Re: [PATCH v4 3/5] lib/bitmap: Introduce bitmap_scatter() and bitmap_gather() helpers

[Andy Shevchenko]

On Thu, Feb 22, 2024 at 03:22:16PM +0100, Herve Codina wrote:
> From: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> 
> These helpers scatters or gathers a bitmap with the help of the mask
> position bits parameter.
> 
> bitmap_scatter() does the following:
>   src:  0000000001011010
>                   ||||||
>            +------+|||||
>            |  +----+||||
>            |  |+----+|||
>            |  ||   +-+||
>            |  ||   |  ||
>   mask: ...v..vv...v..vv
>         ...0..11...0..10
>   dst:  0000001100000010
> 
> and bitmap_gather() performs this one:
>    mask: ...v..vv...v..vv
>    src:  0000001100000010
>             ^  ^^   ^   0
>             |  ||   |  10
>             |  ||   > 010
>             |  |+--> 1010
>             |  +--> 11010
>             +----> 011010
>    dst:  0000000000011010
> 
> bitmap_gather() can the seen as the reverse bitmap_scatter() operation.

> The original work was done by Andy Shevchenko.

Mine SoB is enough for a credit, but thank you :-)

..

> +/**
> + * bitmap_gather - Gather a bitmap according to given mask
> + * @dst: gathered bitmap
> + * @src: scattered bitmap
> + * @mask: mask representing bits to extract from in the scattered bitmap
> + * @nbits: number of bits in each of these bitmaps
> + *
> + * Gathers bitmap with sparse bits according to the given @mask.
> + *
> + * Example:
> + * If @src bitmap = 0x0302, with @mask = 0x1313, @dst will be 0x001a.
> + *
> + * Or in binary form
> + * @src			@mask			@dst
> + * 0000001100000010	0001001100010011	0000000000011010
> + *
> + * (Bits 0, 1, 4, 8, 9, 12 are copied to the bits 0, 1, 2, 3, 4, 5)
> + *
> + * A more 'visual' description of the operation:
> + * mask: ...v..vv...v..vv
> + * src:  0000001100000010
> + *          ^  ^^   ^   0
> + *          |  ||   |  10
> + *          |  ||   > 010
> + *          |  |+--> 1010
> + *          |  +--> 11010
> + *          +----> 011010
> + * dst:  0000000000011010

Cool!

> + * A relationship exists between bitmap_gather() and bitmap_scatter() (See

Either '... (see'
or '(). See'

> + * bitmap_scatter() for the bitmap scatter detailed operations).

> + * Suppose scattered computed using bitmap_scatter(scattered, src, mask, n).
> + * The operation bitmap_gather(result, scattered, mask, n) leads to a result
> + * equal or equivalent to src.

This paragraph...

> + * The result can be 'equivalent' because bitmap_scatter() and bitmap_gather()
> + * are not bijective.


> + * The result and src values are equivalent in that sense that a call to
> + * bitmap_scatter(res, src, mask, n) and a call to bitmap_scatter(res, result,
> + * mask, n) will lead to the same res value.

..seems duplicating this one.

I would drop the latter one.

> + */

-- 
With Best Regards,
Andy Shevchenko