From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758238AbXH2IXQ (ORCPT ); Wed, 29 Aug 2007 04:23:16 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754856AbXH2IXB (ORCPT ); Wed, 29 Aug 2007 04:23:01 -0400 Received: from nic.NetDirect.CA ([216.16.235.2]:58535 "EHLO rubicon.netdirect.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752985AbXH2IXA (ORCPT ); Wed, 29 Aug 2007 04:23:00 -0400 X-Originating-Ip: 72.143.66.27 Date: Wed, 29 Aug 2007 04:12:19 -0400 (EDT) From: "Robert P. J. Day" X-X-Sender: rpjday@localhost.localdomain To: Linux Kernel Mailing List cc: Andrew Morton Subject: [PATCH] Add a "rounddown_pow_of_two" routine to log2.h. Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Net-Direct-Inc-MailScanner-Information: Please contact the ISP for more information X-Net-Direct-Inc-MailScanner: Found to be clean X-Net-Direct-Inc-MailScanner-SpamCheck: not spam, SpamAssassin (not cached, score=-16.8, required 5, autolearn=not spam, ALL_TRUSTED -1.80, BAYES_00 -15.00, INIT_RECVD_OUR_AUTH -20.00, RCVD_IN_SORBS_DUL 20.00) X-Net-Direct-Inc-MailScanner-From: rpjday@mindspring.com Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org To go along with the existing "roundup_pow_of_two" routine, add one for rounding down since that operation appears to crop up on a regular basis in the source tree. Signed-off-by: Robert P. J. Day --- some brief tests indicate that these routines return the correct answer. i'm not sure if i've officially submitted this before or if it's already been accepted in a tree somewhere. diff --git a/include/linux/log2.h b/include/linux/log2.h index 1b8a2c1..ff8835b 100644 --- a/include/linux/log2.h +++ b/include/linux/log2.h @@ -63,6 +63,15 @@ unsigned long __roundup_pow_of_two(unsigned long n) return 1UL << fls_long(n - 1); } +/* + * round down to nearest power of two + */ +static inline __attribute__((const)) +unsigned long __rounddown_pow_of_two(unsigned long n) +{ + return 1UL << (fls_long(n) - 1); +} + /** * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value * @n - parameter @@ -165,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n) __roundup_pow_of_two(n) \ ) +/** + * rounddown_pow_of_two - round the given value down to nearest power of two + * @n - parameter + * + * round the given value down to the nearest power of two + * - the result is undefined when n == 0 + * - this can be used to initialise global variables from constant data + */ +#define rounddown_pow_of_two(n) \ +( \ + __builtin_constant_p(n) ? ( \ + (n == 1) ? 0 : \ + (1UL << ilog2(n)) : \ + __rounddown_pow_of_two(n) \ + ) + #endif /* _LINUX_LOG2_H */ -- ======================================================================== Robert P. J. Day Linux Consulting, Training and Annoying Kernel Pedantry Waterloo, Ontario, CANADA http://crashcourse.ca ========================================================================