Re: [PATCH v17 01/14] bitops: Introduce the for_each_set_clump8 macro

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	Linux PM mailing list <linux-pm@vger.kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	William Breathitt Gray <vilhelm.gray@gmail.com>,
	morten.tiljeset@prevas.dk,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Lukas Wunner <lukas@wunner.de>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Phil Reid <preid@electromag.com.au>,
	sean.nyekjaer@prevas.dk,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v17 01/14] bitops: Introduce the for_each_set_clump8 macro
Date: Thu, 10 Oct 2019 11:07:50 +0300	[thread overview]
Message-ID: <20191010080750.GN32742@smile.fi.intel.com> (raw)
In-Reply-To: <CAMuHMdVrQyt=VJ8outiGEXW78-cY=YUWyeVXN-_MFg75erJ=Yg@mail.gmail.com>

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@gmail.com> wrote:
> > On Thu, Oct 10, 2019 at 9:29 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > On Thu, Oct 10, 2019 at 7:49 AM Andy Shevchenko
> > > <andy.shevchenko@gmail.com> wrote:
> > > > On Thu, Oct 10, 2019 at 5:31 AM Masahiro Yamada
> > > > <yamada.masahiro@socionext.com> wrote:
> > > > > On Thu, Oct 10, 2019 at 3:54 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > > On Wed, Oct 9, 2019 at 7:09 PM Andy Shevchenko
> > > > > > <andriy.shevchenko@linux.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@gmail.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().

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.

> > 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.
> 
> If I call find_next_clump8() to extract a byte, I guess I want to process an u8
> aftwerwards?

Some functions may expect a width-(semi-)dependent types, like regmap_write().
Yes, it's possible to supply u8 there and have an implicit type cast.

-- 
With Best Regards,
Andy Shevchenko

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