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Subject: Re: [PATCH] x86/apic: Use div64_ul() instead of do_div()
From: Thorsten Blum <thorsten.blum@toblux.com>
In-Reply-To: <7e46123775e64898bd7c467328125ee0@AcuMS.aculab.com>
Date: Fri, 1 Mar 2024 13:28:00 +0100
Cc: Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>,
 Dave Hansen <dave.hansen@linux.intel.com>,
 "x86@kernel.org" <x86@kernel.org>,
 "H. Peter Anvin" <hpa@zytor.com>,
 "Peter Zijlstra (Intel)" <peterz@infradead.org>,
 Wei Liu <wei.liu@kernel.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Message-Id: <8ECEC999-A742-488F-99B2-A076EF9CA2B2@toblux.com>
References: <20240227114333.1718-2-thorsten.blum@toblux.com>
 <7e46123775e64898bd7c467328125ee0@AcuMS.aculab.com>
To: David Laight <David.Laight@ACULAB.COM>


> On Feb 29, 2024, at 23:13, David Laight <David.Laight@ACULAB.COM> =
wrote:
>=20
> do_div() does a 64 by 32 divide that gives a 32bit quotient.
> div64_ul() does a much more expensive 64 by 64 divide that
> can generate a 64bit quotient.

Since the dividend and the divisor could (according to the types at =
least) both
be 64-bit values and do_div() does a 64-by-32 division, the quotient =
could
potentially be wrong.

However, if the values don't require a 64-by-64 division (not even on a =
64-bit
system) and the divisor (deltapm) is guaranteed to fit into 32 bits, =
wouldn't it
make sense to change its type from long to int?

Given that acpi_pm_read_early() returns a u32 and is then assigned to =
unsigned
long pm, this might be the source of the issue. Changing pm and deltapm =
from
long to u32 and keeping do_div() as is, would improve the types and =
remove the
Coccinelle warnings...but maybe I'm missing something?

Thanks,
Thorsten=