linux-kernel - Re: [RESEND PATCH v4 1/8] bitops: Introduce the for_each_set

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Date:   Tue, 2 Oct 2018 11:21:42 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     William Breathitt Gray <vilhelm.gray@...il.com>,
        linus.walleij@...aro.org, akpm@...ux-foundation.org,
        linux-gpio@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [RESEND PATCH v4 1/8] bitops: Introduce the for_each_set_clump
 macro

On Tue, Oct 02, 2018 at 09:42:48AM +0200, Rasmus Villemoes wrote:
> On 2018-10-02 03:13, William Breathitt Gray wrote:

> The cover letter says
> 
>   The clump_size argument can be an arbitrary number of bits and is not
>   required to be a multiple of 2.
> 
> by which I assume you mean "power of 2", but either way, the above code
> does not seem to take into account the case where bits_offset +
> clump_size straddles a word boundary, so it wouldn't work for a
> clump_size that does not divide BITS_PER_LONG.

E.g. 3 bits in a clump? Hmm...

Why would we need that? I mean some real use case?

> May I suggest another approach:

You may, of course, but see above and my comments below.

> unsigned long bitmap_get_value(const unsigned long *bitmap, unsigned
> start, unsigned width): Get the value of bitmap[start:start+width] for
> 1<=width<=BITS_PER_LONG (it's up to the caller to ensure this is within
> the defined region). That can almost be an inline
> 
> bitmap_get_value(const unsigned long *bitmap, unsigned start, unsigned
> width)
> {
>   unsigned idx = BIT_WORD(start);
>   unsigned offset = start % BITS_PER_LONG;
>   unsigned long lower = bitmap[idx] >> offset;
>   unsigned long upper = offset <= BITS_PER_LONG - width ? 0 :
> bitmap[idx+1] << (BITS_PER_LONG - offset);
>   return (lower | upper) & GENMASK(width-1, 0)
> }
> 
> Then you can implement the for_each_set_clump by a (IMO) more readable
> 
>   for (i = 0, start = 0; i < num_ports; i++, start += gpio_reg_size) {
>     word_mask = bitmap_get_value(mask, start, gpio_reg_size);
>     if (!word_mask)
>       continue;
>     ...
>   }

I would rather go with two prototypes to get()/set() a clump in the bitmap
in a way when it's aligned and BITS_PER_LONG % clump_size == 0.

unsigned long bitmap_get_clump(unsigned long *src, unsigned int start, unsigned int clump_size)
{
	unsigned int index = BIT_WORD(start);
	unsigned int offset = start % BITS_PER_LONG;


	/* These just for spelling the restrictions */
	WARN_ON(BITS_PER_LONG % clump_size);
	WARN_ON(offset % clump_size);

	/* TODO: take care of clump_size == 64 */
	return (bitmap[index] >> offset) & GENMASK(clump_size - 1, 0);
}

Something similar with set with additional parameter unsigned long value
which has MSB cleared till we reach [clump_size - 1 : 0].

-- 
With Best Regards,
Andy Shevchenko