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bh=m0e4zv4QrLqLCD1Me9b38JfsJRlKwcWAapK9C2V803Y=; b=qUuKqtzcRvnpc7PrP1h4cDuVo/VmQwPivpew0cOJNC1aGPWRXJSBv1ZKU33rZUJuu5Hn8q piA8q3kUzwW/fUDw== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id A64ED3EA63; Mon, 12 Jan 2026 08:15:41 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id ah8sJ62tZGmqcgAAD6G6ig (envelope-from ); Mon, 12 Jan 2026 08:15:41 +0000 Message-ID: <9c61bd17-bd42-4641-a118-9114a5ccdc13@suse.de> Date: Mon, 12 Jan 2026 09:15:41 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/2] bits: introduce ffs_val() To: Petr Tesarik , Yury Norov , Rasmus Villemoes Cc: Richard Henderson , Matt Turner , Magnus Lindholm , Vineet Gupta , Geert Uytterhoeven , "Maciej W. Rozycki" , Thomas Bogendoerfer , Madhavan Srinivasan , Michael Ellerman , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Chris Zankel , Max Filippov , Patrik Jakobsson , Maarten Lankhorst , Maxime Ripard , David Airlie , Simona Vetter , Robin Murphy , Joerg Roedel , Will Deacon , Jakub Kicinski , Andrew Lunn , "David S. Miller" , Eric Dumazet , Paolo Abeni , Oliver Neukum , Arnd Bergmann , Kuan-Wei Chiu , Andrew Morton , Marcel Holtmann , Johan Hedberg , Luiz Augusto von Dentz , Pablo Neira Ayuso , Florian Westphal , linux-kernel@vger.kernel.org References: <9767487fcab7dbe7a7282a48a492171629eb935b.1767975412.git.ptesarik@suse.com> Content-Language: en-US From: Thomas Zimmermann Autocrypt: addr=tzimmermann@suse.de; keydata= xsBNBFs50uABCADEHPidWt974CaxBVbrIBwqcq/WURinJ3+2WlIrKWspiP83vfZKaXhFYsdg XH47fDVbPPj+d6tQrw5lPQCyqjwrCPYnq3WlIBnGPJ4/jreTL6V+qfKRDlGLWFjZcsrPJGE0 BeB5BbqP5erN1qylK9i3gPoQjXGhpBpQYwRrEyQyjuvk+Ev0K1Jc5tVDeJAuau3TGNgah4Yc hdHm3bkPjz9EErV85RwvImQ1dptvx6s7xzwXTgGAsaYZsL8WCwDaTuqFa1d1jjlaxg6+tZsB 9GluwvIhSezPgnEmimZDkGnZRRSFiGP8yjqTjjWuf0bSj5rUnTGiyLyRZRNGcXmu6hjlABEB AAHNJ1Rob21hcyBaaW1tZXJtYW5uIDx0emltbWVybWFubkBzdXNlLmRlPsLAjgQTAQgAOAIb AwULCQgHAgYVCgkICwIEFgIDAQIeAQIXgBYhBHIX+6yM6c9jRKFo5WgNwR1TC3ojBQJftODH AAoJEGgNwR1TC3ojx1wH/0hKGWugiqDgLNXLRD/4TfHBEKmxIrmfu9Z5t7vwUKfwhFL6hqvo lXPJJKQpQ2z8+X2vZm/slsLn7J1yjrOsoJhKABDi+3QWWSGkaGwRJAdPVVyJMfJRNNNIKwVb U6B1BkX2XDKDGffF4TxlOpSQzdtNI/9gleOoUA8+jy8knnDYzjBNOZqLG2FuTdicBXblz0Mf vg41gd9kCwYXDnD91rJU8tzylXv03E75NCaTxTM+FBXPmsAVYQ4GYhhgFt8S2UWMoaaABLDe 7l5FdnLdDEcbmd8uLU2CaG4W2cLrUaI4jz2XbkcPQkqTQ3EB67hYkjiEE6Zy3ggOitiQGcqp j//OwE0EWznS4AEIAMYmP4M/V+T5RY5at/g7rUdNsLhWv1APYrh9RQefODYHrNRHUE9eosYb T6XMryR9hT8XlGOYRwKWwiQBoWSDiTMo/Xi29jUnn4BXfI2px2DTXwc22LKtLAgTRjP+qbU6 3Y0xnQN29UGDbYgyyK51DW3H0If2a3JNsheAAK+Xc9baj0LGIc8T9uiEWHBnCH+RdhgATnWW GKdDegUR5BkDfDg5O/FISymJBHx2Dyoklv5g4BzkgqTqwmaYzsl8UxZKvbaxq0zbehDda8lv hFXodNFMAgTLJlLuDYOGLK2AwbrS3Sp0AEbkpdJBb44qVlGm5bApZouHeJ/+n+7r12+lqdsA EQEAAcLAdgQYAQgAIAIbDBYhBHIX+6yM6c9jRKFo5WgNwR1TC3ojBQJftOH6AAoJEGgNwR1T C3ojVSkIALpAPkIJPQoURPb1VWjh34l0HlglmYHvZszJWTXYwavHR8+k6Baa6H7ufXNQtThR yIxJrQLW6rV5lm7TjhffEhxVCn37+cg0zZ3j7zIsSS0rx/aMwi6VhFJA5hfn3T0TtrijKP4A SAQO9xD1Zk9/61JWk8OysuIh7MXkl0fxbRKWE93XeQBhIJHQfnc+YBLprdnxR446Sh8Wn/2D Ya8cavuWf2zrB6cZurs048xe0UbSW5AOSo4V9M0jzYI4nZqTmPxYyXbm30Kvmz0rYVRaitYJ 4kyYYMhuULvrJDMjZRvaNe52tkKAvMevcGdt38H4KSVXAylqyQOW5zvPc4/sq9c= In-Reply-To: <9767487fcab7dbe7a7282a48a492171629eb935b.1767975412.git.ptesarik@suse.