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From: Willy Tarreau <w@1wt.eu>
To: "Pali Rohár" <pali@kernel.org>
Cc: "Geert Uytterhoeven" <geert@linux-m68k.org>,
	"Michael Turquette" <mturquette@baylibre.com>,
	"Stephen Boyd" <sboyd@kernel.org>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Andrew Lunn" <andrew@lunn.ch>,
	"Gregory Clement" <gregory.clement@bootlin.com>,
	"Sebastian Hesselbarth" <sebastian.hesselbarth@gmail.com>,
	"Vladimir Vid" <vladimir.vid@sartura.hr>,
	"Marek Behún" <kabel@kernel.org>,
	linux-clk <linux-clk@vger.kernel.org>,
	"open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 07/11] math64: New DIV_U64_ROUND_CLOSEST helper
Date: Fri, 25 Jun 2021 17:50:08 +0200	[thread overview]
Message-ID: <20210625155008.GB16901@1wt.eu> (raw)
In-Reply-To: <20210625153803.u6uesckcqyvvo7dl@pali>

On Fri, Jun 25, 2021 at 05:38:03PM +0200, Pali Rohár wrote:
> On Friday 25 June 2021 17:22:31 Geert Uytterhoeven wrote:
> > > +/*
> > > + * DIV_U64_ROUND_CLOSEST - unsigned 64bit divide with 32bit divisor rounded to nearest integer
> > > + * @dividend: unsigned 64bit dividend
> > > + * @divisor: unsigned 32bit divisor
> > > + *
> > > + * Divide unsigned 64bit dividend by unsigned 32bit divisor
> > > + * and round to closest integer.
> > > + *
> > > + * Return: dividend / divisor rounded to nearest integer
> > > + */
> > > +#define DIV_U64_ROUND_CLOSEST(dividend, divisor)       \
> > > +       ({ u32 _tmp = (divisor); div_u64((u64)(dividend) + _tmp / 2, _tmp); })
> > 
> > Given "dividend" should already be an unsigned 64-bit value, I don't
> > think the cast to "u64" is needed. Similar macros in this file also
> > don't have the cast.
> 
> It is just to ensure that plus operation between dividend and _tmp is
> evaluated in 64-bit context to prevent overflow. Just a case when user
> calls this macro with 32-bit dividend param. As it is a macro (and not
> inline function) type is not automatically enforced.

I agree, a large u32 argument added to _tmp/2 could overflow and remain
32 bits, yielding an incorrect result. The cast is mandatory here (and
will either emit no code, or be useful).

The only trap I'm seeing is if a negative signed int is passed in dividend,
it will be sign-extended and will give a large u64 value. A preliminary
u32 cast could avoid this but would break valid u64 arguments, and I'd
claim we never know what the user wants if this happens in the first place.

Willy

  reply	other threads:[~2021-06-25 15:50 UTC|newest]

