linux-kernel - Re: [RFC PATCH 1/2] bits: introduce ffs

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Message-ID: <aWFaiEvJyj_35-NK@yury>
Date: Fri, 9 Jan 2026 14:44:08 -0500
From: Yury Norov <ynorov@...dia.com>
To: Petr Tesarik <ptesarik@...e.com>
Cc: Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Richard Henderson <richard.henderson@...aro.org>,
	Matt Turner <mattst88@...il.com>,
	Magnus Lindholm <linmag7@...il.com>,
	Vineet Gupta <vgupta@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"Maciej W. Rozycki" <macro@...am.me.uk>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
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	Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
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	Thomas Zimmermann <tzimmermann@...e.de>,
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	Pablo Neira Ayuso <pablo@...filter.org>,
	Florian Westphal <fw@...len.de>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] bits: introduce ffs_val()

On Fri, Jan 09, 2026 at 08:27:11PM +0100, Petr Tesarik wrote:
> On Fri, 9 Jan 2026 13:26:30 -0500
> Yury Norov <ynorov@...dia.com> wrote:

...

> > In bitops world, something_mask() has a very clear meaning. Consider
> > GENMASK(), BITMAP_FIRST_BYTE_MASK(), __BF_FIELD_CHECK_MASK(), and so
> > on.
> > 
> > lsb_mask(), or LSB_MASK() if you prefer, is just right.
> 
> I assume you mean BITMAP_FIRST_WORD_MASK(), not btrfs-specific
> BITMAP_FIRST_BYTE_MASK(), defined in fs/btrfs/extent_io.h.
> 
> But then it nicely illustrate exactly what I mean:

They all have a consistent meaning:

>   BIT_MASK(4) = 0x0000000000000010

A mask constructed from bit #4.

>   GENMASK(4, 0) = 0x000000000000001f

A mask constructed from bits #0..4.

>   BITMAP_FIRST_WORD_MASK(4) == 0xfffffffffffffff0

A mask constructed from all bits starting #4.

Don't see any misalignment here.
 
> > > The only issue is that LSB may also refer to least-significant BYTE. :-(  
> > 
> > It will never mean byte because it hosts in bitops.h, not byteops.h
> 
> I'm afraid that identifiers from various sources end up next to each
> other and they do not carry tags where each of them came from...

You can name it lsb_bitmask() if you want, either way works. But to me
shorter is better.