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 33F6DC4167B for ; Fri, 4 Dec 2020 11:39:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 07DDE229F0 for ; Fri, 4 Dec 2020 11:39:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730072AbgLDLjb (ORCPT ); Fri, 4 Dec 2020 06:39:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39332 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725999AbgLDLja (ORCPT ); Fri, 4 Dec 2020 06:39:30 -0500 Received: from gofer.mess.org (gofer.mess.org [IPv6:2a02:8011:d000:212::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 69654C0613D1; Fri, 4 Dec 2020 03:38:50 -0800 (PST) Received: by gofer.mess.org (Postfix, from userid 1000) id 2613AC63B3; Fri, 4 Dec 2020 11:38:47 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mess.org; s=2020; t=1607081927; bh=Az/uDrcMyA9/dLYwyl4sQ5Fgc9q8oK/Kxxi0WSvZST4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=VuaTLHk6t5PWIq8CN0UUidPilVW6Fw6sSC3EJL4dWj62vDf1kYjZJmlJFPz4zQQFS yLQWocauR2jIvec/j0HKqlxH9wgKBSMlE5y60gHsBGw/vWlQPurXXpy5h+O72xUZA4 FmRlg51yNiXA3mfHemg8p/hgd7CiuB23rczJij48Q3yIiSrESCaYrG2so7bMxr0+/I Ac5UOPYrayS3tkULu6uAw0J+LMPO3jvq7DRKfXbMQjSTJkKLUiYZZv61bLHtJyZYPv aNGbOr2KpTr4Yr1xFJ0Dztrxr2w19JxhKVtbgh3T9M0RNZvKI/7gItixlhWODJHops 5ndnB27gqImsg== Date: Fri, 4 Dec 2020 11:38:46 +0000 From: Sean Young To: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= Cc: Lino Sanfilippo , 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: <20201204113846.GA6547@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> <20201204084417.GA2154@gofer.mess.org> <20201204111326.qjux6k2472dmukot@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201204111326.qjux6k2472dmukot@pengutronix.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 12:13:26PM +0100, Uwe Kleine-König wrote: > On Fri, Dec 04, 2020 at 08:44:17AM +0000, Sean Young wrote: > > 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_MIN || value > U32_MAX) > > return -EINVAL; > > Given that value is a u32, value > U32_MAX will never trigger. I meant that value is declared u64 as well ("assign to u64"). > Maybe checking period before doing the division is more sensible. That could introduce rounding errors, exactly why PERIOD_MIN was introduced. > > > > 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. > > IMO it's not an option to say: pwm-driver A is used for IR, so A's > .apply uses round-nearest and pwm-driver B is used for $somethingelse, > so B's .apply uses round-down. I'm not saying that one driver should have one it one way and another driver another way. > To be a sensible API pwm_apply_state > should have a fixed behaviour. I consider round-down the sensible > choice (because it is easier to implmement the other options with this) It's not sensible when it's wrong about half the time. Why is is easier to implement? > and for consumers like the IR stuff we need to provide some more > functions to allow it selecting a better suited state. Something like: > > pwm_round_state_nearest(pwm, { .period = 2198, .. }, &state) > > which queries the hardwares capabilities and then assigns state.period = > 2200 instead of 2100. This is very elaborate and surely not "easier to implement". Why not just do the right thing in the first place and round-closest? > Where can I find the affected (consumer) driver? So there is the pwm-ir-tx driver. The infrared led is directly connected to the pwm output pin, so that's all there is. Thanks, Sean