From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8F37BC433FE for ; Fri, 4 Dec 2020 08:45:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4014C22525 for ; Fri, 4 Dec 2020 08:45:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729099AbgLDIpD (ORCPT ); Fri, 4 Dec 2020 03:45:03 -0500 Received: from gofer.mess.org ([88.97.38.141]:49175 "EHLO gofer.mess.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727056AbgLDIpC (ORCPT ); Fri, 4 Dec 2020 03:45:02 -0500 Received: by gofer.mess.org (Postfix, from userid 1000) id EBCAFC64ED; Fri, 4 Dec 2020 08:44:17 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mess.org; s=2020; t=1607071457; bh=0GRGIK4YsIhbmKXjmZr8ocLCjLu/Ct8TXGitNBF/kEg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=jmcX1xwmjgUKHnX93Lw6UIv3GhF+t14P/YJ2GQNB6PzJUS82dgp1ZDh0k6FK/IneF KRZ7gZaGXzfm3577wvxNLOlFQ/xAFLyixCpnccbcQ9WHyidzvl3w6+kRoNp7Ga92r9 fPKpVeGnmfpyXXA2vUkgBWBujSFR4LAhQiIAvWJ5DOmnlB0AhLRBDQAyCQInQ8eJLr +6lKh6wJeyr9dWX9qySYLycjvz14Ij3kou9t3fCYzvfRzy6jPcDSkOd1lzzvV0DsJx YntUspU35hmS1VWYYzzDaV0ZEraw4Eqea9nML1Pb0V9OwVIksA8sfICX4BwMatPxlN HDKQrm3r3ZHLA== Date: Fri, 4 Dec 2020 08:44:17 +0000 From: Sean Young To: Lino Sanfilippo Cc: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= , thierry.reding@gmail.com, lee.jones@linaro.org, nsaenzjulienne@suse.de, f.fainelli@gmail.com, rjui@broadcom.com, sbranden@broadcom.com, bcm-kernel-feedback-list@broadcom.com, linux-pwm@vger.kernel.org, linux-rpi-kernel@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic configuration Message-ID: <20201204084417.GA2154@gofer.mess.org> References: <202011281128.54eLfMWr-lkp@intel.com> <1606564926-19555-1-git-send-email-LinoSanfilippo@gmx.de> <20201129181050.p6rkif5vjoumvafm@pengutronix.de> <4683237c-7b40-11ab-b3c0-f94a5dd39b4d@gmx.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4683237c-7b40-11ab-b3c0-f94a5dd39b4d@gmx.de> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Fri, Dec 04, 2020 at 12:42:15AM +0100, Lino Sanfilippo wrote: > > You're storing an unsigned long long (i.e. 64 bits) in an u32. If > > you are sure that this won't discard relevant bits, please explain > > this in a comment for the cursory reader. > > What about an extra check then to make sure that the period has not been truncated, > e.g: > > value = DIV_ROUND_CLOSEST_ULL(state->period, scaler); > > /* dont accept a period that is too small or has been truncated */ > if ((value < PERIOD_MIN) || > (value != DIV_ROUND_CLOSEST_ULL(state->period, scaler))) > return -EINVAL; Rather than doing another 64 bit division which is expensive (esp on 32 bit kernels), you could assign to u64 and check: if (value < PERIOD || value > U32_MAX) return -EINVAL; > > Also note that round_closed is probably wrong, as .apply() is > > supposed to round down the period to the next achievable period. (But > > fixing this has to do done in a separate patch.) > > According to commit 11fc4edc4 rounding to the closest integer has been introduced > to improve precision in case that the pwm controller is used by the pwm-ir-tx driver. > I dont know how strong the requirement is to round down the period in apply(), but I > can imagine that this may be a good reason to deviate from this rule. > (CCing Sean Young who introduced DIV_ROUND_CLOSEST) There was a problem where the carrier is incorrect for some IR hardware which uses a carrier of 455kHz. With periods that small, rounding errors do really matter and rounding down might cause problems. A policy of rounding down the carrier is not the right thing to do for pwm-ir-tx, and such a change will probably break pwm-ir-tx in some edge cases. Thanks Sean