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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CF80BC4332F for ; Mon, 6 Nov 2023 07:27:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230173AbjKFH1C convert rfc822-to-8bit (ORCPT ); Mon, 6 Nov 2023 02:27:02 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45688 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229841AbjKFH1B (ORCPT ); Mon, 6 Nov 2023 02:27:01 -0500 Received: from fd01.gateway.ufhost.com (fd01.gateway.ufhost.com [61.152.239.71]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2183C112; Sun, 5 Nov 2023 23:26:57 -0800 (PST) Received: from EXMBX165.cuchost.com (unknown [175.102.18.54]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "EXMBX165", Issuer "EXMBX165" (not verified)) by fd01.gateway.ufhost.com (Postfix) with ESMTP id BC38A24DE73; Mon, 6 Nov 2023 15:26:48 +0800 (CST) Received: from EXMBX168.cuchost.com (172.16.6.78) by EXMBX165.cuchost.com (172.16.6.75) with Microsoft SMTP Server (TLS) id 15.0.1497.42; Mon, 6 Nov 2023 15:26:48 +0800 Received: from [192.168.120.47] (171.223.208.138) by EXMBX168.cuchost.com (172.16.6.78) with Microsoft SMTP Server (TLS) id 15.0.1497.42; Mon, 6 Nov 2023 15:26:47 +0800 Message-ID: <6b57eb4b-241f-4530-b639-5a2e2c55cfd3@starfivetech.com> Date: Mon, 6 Nov 2023 15:26:45 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/4] pwm: opencores: Add PWM driver support To: =?UTF-8?Q?Uwe_Kleine-K=C3=B6nig?= CC: , , , , "Emil Renner Berthing" , Rob Herring , Thierry Reding , Philipp Zabel , "Krzysztof Kozlowski" , Conor Dooley , Hal Feng , Paul Walmsley , Palmer Dabbelt , Albert Ou References: <20231020103741.557735-1-william.qiu@starfivetech.com> <20231020103741.557735-3-william.qiu@starfivetech.com> <20231020112539.gctx5uj2rrhryulo@pengutronix.de> <7d64ea1d-b573-4a69-ba0c-4cbfab638c5e@starfivetech.com> <20231102113016.jgsh7jru6vjv4vsp@pengutronix.de> Content-Language: en-US From: William Qiu In-Reply-To: <20231102113016.jgsh7jru6vjv4vsp@pengutronix.de> Content-Type: text/plain; charset="UTF-8" X-Originating-IP: [171.223.208.138] X-ClientProxiedBy: EXCAS066.cuchost.com (172.16.6.26) To EXMBX168.cuchost.com (172.16.6.78) X-YovoleRuleAgent: yovoleflag Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-pwm@vger.kernel.org On 2023/11/2 19:30, Uwe Kleine-König wrote: > Hello William, > > On Wed, Nov 01, 2023 at 10:22:44AM +0800, William Qiu wrote: >> >> >> On 2023/10/20 19:25, Uwe Kleine-König wrote: >> >> + void __iomem *base = pwm->data->get_ch_base ? >> >> + pwm->data->get_ch_base(pwm->regs, dev->hwpwm) : pwm->regs; >> >> + u32 period_data, duty_data, ctrl_data; >> >> + >> >> + period_data = readl(REG_OCPWM_LRC(base)); >> >> + duty_data = readl(REG_OCPWM_HRC(base)); >> >> + ctrl_data = readl(REG_OCPWM_CTRL(base)); >> >> + >> >> + state->period = DIV_ROUND_CLOSEST_ULL((u64)period_data * NSEC_PER_SEC, pwm->clk_rate); >> >> + state->duty_cycle = DIV_ROUND_CLOSEST_ULL((u64)duty_data * NSEC_PER_SEC, pwm->clk_rate); >> > >> > Please test your driver with PWM_DEBUG enabled. The rounding is wrong >> > here. >> >> The conclusion after checking is: when the period or duty_cycle value set >> by the user is not divisible (1000000000/49.5M), there will be an error. >> This error is due to hardware accuracy. So why is rounding is wrong? >> rockchip also has a similar implementation drivers/pwm/ pwm-rockchip.c > > I fail to follow. Where is an error? > > The general policy (for new drivers at least) is to implement the > biggest period possible not bigger than the requested period. That means > that .apply must round down and to make .apply ∘ .get_state idempotent > .get_state must round up to match. > > Assuming a clkrate of 49500000 Hz the actual period for REG_OCPWM_LRC = > 400 is 8080.808ns and for REG_OCPWM_LRC = 401 is 8101.010. > > So with REG_OCPWM_LRC = 401 .get_state should report state.period = 8102 > [ns] because if you call .apply with .period = 8101 [ns] you're supposed > to use REG_OCPWM_LRC = 400. > > Rounding using DIV_ROUND_CLOSEST doesn't give consistent behaviour in > some cases. Consider a PWM that can implement the following periods (and > none in between): > > 20.1 ns > 20.4 ns > 21.7 ns > > With round-to-nearest a request to configure 21 ns will yield 20.4 ns. > If you call .get_state there the driver will return 20 ns. However > configuring 20 ns results in a period of 20.1 ns. > > With rounding as requested above you get a consistent behaviour. After > .apply_state(period=21) .get_state() returns period=21. > > Best regards > Uwe > I see, then we'll use DIV_ROUND_DOWN_ULL for .apply() and DIV_ROUND_UP_ULL for .get_state(). Thank you for your answer. Best regards, William