do_div(delta, ms);
>+	div64_ul(delta, ms);
> 	return delta;
> }
>=20

Are you sure this is not a false positive? This is a *much* more expensive=
 operation on 32 bits=2E