linux-kernel - Re: [PATCH v4 0/8] Introduce the for_each_set

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Date:   Wed, 16 May 2018 16:03:51 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-arch@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/8] Introduce the for_each_set_clump macro

On Tue, May 15, 2018 at 6:22 PM, William Breathitt Gray
<vilhelm.gray@...il.com> wrote:

> For example, suppose you would like to iterate over a 16-bit integer 4
> bits at a time, skipping over 4-bit groups with no set bit, where XXXX
> represents the current 4-bit group:
>
>     Example:        1011 1110 0000 1111
>     First loop:     1011 1110 0000 XXXX
>     Second loop:    1011 XXXX 0000 1111
>     Third loop:     XXXX 1110 0000 1111
>
> Each iteration of the loop returns the next 4-bit group that has at
> least one set bit.
>
> The for_each_set_clump macro has six parameters:
>
>     * clump: set to current clump index for the iteration
>     * index: set to current bitmap word index for the iteration
>     * offset: bits offset of the found clump in the bitmap word
>     * bits: bitmap to search within
>     * size: bitmap size in number of clumps
>     * clump_size: clump size in number of bits
>
> The clump_size argument can be an arbitrary number of bits and is not
> required to be a multiple of 2.

I must say I'm impressed. Very nice arithmetics going on there.

If I can get some ACK for the bitops patch I'd be happy to merge
it all through the GPIO tree. The users are pretty clear cut.

BTW: if I could, I would pull out Donald Knuth's "The Art of Computer
Programming vol 4A" chapter 7.1.3 "Bitwise Tricks and Techniques"
to see what he has to say about the subject, but I don't have
that book as it turns out.

Yours,
Linus Walleij