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=-5.0 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 361A7C4361B for ; Mon, 7 Dec 2020 18:19:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EBB8423877 for ; Mon, 7 Dec 2020 18:19:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726483AbgLGSSs (ORCPT ); Mon, 7 Dec 2020 13:18:48 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47666 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725816AbgLGSSr (ORCPT ); Mon, 7 Dec 2020 13:18:47 -0500 Received: from gofer.mess.org (gofer.mess.org [IPv6:2a02:8011:d000:212::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 79D00C061749; Mon, 7 Dec 2020 10:18:07 -0800 (PST) Received: by gofer.mess.org (Postfix, from userid 1000) id 7AFADC638E; Mon, 7 Dec 2020 18:18:03 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mess.org; s=2020; t=1607365083; bh=aTSDI8UqWPGA6F2iy6N5PWwkwrk6VMgzATi9jVD/WNU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=GI8vmkvI1DBeRKhC57wPyvnLU6musqd5kvAq1Zl5RKoyoi9e27aT/whYI4W56Gs8b 6EPgv0e9UkLz5xVRuqeJiBSDObvKaxoaU9L9P+OhCtxRT35B+o4HpMS/n8RINQsF3o 3ylyefqK6ds7O8zulImV220psCcopEcbddDhg5tQ/7gufd4iu9BFxW+9jpGkbTFH7N BnJ4qbBEGizs6u+TOI5ptifIXzVVYCh0ls9OuibKUxD+1fYFkCCCNPRq0spkaxG75F RkLMrn+tYfx15eGtT49xgmZxXGQnwaEwCMne1cdux/a5H755m3IHlpQY3qV+msr6nK GMVerLzy5n0MQ== Date: Mon, 7 Dec 2020 18:18:03 +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: <20201207181803.GA18865@gofer.mess.org> References: <20201204084417.GA2154@gofer.mess.org> <20201204111326.qjux6k2472dmukot@pengutronix.de> <20201204113846.GA6547@gofer.mess.org> <20201204232834.xzsafkzfmfpw7pqz@pengutronix.de> <20201205173444.GA1265@gofer.mess.org> <20201205192510.o76pjs3yc524nwvm@pengutronix.de> <20201206141941.GA24807@gofer.mess.org> <20201207081628.tm3yg7az5k5sbivu@pengutronix.de> <20201207094320.GA10460@gofer.mess.org> <20201207135209.htp7plyotjxp37q2@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201207135209.htp7plyotjxp37q2@pengutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Uwe, On Mon, Dec 07, 2020 at 02:52:09PM +0100, Uwe Kleine-König wrote: > On Mon, Dec 07, 2020 at 09:43:20AM +0000, Sean Young wrote: > > On Mon, Dec 07, 2020 at 09:16:28AM +0100, Uwe Kleine-König wrote: > > > On Sun, Dec 06, 2020 at 02:19:41PM +0000, Sean Young wrote: > > > > On Sat, Dec 05, 2020 at 08:25:10PM +0100, Uwe Kleine-König wrote: > > > > > On Sat, Dec 05, 2020 at 05:34:44PM +0000, Sean Young wrote: > > > > > > What real life uses-cases are there for round down? If you want to round > > > > > > down, is there any need for round up? > > > > > > > > > > The scenario I have in mind is for driving a motor. I have to admit > > > > > however that usually the period doesn't matter much and it's the > > > > > duty_cycle that defines the motor's speed. So for this case the > > > > > conservative behaviour is round-down to not make the motor run faster > > > > > than expected. > > > > > > > > I am reading here that for driving motors, only the duty cycle matters, > > > > not the period. > > > > > > There is an upper limit (usually around 1 ms) for the period, but if you > > > choose 0.1 ms or 0.001 ms doesn't matter much AFAICT. > > > > > > @Thierry: Do you have further use cases in mind? > > I asked in the hardware department of the company I work for and they > had another usecase: Motors where for example a 1 ms pulse means "move > forwards" and 2 ms means "move backwards". They had the same idea as I > had: You want to know beforehand the result of a given > pwm_apply_state(). That sounds good, that would be nice. > > > > > For other usecases (fan, backlight, LED) exactness typically doesn't > > > > > matter that much. > > > > > > > > So, the use-cases you have are driving motor, fan, backlight, and led. > > > > And in all these cases the exact Hz does not matter. > > > > > > > > The only uses case where the exact Hz does matter is pwm-ir-tx. > > > > > > > > So, I gather there are no use-cases for round-down. Yes, should round-down > > > > be needed, then this is more difficult to implement if the driver always > > > > does a round-closest. But, since there is no reason to have round-down, > > > > this is all academic. > > > > > > > > Your policy of forcing new pwm drivers to use round-down is breaking > > > > pwm-ir-tx. > > > > > > So you're indeed suggesting that the "right" rounding strategy for > > > lowlevel drivers should be: > > > > > > - Use the period length closest to the requested period (in doubt round > > > down?) > > > - With the chosen period length use the biggest duty_cycle not bigger > > > than the requested duty_cycle. > > > > > > While this seems technically fine I think for maintenance this is a > > > nightmare. > > > > > > My preference would be to stick to the rounding strategy we used so far > > > (i.e.: > > > > > > - Use the biggest period length not bigger than the requested period > > > - With the chosen period length use the biggest duty_cycle not bigger > > > than the requested duty_cycle. > > > > > > ) and for pwm-ir-tx add support to the PWM API to still make it possible > > > (and easy) to select the best setting. > > > > > > The reasons why I think that this rounding-down strategy is the best > > > are (in order of importance): > > > > > > - It is easier to implement correctly [1] > > > > Yes, you are right. You have given a great example where a simple > > DIV_ROUND_CLOSEST() does not give the result you want. > > > > > - Same rounding method for period and duty cycle > > > - most drivers already do this (I think) > > > > > > The (IMHO nice) result would then mean: > > > > > > - All consumers can get the setting they want; and > > > > Once there is a nice pwm api for selecting round-nearest, then yes. > > > > For the uses cases you've given, fan, backlight, and led a round-nearest > > is the rounding they would want, I would expect. > > maybe, yes. Maybe it is also not important enough to spend the extra > cycles getting round nearest and so sticking to round-down is good > enough. > > > > - Code in lowlevel drivers is simple and the complexity is in common > > > code and so a single place. > > > > > > And it would also allow the pwm-ir-tx driver to notice if the PWM to be > > > used can for example only support frequencies under 400 kHz. > > > > I doubt pwm-ir-tx cares about this, however it is a nice-to-have. It would > > also be nice if the rounding could be used with atomic configuration > > as well. > > I cannot follow, you created 11fc4edc483bea8bf0efa0cc726886d2342f6fa6 > because 476.2 Mhz was too bad. So you seem to be interested in > deviations and part of the problem is that you don't get feedback about > how your request is fulfilled. Right, that's true. > > Please let me know when/if this new API exists for pwm so that pwm-ir-tx > > can select the right rounding. > > Given that the bcm2835 driver is quite trivial I would be happy to > create a series that "fixes" the driver to round down and provide a > prototype for pwm_round_nearest for you to test on pwm-ir-tx. A willing > tester and a real use-case were the single two things that stopped me > investing time here. pwm-ir-tx does not just use the bcm2845 driver/rpi. There is the Firefly ROC-RK3308-CC board which uses pwm-ir-tx with a different pwm dirver. Also all you need is a infrared led, and a resistor to stop the led from burning out, to create your own infrared emitter. So, users can easily add pwm-ir-tx to their systems. Having said that I'm happy to test the rpi. I would attach a logic analyser and check the period. Sean