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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 13 Mar 2019 11:11:08 +0100
Message-ID: <CAMuHMdXC5QffaOj479j3WCt5_GH2jeQnGrPoCrRPoFkHmFeDfw@mail.gmail.com>
Subject: Re: [PATCH] um: remove uses of variable length arrays
To:     Bartosz Golaszewski <brgl@bgdev.pl>
Cc:     Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Jeff Dike <jdike@addtoit.com>,
        Richard Weinberger <richard@nod.at>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        linux-um@lists.infradead.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Bartosz,

On Wed, Mar 13, 2019 at 10:50 AM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> śr., 13 mar 2019 o 10:45 Anton Ivanov
> <anton.ivanov@cambridgegreys.com> napisał(a):
> > PID can be more than 5 digits nowdays.
>
> Do we have a macro in the kernel that yields the minimum size of an
> array to fit a number? I can't find it. I vagely remember seeing
> something like this in systemd tree. If it doesn't exist in the kernel
> then maybe we should copy it over.

1 + const_ilog2(n) / log2(10) = 1 + const_ilog2(n) / 3.321928

Using fixed point calculations, that can be approximated as

    1 + 19728 * const_ilog2(n) >> 16
    1 + 77 * const_ilog2(n) >> 8

(the factors might be one off)

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