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Score: -3.01 X-Spamd-Result: default: False [-3.01 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; FREEMAIL_TO(0.00)[suse.com,gmail.com,rasmusvillemoes.dk]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; MIME_TRACE(0.00)[0:+]; FUZZY_RATELIMITED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCPT_COUNT_TWELVE(0.00)[40]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; ARC_NA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; FREEMAIL_CC(0.00)[linaro.org,gmail.com,kernel.org,linux-m68k.org,orcam.me.uk,alpha.franken.de,linux.ibm.com,ellerman.id.au,zankel.net,linux.intel.com,ffwll.ch,arm.com,8bytes.org,lunn.ch,davemloft.net,google.com,redhat.com,neukum.org,arndb.de,linux-foundation.org,holtmann.org,netfilter.org,strlen.de,vger.kernel.org]; RCVD_COUNT_TWO(0.00)[2]; MID_RHS_MATCH_FROM(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TAGGED_RCPT(0.00)[netdev]; R_RATELIMIT(0.00)[to_ip_from(RLqbkuqg11osc55coksncbnarj)]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.de:+]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim,suse.de:mid,suse.com:url,suse.com:email] X-Spam-Level: X-Rspamd-Action: no action X-Rspamd-Queue-Id: AD2313368A X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Flag: NO Hi Petr Am 09.01.26 um 17:37 schrieb Petr Tesarik: > Introduce a macro that can efficiently extract the least significant > non-zero bit from a value. > > Interestingly, this bit-twiddling trick is open-coded in some places, but > it also appears to be little known, leading to various inefficient > implementations in other places. Let's make it part of the standard bitops > arsenal. > > Define the macro in a separate header file included from , > to allow using it in very low-level header files that may not want to > include all of . > > Signed-off-by: Petr Tesarik > --- > MAINTAINERS | 1 + > include/linux/bitops.h | 1 + > include/linux/ffs_val.h | 21 +++++++++++++++++++++ > 3 files changed, 23 insertions(+) > create mode 100644 include/linux/ffs_val.h > > diff --git a/MAINTAINERS b/MAINTAINERS > index a0dd762f5648b..8f15c76a67ea2 100644 > --- a/MAINTAINERS > +++ b/MAINTAINERS > @@ -4466,6 +4466,7 @@ F: arch/*/lib/bitops.c > F: include/asm-generic/bitops > F: include/asm-generic/bitops.h > F: include/linux/bitops.h > +F: include/linux/ffs_val.h > F: lib/hweight.c > F: lib/test_bitops.c > F: tools/*/bitops* > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index ea7898cc59039..209f0c3e07b9e 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -4,6 +4,7 @@ > > #include > #include > +#include > #include > > #include > diff --git a/include/linux/ffs_val.h b/include/linux/ffs_val.h > new file mode 100644 > index 0000000000000..193ec86d2b53b > --- /dev/null > +++ b/include/linux/ffs_val.h > @@ -0,0 +1,21 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +#ifndef _ASM_LINUX_FFS_VAL_H_ > +#define _ASM_LINUX_FFS_VAL_H_ > + > +/** > + * ffs_val - find the value of the first set bit > + * @x: the value to search > + * > + * Unlike ffs(), which returns a bit position, ffs_val() returns the bit > + * value itself. This sentence was confusing me at first, because the individual bit's value is always '1'. Maybe say something more descriptive, such as 'ffs_val returns the value resulting from that bit's position.' > + * > + * Returns: > + * least significant non-zero bit, 0 if all bits are zero Same here. > + */ > +#define ffs_val(x) \ > +({ \ > + const typeof(x) val__ = (x); \ > + val__ & -val__; \ Is this construct supposed to work with signed integers and/or negative numbers? I assume that two's complement can be expected nowadays, but for LONG_MIN it returns zero AFAICT. The documentation should mention these cases. Best regards Thomas > +}) > + > +#endif /* _ASM_LINUX_FFS_VAL_H_ */ -- -- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)