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-24 22:48 [PATCH 00/10] serial: mvebu-uart: Fixes and new support for higher baudrates Pali Rohár
2021-06-24 22:49 ` [PATCH 01/10] serial: mvebu-uart: fix calculation of clock divisor Pali Rohár
2021-06-24 22:49 ` [PATCH 02/10] serial: mvebu-uart: do not allow changing baudrate when uartclk is not available Pali Rohár
2021-06-24 22:49 ` [PATCH 03/10] serial: mvebu-uart: correctly calculate minimal possible baudrate Pali Rohár
2021-06-24 22:49 ` [PATCH 04/10] dt-bindings: mvebu-uart: fix documentation Pali Rohár
2021-06-24 22:49 ` [PATCH 05/10] arm64: dts: marvell: armada-37xx: Fix reg for standard variant of UART Pali Rohár
2021-06-24 22:49 ` [PATCH 06/10] serial: mvebu-uart: remove unused member nb from struct mvebu_uart Pali Rohár
2021-06-24 22:49 ` [PATCH 07/10] serial: mvebu-uart: implement UART clock driver for configuring UART base clock Pali Rohár
2021-06-24 22:49 ` [PATCH 08/10] dt-bindings: mvebu-uart: document DT bindings for marvell,armada-3700-uart-clock Pali Rohár
2021-06-24 22:49 ` [PATCH 09/10] arm64: dts: marvell: armada-37xx: add device node for UART clock and use it Pali Rohár
2021-06-24 22:49 ` [PATCH 10/10] serial: mvebu-uart: implement support for baudrates higher than 230400 Pali Rohár
2021-06-25 14:36 ` [PATCH v2 00/11] serial: mvebu-uart: Fixes and new support for higher baudrates Pali Rohár
2021-06-25 14:36   ` [PATCH v2 01/11] serial: mvebu-uart: fix calculation of clock divisor Pali Rohár
2021-06-25 14:36   ` [PATCH v2 02/11] serial: mvebu-uart: do not allow changing baudrate when uartclk is not available Pali Rohár
2021-06-25 14:36   ` [PATCH v2 03/11] serial: mvebu-uart: correctly calculate minimal possible baudrate Pali Rohár
2021-06-25 14:36   ` [PATCH v2 04/11] dt-bindings: mvebu-uart: fix documentation Pali Rohár
2021-06-25 14:36   ` [PATCH v2 05/11] arm64: dts: marvell: armada-37xx: Fix reg for standard variant of UART Pali Rohár
2021-06-25 14:36   ` [PATCH v2 06/11] serial: mvebu-uart: remove unused member nb from struct mvebu_uart Pali Rohár
2021-06-25 14:36   ` [PATCH v2 07/11] math64: New DIV_U64_ROUND_CLOSEST helper Pali Rohár
2021-06-25 15:22     ` Geert Uytterhoeven
2021-06-25 15:38       ` Pali Rohár
2021-06-25 15:50         ` Willy Tarreau [this message]
2021-06-25 17:39           ` Geert Uytterhoeven
2021-06-25 17:44             ` Pali Rohár
2021-06-25 18:11               ` Geert Uytterhoeven
2021-07-19 12:45         ` Andy Shevchenko
2021-06-25 14:36   ` [PATCH v2 08/11] serial: mvebu-uart: implement UART clock driver for configuring UART base clock Pali Rohár
2021-06-25 14:36   ` [PATCH v2 09/11] dt-bindings: mvebu-uart: document DT bindings for marvell,armada-3700-uart-clock Pali Rohár
2021-06-25 14:36   ` [PATCH v2 10/11] arm64: dts: marvell: armada-37xx: add device node for UART clock and use it Pali Rohár
2021-06-25 14:36   ` [PATCH v2 11/11] serial: mvebu-uart: implement support for baudrates higher than 230400 Pali Rohár
2021-07-17 12:38 ` [PATCH v3 0/5] serial: mvebu-uart: Support for higher baudrates Pali Rohár
2021-07-17 12:38   ` [PATCH v3 1/5] math64: New DIV_U64_ROUND_CLOSEST helper Pali Rohár
2021-07-19 12:47     ` Andy Shevchenko
2021-07-22 21:57       ` Pali Rohár
2021-07-24 11:38         ` David Laight
2021-07-17 12:38   ` [PATCH v3 2/5] serial: mvebu-uart: implement UART clock driver for configuring UART base clock Pali Rohár
2021-07-17 17:26     ` Andrew Lunn
2021-07-17 18:05       ` Pali Rohár
2021-07-24  9:48         ` Pali Rohár
2021-07-24 16:33           ` Andrew Lunn
2021-07-25 12:14             ` Pali Rohár
2021-07-17 12:38   ` [PATCH v3 3/5] dt-bindings: mvebu-uart: document DT bindings for marvell,armada-3700-uart-clock Pali Rohár
2021-07-17 17:30     ` [PATCH v3 3/5] dt-bindings: mvebu-uart: document DT bindings for marvell, armada-3700-uart-clock Andrew Lunn
2021-08-02 14:45       ` Pali Rohár
2021-07-17 12:38   ` [PATCH v3 4/5] arm64: dts: marvell: armada-37xx: add device node for UART clock and use it Pali Rohár
2021-07-17 12:38   ` [PATCH v3 5/5] serial: mvebu-uart: implement support for baudrates higher than 230400 Pali Rohár
2021-08-02 14:45 ` [PATCH v4 0/6] serial: mvebu-uart: Support for higher baudrates Pali Rohár
2021-08-02 14:45   ` [PATCH v4 1/6] math64: New DIV_U64_ROUND_CLOSEST helper Pali Rohár
2021-08-02 14:45   ` [PATCH v4 2/6] serial: mvebu-uart: implement UART clock driver for configuring UART base clock Pali Rohár
2021-08-02 14:45   ` [PATCH v4 3/6] dt-bindings: mvebu-uart: document DT bindings for marvell,armada-3700-uart-clock Pali Rohár
2021-08-06  0:30     ` Stephen Boyd
2021-08-06  8:28       ` Pali Rohár
2021-08-12 19:33         ` Stephen Boyd
2021-08-12 20:08           ` Pali Rohár
2021-08-02 14:45   ` [PATCH v4 4/6] dt-bindings: mvebu-uart: update information about UART clock Pali Rohár
2021-08-02 14:45   ` [PATCH v4 5/6] arm64: dts: marvell: armada-37xx: add device node for UART clock and use it Pali Rohár
2021-08-02 14:45   ` [PATCH v4 6/6] serial: mvebu-uart: implement support for baudrates higher than 230400 Pali Rohár
2021-08-09 14:53 ` [PATCH v5 0/6] serial: mvebu-uart: Support for higher baudrates Pali Rohár
2021-08-09 14:53   ` [PATCH v5 1/6] math64: New DIV_U64_ROUND_CLOSEST helper Pali Rohár
2021-08-09 14:53   ` [PATCH v5 2/6] serial: mvebu-uart: implement UART clock driver for configuring UART base clock Pali Rohár
2021-08-09 14:53   ` [PATCH v5 3/6] dt-bindings: mvebu-uart: document DT bindings for marvell,armada-3700-uart-clock Pali Rohár
2021-08-09 14:53   ` [PATCH v5 4/6] dt-bindings: mvebu-uart: update information about UART clock Pali Rohár
2021-08-09 14:53   ` [PATCH v5 5/6] arm64: dts: marvell: armada-37xx: add device node for UART clock and use it Pali Rohár
2021-08-09 14:53   ` [PATCH v5 6/6] serial: mvebu-uart: implement support for baudrates higher than 230400 Pali Rohár
2021-08-20 17:22   ` [PATCH v5 0/6] serial: mvebu-uart: Support for higher baudrates Pali Rohár
2021-08-31  8:35     ` Pali Rohár
2021-09-21  7:41       ` Greg Kroah-Hartman

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