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Date:   Wed, 25 Oct 2023 19:36:06 +0200
From:   Sebastian Reichel <sebastian.reichel@collabora.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org,
        Vasily Gorbik <gor@linux.ibm.com>,
        Niklas Schnelle <schnelle@linux.ibm.com>, kernel@collabora.com
Subject: Re: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO_OVERFLOW
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Hi,

On Tue, Oct 24, 2023 at 12:53:03PM -1000, Linus Torvalds wrote:
> On Tue, 24 Oct 2023 at 09:32, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > I would really prefer to just make our regular DIV_ROUND_UP() DTRT.  Bu=
t:
> >
> >  - people do use it with complex first arguments (ie function calls
> > etc) that we don't want to evaluate twice
> >
> >  - we can't make it an inline function, because the types aren't fixed
> >
> >  - we can't even use a statement expression and __auto_type, because
> > these things are used in type definitions etc and need to be constant
> > expressions
>=20
> Ok. I have a potential beginning of a solution.
>=20
> It is unbelievably disgustingly complicated. But it might approach
> being correct.
>=20
> And by that "it might approach being correct" I obviously mean "this
> is untested, but builds at least some kernel code".
>=20
> I'm almost certain it will fail on more complex cases, because I
> already found a lot of questionable stuff that was simply hidden by
> the old macro just silently doing the C arithmetic type conversions,
> and this thing does type handling manually.
>=20
> I'm hoping that somebody will go "Linus, you're just being
> *completely* silly, it's much easier to do XYZ".

I think your new __div_round_up_ull() should also be used by
DIV_ROUND_UP_ULL, which is defined in linux/math.h.

-- Sebastian

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