linux-kernel - Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}

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Date:   Mon, 17 Jul 2023 16:14:57 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, linux@...musvillemoes.dk,
        yury.norov@...il.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}_value()

>
> > Cc: Arnd Bergmann <arnd@...db.de>
>
> You can use --cc to `git send-email` instead of polluting the commit message.

Right. But as far as I can tell, certain kernel devs prefer to be CCed
on the whole series, whereas others do not want to see anything but
the actual patch they were interested in.
I am not sure about Arnd's preferences, so I just decided to keep the
tag from the original patch by Syed Nayyar Waris (which I also
consider to be an indication of the fact "that potentially interested
parties have been included in the discussion" per
https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by)

> > Signed-off-by: Syed Nayyar Waris <syednwaris@...il.com>
> > Signed-off-by: William Breathitt Gray <william.gray@...aro.org>
> > Link: https://lore.kernel.org/lkml/fe12eedf3666f4af5138de0e70b67a07c7f40338.1592224129.git.syednwaris@gmail.com/
> > Suggested-by: Yury Norov <yury.norov@...il.com>
> > Signed-off-by: Alexander Potapenko <glider@...gle.com>
>
> With above, I think you can also add Co-developed-by (as the changes were
> made).

Ok, will do.

> ...
>
> > +static inline void bitmap_set_value(unsigned long *map,
> > +                                 unsigned long value,
> > +                                 unsigned long start, unsigned long nbits)
> > +{
> > +     const size_t index = BIT_WORD(start);
> > +     const unsigned long offset = start % BITS_PER_LONG;
> > +     const unsigned long space = BITS_PER_LONG - offset;
> > +
> > +     value &= GENMASK(nbits - 1, 0);
> > +
> > +     if (space >= nbits) {
>
> > +             map[index] &= ~(GENMASK(nbits + offset - 1, offset));
>
> I remember that this construction may bring horrible code on some architectures
> with some version(s) of the compiler (*).

Wow, even the trunk Clang and GCC seem to generate better code for
your version of this line: https://godbolt.org/z/36Kqxhe6j

> To fix that I found an easy refactoring:
>
>                 map[index] &= ~(GENMASK(nbits, 0) << offset));
>

I'll take this one.

> (*) don't remember the actual versions, though, but anyway...
>
> > +             map[index] |= value << offset;
> > +             return;
> > +     }
> > +     map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
> > +     map[index] |= value << offset;
> > +     map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> > +     map[index + 1] |= (value >> space);
> > +}
>
> --
> With Best Regards,
> Andy Shevchenko
>
>


-- 
Alexander Potapenko
Software Engineer

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