linux-kernel - Re: [PATCH v1 1/1] bitmap.h: add const modifier to bitmap_next_[set/clear]

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Message-ID: <YQFfvoZn/DlYRJkI@yury-ThinkPad>
Date:   Wed, 28 Jul 2021 06:46:38 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Andrea Merello <andrea.merello@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrea Merello <andrea.merello@....it>
Subject: Re: [PATCH v1 1/1] bitmap.h: add const modifier to
 bitmap_next_[set/clear]_region() argument

On Wed, Jul 28, 2021 at 08:39:15AM +0200, Andrea Merello wrote:
> Il giorno mar 27 lug 2021 alle ore 17:26 Yury Norov
> <yury.norov@...il.com> ha scritto:
> >
> > On Tue, Jul 27, 2021 at 02:48:42PM +0300, Andy Shevchenko wrote:
> > > On Tue, Jul 27, 2021 at 11:44:41AM +0200, Andrea Merello wrote:
> > > > Those two functions don't modify the bitmap, so their bitmap argument
> > > > should be const. This patch add this.
> >
> > Thanks Andrea.
> >
> > Acked-by: Yury Norov <yury.norov@...il.com>
> >
> > > Constification is always a good thing. No objections from me,
> > > although Yuri is doing something with them in one of his patch series.
> >
> > I try to remove those two because in practice they bring more mess
> > than good. All real use-cases for bitmap_next_{set,clear}_region
> > relate to iterating the whole bitmap, ie nobody just wants to find a
> > next region. Untill recently there was only a single user of the API,
> > so I easily reworked the code to use find_first_bit/find_next bit and
> > by chance return faster.
> 
> Ah, that's interesting to know: I'm working on a new driver that used
> find_next_zero_bit() and find_next_bit() [0]; Andy told me that my
> algorithm could be rewritten exploiting some bitmap_* helpers, so I
> rewrote my code using bitmap_for_each_set_region(), which in turn uses
> bitmap_next_set_region().
> 
> Should I revert to my previous implementation that uses
> find_next_zero_bit() and find_next_bit() ?
> 
> [0] https://lore.kernel.org/linux-iio/20210715141742.15072-3-andrea.merello@gmail.com/

No, you're doing right. I just reimplemented and renamed
bitmap_for_each_set_region to unify with other similar functions. The
transition is very stragthforward, so you have nothing to worry on.

https://patchwork.kernel.org/project/linux-mmc/patch/20210719021755.883182-3-yury.norov@gmail.com/

> 
> > https://github.com/norov/linux/commit/1c870b5c3fcd2eea9b351a1e0af8d1e93be78e1e
> >
> > Recently in next-20210716, there appeared another user in fs/btrfs/extent_io.c:
> > find_next_dirty_byte(). The fun is that in that case the length of bitmap
> > is 16 bit, so it's probably simpler to return the bitmap by value,
> > instead of calling prologue code up to 8 times. Anyways, I'll contact
> > authors of the find_next_dirty_byte() and ask if it's possible to rework
> > their code.
> >
> > > Yuri, do you have a public repo / branch that people can base their changes to
> > > bitmap stuff against of?
> >
> > I collected not yet upstreamed bitmap patches here:
> >
> > https://github.com/norov/linux/commits/bitmap-20210716
> 
> Looks like also bitmap_for_each_set_region() went away indeed ?

Just replaced with for_each_{set,clear}_bitrange.

Thanks,
Yury