linux-kernel - Re: [PATCH v17 01/14] bitops: Introduce the for_each_set

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Date:   Thu, 10 Oct 2019 10:21:45 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux PM mailing list <linux-pm@...r.kernel.org>,
        Phil Reid <preid@...ctromag.com.au>,
        Lukas Wunner <lukas@...ner.de>, sean.nyekjaer@...vas.dk,
        morten.tiljeset@...vas.dk, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v17 01/14] bitops: Introduce the for_each_set_clump8 macro

Hi Andy,

On Thu, Oct 10, 2019 at 10:08 AM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Thu, Oct 10, 2019 at 09:49:51AM +0200, Geert Uytterhoeven wrote:
> > On Thu, Oct 10, 2019 at 9:42 AM Andy Shevchenko
> > <andy.shevchenko@...il.com> wrote:
> > > On Thu, Oct 10, 2019 at 9:29 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > > > On Thu, Oct 10, 2019 at 7:49 AM Andy Shevchenko
> > > > <andy.shevchenko@...il.com> wrote:
> > > > > On Thu, Oct 10, 2019 at 5:31 AM Masahiro Yamada
> > > > > <yamada.masahiro@...ionext.com> wrote:
> > > > > > On Thu, Oct 10, 2019 at 3:54 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > > > > > > On Wed, Oct 9, 2019 at 7:09 PM Andy Shevchenko
> > > > > > > <andriy.shevchenko@...ux.intel.com> wrote:
> > > > > > > > On Thu, Oct 10, 2019 at 01:28:08AM +0900, Masahiro Yamada wrote:
> > > > > > > > > On Thu, Oct 10, 2019 at 12:27 AM William Breathitt Gray
> > > > > > > > > <vilhelm.gray@...il.com> wrote:
>
> > > > > > > > > Why is the return type "unsigned long" where you know
> > > > > > > > > it return the 8-bit value ?
> > > > > > > >
> > > > > > > > Because bitmap API operates on unsigned long type. This is not only
> > > > > > > > consistency, but for sake of flexibility in case we would like to introduce
> > > > > > > > more calls like clump16 or so.
> > > > > > >
> > > > > > > TBH, that doesn't convince me: those functions explicitly take/return an
> > > > > > > 8-bit value, and have "8" in their name.  The 8-bit value is never
> > > > > > > really related to, retrieved from, or stored in a full "unsigned long"
> > > > > > > element of a bitmap, only to/from/in a part (byte) of it.
> > > > > > >
> > > > > > > Following your rationale, all of iowrite{8,16,32,64}*() should take an
> > > > > > > "unsigned long" value, too.
> > > > > >
> > > > > > Using u8/u16/u32/u64 looks more consistent with other bitmap helpers.
> > > > > >
> > > > > > void bitmap_from_arr32(unsigned long *bitmap, const u32 *buf, unsigned
> > > > > > int nbits);
> > > > > > void bitmap_to_arr32(u32 *buf, const unsigned long *bitmap, unsigned int nbits);
> > > > > > static inline void bitmap_from_u64(unsigned long *dst, u64 mask);
> > > > > >
> > > > > > If you want to see more examples from other parts,
> > > > >
> > > > > Geert's and yours examples both are not related. They are about
> > > > > fixed-width properies when we know that is the part of protocol.
> > > > > Here we have no protocol which stricts us to the mentioned fixed-width types.
> > > >
> > > > Yes you have: they are functions to store/retrieve an 8-bit value from
> > > > the middle of the bitmap, which is reflected in their names ("clump8",
> > > > "value8").
> > > > The input/output value is clearly separated from the actual bitmap,
> > > > which is referenced by the "unsigned long *".
> > > >
> > > > If you add new "value16" functions, they will be intended to store/retrieve
> > > > 16-bit values.
> > >
> > > And if I add 4-bit, 12-bit or 24-bit values, what should I use?
> >
> > Whatever is needed to store that?
> > I agree "unsigned long" is appropriate for a generic function to extract a
> > bit field of 1 to BITS_PER_LONG bits.
> >
> > > > Besides, if retrieving an 8-bit value requires passing an
> > > > "unsigned long *", the caller needs two variables: one unsigned long to
> > > > pass the address of, and one u8 to copy the returned value into.
> > >
> > > Why do you need a temporary variable? In some cases it might make
> > > sense, but in general simple cases I don't see what you may achieve
> > > with it.
> >
> > Because find_next_clump8() takes a pointer to store the output value.
>
> So does regmap_read().

I believe that one is different, as it is a generic function, and the
width of the
returned value depends on the regmap config.

> 8 appeared there during review when it has been proposed to optimize to 8-bit
> clumps as most of the current users utilize it. The initial idea was to be
> bit-width agnostic. And with current API it's possible to easy convert to other
> formats later if we need.

"optimized for 8-bit clumps" and "out-of-line function that takes an
unsigned long pointer for an output parameter" don't match well, IMHO.

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