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

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Date:   Thu, 10 Oct 2019 10:41:49 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.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

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:
> > > > > > >
> > > > > > > This macro iterates for each 8-bit group of bits (clump) with set bits,
> > > > > > > within a bitmap memory region. For each iteration, "start" is set to the
> > > > > > > bit offset of the found clump, while the respective clump value is
> > > > > > > stored to the location pointed by "clump". Additionally, the
> > > > > > > bitmap_get_value8 and bitmap_set_value8 functions are introduced to
> > > > > > > respectively get and set an 8-bit value in a bitmap memory region.
> > > > >
> > > > > > 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.
> > > >
> > >
> > > +1
> > >
> > > 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?

> 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.

I looked at bitmap.h and see few functions may have benefited of
actually eliminating a use of long -> u8 -> long conversion.

Here is the question what we are mostly doing after we got a clump out
of bitmap.

> > So, I can tell an opposite, your arguments didn't convince me.
> >
> > Imagine the function which does an or / and / xor operation on bitmap.
> > Now, when I supply unsigned long, I will see
> > operations on one type in _one_ function independently of the size.
> > Your proposal will make an unneded churn.
>
> Depends on what kind of value you will use to do the logical operation
> with the bitmap:
>   - Full bitmap => unsigned long * + size,
>   - Single bitmap "word" => unsigned long,
>   - 8-bit value => u8,
>   - 16-bit value => u16

-- 
With Best Regards,
Andy Shevchenko