From: "Pali Rohár" <pali@kernel.org>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Willy Tarreau" <w@1wt.eu>,
"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 19:44:48 +0200 [thread overview]
Message-ID: <20210625174448.kje4pvg5ixpu3vaw@pali> (raw)
In-Reply-To: <CAMuHMdWfE7UKkp6=MdAVgHywFjDK3SN4m9+d_6AmJTB=qBEqFw@mail.gmail.com>
On Friday 25 June 2021 19:39:10 Geert Uytterhoeven wrote:
> Hi Willy,
>
> On Fri, Jun 25, 2021 at 5:50 PM Willy Tarreau <w@1wt.eu> wrote:
> > 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).
>
> Fair enough.
> So we want to add a cast to DIV64_U64_ROUND_CLOSEST() above, too?
For DIV64_U64_ROUND_CLOSEST() it is not needed. divisor is copied into
u64 _tmp variable and therefore "(dividend) + _tmp / 2" is already
evaluated in 64-bit context even when dividend is only 32-bit.
The only trap is that negative value as written below.
> > 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.
>
> Yep.
>
> Gr{oetje,eeting}s,
>
> Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
> -- Linus Torvalds
next prev parent reply other threads:[~2021-06-25 17:44 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
2021-06-25 17:39 ` Geert Uytterhoeven
2021-06-25 17:44 ` Pali Rohár [this message]
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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