From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752999AbcKSEu3 (ORCPT ); Fri, 18 Nov 2016 23:50:29 -0500 Received: from mail-pg0-f48.google.com ([74.125.83.48]:36034 "EHLO mail-pg0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752913AbcKSEu1 (ORCPT ); Fri, 18 Nov 2016 23:50:27 -0500 From: John Stultz To: lkml Cc: Chris Metcalf , Richard Cochran , Ingo Molnar , Prarit Bhargava , Thomas Gleixner , John Stultz Subject: [PATCH 4/4] timekeeping: clocksource_cyc2ns: Document intended range limitation Date: Fri, 18 Nov 2016 20:50:14 -0800 Message-Id: <1479531014-25264-5-git-send-email-john.stultz@linaro.org> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1479531014-25264-1-git-send-email-john.stultz@linaro.org> References: <1479531014-25264-1-git-send-email-john.stultz@linaro.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Chris Metcalf The "cycles" argument should not be an absolute clocksource cycle value, as the implementation's arithmetic will overflow relatively easily with wide (64 bit) clocksource counters. For performance, the implementation is simple and fast, since the function is intended for only relatively small delta values of clocksource cycles. Cc: Richard Cochran Cc: Ingo Molnar Cc: Prarit Bhargava Cc: Thomas Gleixner Signed-off-by: Chris Metcalf [jstultz: Fixed up to merge against HEAD & commit message tweaks] Signed-off-by: John Stultz --- include/linux/clocksource.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h index 0839818..0881bca 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -169,7 +169,10 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant) * @mult: cycle to nanosecond multiplier * @shift: cycle to nanosecond divisor (power of two) * - * Converts cycles to nanoseconds, using the given mult and shift. + * Converts clocksource cycles to nanoseconds, using the given mult and shift. + * The code is optimized for performance and not intended to work + * with absolute clocksource cycles, as it will easily overflow, + * but just intended for relative (delta) clocksource cycles. * * XXX - This could use some mult_lxl_ll() asm optimization */ -- 2.7.4