linux-kernel - Re: [PATCH v1 2/5] lib/bitmap: Introduce bitmap_scatter() and bitmap

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Message-ID: <ZRQbpSiwsS886cvH@smile.fi.intel.com>
Date:   Wed, 27 Sep 2023 15:10:13 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        linux-gpio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Shubhrajyoti Datta <shubhrajyoti.datta@....com>,
        Srinivas Neeli <srinivas.neeli@....com>,
        Michal Simek <michal.simek@....com>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Marek Behún <kabel@...nel.org>
Subject: Re: [PATCH v1 2/5] lib/bitmap: Introduce bitmap_scatter() and
 bitmap_gather() helpers

On Tue, Sep 26, 2023 at 07:10:33PM -0700, Yury Norov wrote:

...

> So, if mask is 0b01, and src is 0b10, the output will be 0b00.

Correct. This how it must work.

> To me it sounds like you've gathered nothing, while the intention
> was to gather all source bits to bit #0.

No, the idea is to gather bits positions of which are provided by a mask
from source to a destination, where the positions are sequential.

> This is my understanding
> of the word 'gather',

You should get the mask meaning. It's not the bit provider, it's a bit
positions provider.

>	and this is how bitmap_remap() works.

It's NOT a replacement of bitmap_remap(). It's specifically written in the
commit message that domain of these APIs is when @old or @new (in case of
bitmap_remap() API) equals 2^n - 1, where n is amount of bits we consider.

...

> If you claim you're replacing bitmap_remap(),
> you should correctly handle
> the above case; when src == dst; when mask is empty, and probably more...

I don't care about corner cases of bitmap_remap(), and we can solve the issue
when it comes. Currently there is no issue with the all users that need these
API as they use different addresses.

-- 
With Best Regards,
Andy Shevchenko