linux-kernel - Re: [RFC PATCH 0/2] Helper to isolate least-significant bit

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Message-ID: <aWE4-zTTvHC4zhvj@google.com>
Date: Sat, 10 Jan 2026 01:20:59 +0800
From: Kuan-Wei Chiu <visitorckw@...il.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>,
	Chris Zankel <chris@...kel.net>, Max Filippov <jcmvbkbc@...il.com>,
	Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Oliver Neukum <oliver@...kum.org>, Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marcel Holtmann <marcel@...tmann.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	Luiz Augusto von Dentz <luiz.dentz@...il.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Florian Westphal <fw@...len.de>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] Helper to isolate least-significant bit

Hi Petr,

On Fri, Jan 09, 2026 at 05:41:34PM +0100, Petr Tesarik wrote:
> Isolation of the least significant bit can be achieved with 3 basic
> ALU operations which are already open-coded in various places in the
> kernel.
> 
> However, since other places less efficient constructs, for example
> `1UL << ffs(x)`, I assume the trick is known only to some authors, and
> it's worth adding a helper to promote its use.

Just out of curiosity, are there any existing users employing 1 <<
ffs(x) (or other inefficient variants) in performance-critical
hotpaths?

>From a quick grep, I only found one instance in drivers/clk/ti/mux.c
matching the 1 << ffs(x) pattern. However, this doesn't appear to be a
bottleneck since it is followed by ti_clk_ll_ops->clk_writel(...). The
latency of the MMIO write would likely overshadow the savings of a few
ALU cycles.

Additionally, it seems that patch #2 focuses on replacing the x & -x
implementation with the new API, rather than converting inefficient
constructs like 1 << ffs(x) to use ffs_val().

Regards,
Kuan-Wei

> 
> Petr Tesarik (2):
>   bits: introduce ffs_val()
>   treewide, bits: use ffs_val() where it is open-coded
> 
>  arch/alpha/include/asm/bitops.h              |  2 +-
>  arch/alpha/kernel/core_cia.c                 |  2 +-
>  arch/alpha/kernel/smp.c                      |  2 +-
>  arch/arc/include/asm/bitops.h                |  2 +-
>  arch/m68k/include/asm/bitops.h               | 12 +++++------
>  arch/mips/dec/ecc-berr.c                     |  2 +-
>  arch/mips/include/asm/bitops.h               |  4 ++--
>  arch/mips/pci/pci-malta.c                    |  4 ++--
>  arch/powerpc/include/asm/bitops.h            |  4 ++--
>  arch/powerpc/kvm/e500_mmu_host.c             |  2 +-
>  arch/powerpc/lib/sstep.c                     |  2 +-
>  arch/powerpc/xmon/ppc-dis.c                  |  3 ++-
>  arch/powerpc/xmon/ppc-opc.c                  |  6 +++---
>  arch/s390/include/asm/bitops.h               |  2 +-
>  arch/xtensa/include/asm/bitops.h             |  6 +++---
>  arch/xtensa/kernel/traps.c                   |  2 +-
>  drivers/gpu/drm/gma500/psb_intel_sdvo.c      |  2 +-
>  drivers/iommu/dma-iommu.c                    |  2 +-
>  drivers/net/ethernet/netronome/nfp/bpf/jit.c |  2 +-
>  drivers/net/usb/cdc_ncm.c                    |  4 ++--
>  include/asm-generic/div64.h                  |  4 ++--
>  include/linux/bitfield.h                     |  3 ++-
>  include/linux/bitops.h                       |  1 +
>  include/linux/ffs_val.h                      | 21 ++++++++++++++++++++
>  include/linux/log2.h                         |  2 +-
>  include/linux/min_heap.h                     |  5 +++--
>  lib/math/gcd.c                               |  4 ++--
>  lib/sort.c                                   |  3 ++-
>  net/bluetooth/mgmt.c                         |  2 +-
>  net/netfilter/nft_set_pipapo.c               |  2 +-
>  30 files changed, 70 insertions(+), 44 deletions(-)
>  create mode 100644 include/linux/ffs_val.h
> 
> -- 
> 2.52.0
>