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

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Message-ID: <20231025173606.i2e74tfpbo6as5u4@mercury.elektranox.org>
Date:   Wed, 25 Oct 2023 19:36:06 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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

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@...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".

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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