linux-kernel - Re: [PATCH 00/17] Non-const bitfield helper conversions

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Message-ID: <CAMuHMdWKHoBWm2XQmKwB0q8Ya8KzpCgA29D0igXJrGY8=3e8_A@mail.gmail.com>
Date:   Tue, 23 Nov 2021 09:20:23 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Paul Walmsley <paul@...an.com>,
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        Stephen Boyd <sboyd@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Tero Kristo <kristo@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Lorenzo Bianconi <lorenzo.bianconi83@...il.com>,
        Benoit Parrot <bparrot@...com>,
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        Adrian Hunter <adrian.hunter@...el.com>,
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Subject: Re: [PATCH 00/17] Non-const bitfield helper conversions

Hi Alexandre,

On Mon, Nov 22, 2021 at 6:50 PM Alexandre Belloni
<alexandre.belloni@...tlin.com> wrote:
> On 22/11/2021 16:53:53+0100, Geert Uytterhoeven wrote:
> > The existing FIELD_{GET,PREP}() macros are limited to compile-time
> > constants.  However, it is very common to prepare or extract bitfield
> > elements where the bitfield mask is not a compile-time constant.
> > To avoid this limitation, the AT91 clock driver already has its own
> > field_{prep,get}() macros.
>
> My understanding was that this (being compile time only) was actually
> done on purpose. Did I misunderstand?

Yes, it was done on purpose, to maximize type safety.

However, this imposes a severe limitation: we cannot use them in case
the mask is non-const (i.e. stored in some data structure).  This
is a quite common use-case, given the examples I found and converted,
and given you added field_{get,prep}() to the AT91 clock driver.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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