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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 20BD6C4332E for ; Thu, 19 Mar 2020 20:51:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F330320775 for ; Thu, 19 Mar 2020 20:51:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727544AbgCSUvS (ORCPT ); Thu, 19 Mar 2020 16:51:18 -0400 Received: from alexa-out-sd-01.qualcomm.com ([199.106.114.38]:14562 "EHLO alexa-out-sd-01.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727141AbgCSUvS (ORCPT ); Thu, 19 Mar 2020 16:51:18 -0400 Received: from unknown (HELO ironmsg01-sd.qualcomm.com) ([10.53.140.141]) by alexa-out-sd-01.qualcomm.com with ESMTP; 19 Mar 2020 13:51:17 -0700 Received: from gurus-linux.qualcomm.com ([10.46.162.81]) by ironmsg01-sd.qualcomm.com with ESMTP; 19 Mar 2020 13:51:17 -0700 Received: by gurus-linux.qualcomm.com (Postfix, from userid 383780) id 6A02F4B82; Thu, 19 Mar 2020 13:51:17 -0700 (PDT) Date: Thu, 19 Mar 2020 13:51:17 -0700 From: Guru Das Srinagesh To: Arnd Bergmann Cc: Linux PWM List , Thierry Reding , Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= , Subbaraman Narayanamurthy , "linux-kernel@vger.kernel.org" , Alexander Shiyan Subject: Re: [PATCH v9 04/11] pwm: clps711x: Use 64-bit division macro Message-ID: <20200319205117.GA17255@codeaurora.org> References: <587f9ccae68ad7e1ce97fa8da6037292af1a5095.1584473399.git.gurus@codeaurora.org> <20200317233003.GA11350@codeaurora.org> <20200318170010.GA26509@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 18, 2020 at 08:38:29PM +0100, Arnd Bergmann wrote: > On Wed, Mar 18, 2020 at 6:00 PM Guru Das Srinagesh wrote: > > > > On Wed, Mar 18, 2020 at 10:49:34AM +0100, Arnd Bergmann wrote: > > > On Wed, Mar 18, 2020 at 12:30 AM Guru Das Srinagesh > > > wrote: > > > > > > > > On Tue, Mar 17, 2020 at 11:22:06PM +0100, Arnd Bergmann wrote: > > > > > > diff --git a/drivers/pwm/pwm-clps711x.c b/drivers/pwm/pwm-clps711x.c > > > > > > index 924d39a..ba9500a 100644 > > > > > > --- a/drivers/pwm/pwm-clps711x.c > > > > > > +++ b/drivers/pwm/pwm-clps711x.c > > > > > > @@ -43,7 +43,7 @@ static void clps711x_pwm_update_val(struct clps711x_chip *priv, u32 n, u32 v) > > > > > > static unsigned int clps711x_get_duty(struct pwm_device *pwm, unsigned int v) > > > > > > { > > > > > > /* Duty cycle 0..15 max */ > > > > > > - return DIV_ROUND_CLOSEST(v * 0xf, pwm->args.period); > > > > > > + return DIV64_U64_ROUND_CLOSEST(v * 0xf, pwm->args.period); > > > > > > } > > > > > > > > > > Is it actually going to exceed U32_MAX? If not, a type cast may be > > > > > more appropriate here than the expensive 64-bit division. > > > > > > > > With the final change in this patch series, the framework will support > > > > periods that exceed U32_MAX. My concern is that using a typecast would > > > > mean that in those cases, this driver will not support > U32_MAX values. > > > > Using DIV64_U64_ROUND_CLOSEST makes the driver future proof and able to > > > > handle > U32_MAX values correctly. What do you think? > > > > > > Ah, so if the period can actually be larger than U32_MAX, you need to > > > handle that case. However, I see that the divident in this code (v * 0xf) > > > is still a 32-bit number, so a correct and efficient implementation could be > > > > > > if (pwm->args.period > (UINT_MAX / 0xf)) > > > > Shouldn't the if condition be the following? Or am I missing > > something here? > > > > if (pwm->args.period > (UINT_MAX / (v * 0xf))) > > ^^^^^^^^^ > > That would require performing a division, which is an external function > call on ARMv4. My version just checks for an upper bound and completely > avoids the division. You could also just check for ">UINT_MAX" if you > find that clearer. Thanks, have checked for UINT_MAX in v10 of my patchset that I just uploaded. Could you please review it? Thank you. Guru Das.