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

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Message-ID: <cover.1767975412.git.ptesarik@suse.com>
Date: Fri,  9 Jan 2026 17:41:34 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: 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>,
	Kuan-Wei Chiu <visitorckw@...il.com>,
	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,
	Petr Tesarik <ptesarik@...e.com>
Subject: [RFC PATCH 0/2] Helper to isolate least-significant bit

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.

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