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=-2.3 required=3.0 tests=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 9F3BEC2BA2B for ; Thu, 9 Apr 2020 02:40:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7A9F12087E for ; Thu, 9 Apr 2020 02:40:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726654AbgDICkq (ORCPT ); Wed, 8 Apr 2020 22:40:46 -0400 Received: from alexa-out-sd-01.qualcomm.com ([199.106.114.38]:26320 "EHLO alexa-out-sd-01.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726523AbgDICkq (ORCPT ); Wed, 8 Apr 2020 22:40:46 -0400 Received: from unknown (HELO ironmsg05-sd.qualcomm.com) ([10.53.140.145]) by alexa-out-sd-01.qualcomm.com with ESMTP; 08 Apr 2020 19:40:46 -0700 Received: from gurus-linux.qualcomm.com ([10.46.162.81]) by ironmsg05-sd.qualcomm.com with ESMTP; 08 Apr 2020 19:40:46 -0700 Received: by gurus-linux.qualcomm.com (Postfix, from userid 383780) id D817A4C51; Wed, 8 Apr 2020 19:40:45 -0700 (PDT) Date: Wed, 8 Apr 2020 19:40:45 -0700 From: Guru Das Srinagesh To: David Laight Cc: "linux-pwm@vger.kernel.org" , Thierry Reding , Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= , Subbaraman Narayanamurthy , "linux-kernel@vger.kernel.org" , Michael Turquette , Stephen Boyd , "linux-clk@vger.kernel.org" Subject: Re: [PATCH v8 01/12] clk: pwm: Use 64-bit division function Message-ID: <20200409024045.GA1147@codeaurora.org> References: <338966686a673c241905716c90049993e7bb7d6a.1583889178.git.gurus@codeaurora.org> <7506bc2972324fd286dac6327ec73a3a@AcuMS.aculab.com> <20200312020938.GA14827@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 Thu, Mar 12, 2020 at 09:14:09AM +0000, David Laight wrote: > From: Guru Das Srinagesh > > Sent: 12 March 2020 02:10 > > On Wed, Mar 11, 2020 at 04:58:24PM +0000, David Laight wrote: > > > From: Guru Das Srinagesh > > > > Sent: 11 March 2020 01:41 > > > > > > > > Since the PWM framework is switching struct pwm_args.period's datatype > > > > to u64, prepare for this transition by using div64_u64 to handle a > > > > 64-bit divisor. > > > > > ... > > > > --- a/drivers/clk/clk-pwm.c > > > > +++ b/drivers/clk/clk-pwm.c > > > > @@ -89,7 +89,7 @@ static int clk_pwm_probe(struct platform_device *pdev) > > > > } > > > > > > > > if (of_property_read_u32(node, "clock-frequency", &clk_pwm->fixed_rate)) > > > > - clk_pwm->fixed_rate = NSEC_PER_SEC / pargs.period; > > > > + clk_pwm->fixed_rate = div64_u64(NSEC_PER_SEC, pargs.period); > > > > > > That cannot be needed, a 32 bit division is fine. > > > > Could you please explain why? I think the use of this function is > > warranted in order to handle the division properly with a 64-bit > > divisor. > ... > > > I'd assign pargs.period to an 'unsigned int' variable > > > prior to the division (I hate casts - been bitten by them in the past.). > > > > Wouldn't this truncate the 64-bit value? The intention behind this patch > > is to allow the processing of 64-bit values in full. > > You are dividing a 32bit constant by a value. > If pargs.period is greater than 2^32 the result is zero. Correction: if pargs.period is greater than NSEC_PER_SEC, not 2^32. > I think you divide by 'fixed_rate' a bit later on - better not be zero. I am adding an explicit check in v12 to ensure fixed_rate is not zero. If during the calculation it is found to be zero, probing will fail. I think with this modification, this v8 version of this change makes sense to use. Thank you. Guru Das.