linux-kernel - Re: [PATCH v2 07/11] math64: New DIV_U64_ROUND

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Message-ID: <CAHp75Vdzb9yhm490vAhL7O0S+5FPR=sM1Tohqi70xYV+bXVr8g@mail.gmail.com>
Date:   Mon, 19 Jul 2021 15:45:08 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Pali Rohár <pali@...nel.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Lunn <andrew@...n.ch>,
        Gregory Clement <gregory.clement@...tlin.com>,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
        Vladimir Vid <vladimir.vid@...tura.hr>,
        Marek Behún <kabel@...nel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 07/11] math64: New DIV_U64_ROUND_CLOSEST helper

On Fri, Jun 25, 2021 at 6:39 PM Pali Rohár <pali@...nel.org> wrote:
> On Friday 25 June 2021 17:22:31 Geert Uytterhoeven wrote:
> > On Fri, Jun 25, 2021 at 4:37 PM Pali Rohár <pali@...nel.org> wrote:

...

> > > +/*
> > > + * DIV_U64_ROUND_CLOSEST - unsigned 64bit divide with 32bit divisor rounded to nearest integer

> > > + * @dividend: unsigned 64bit dividend

(1)

> > > + * @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.

This contradicts (1).

> As it is a macro (and not
> inline function) type is not automatically enforced.
>
> DIV_S64_ROUND_CLOSEST macro assigns its argument into temporary 64-bit
> variable which then ensures usage of 64-bit arithmetic operations. Same
> applies for DIV64_U64_ROUND_CLOSEST and DIV64_U64_ROUND_UP macros.
>
> So this is reason why I added explicit cast to u64.

I don't see the reason for casting in the current code. Probably you
need to rephrase documentation to explain why it's there.

-- 
With Best Regards,
Andy Shevchenko