linux-kernel - Re: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO

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Message-ID: <CAHk-=wjO5ivM6k7iMiThO9JfxH0dhLe=mcC4TQwReU0nBCnWpg@mail.gmail.com>
Date:   Tue, 24 Oct 2023 12:53:03 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sebastian Reichel <sebastian.reichel@...labora.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>, kernel@...labora.com
Subject: Re: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO_OVERFLOW

On Tue, 24 Oct 2023 at 09:32, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I would really prefer to just make our regular DIV_ROUND_UP() DTRT.  But:
>
>  - 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

Ok. I have a potential beginning of a solution.

It is unbelievably disgustingly complicated. But it might approach
being correct.

And by that "it might approach being correct" I obviously mean "this
is untested, but builds at least some kernel code".

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.

I'm hoping that somebody will go "Linus, you're just being
*completely* silly, it's much easier to do XYZ".

            Linus

View attachment "0001-Introduce-complicated-non-overflowing-DIV_ROUND_UP.patch" of type "text/x-patch" (5557 bytes)