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 EDC57C433FE for ; Mon, 7 Dec 2020 09:44:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A94D8225A9 for ; Mon, 7 Dec 2020 09:44:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726178AbgLGJoD (ORCPT ); Mon, 7 Dec 2020 04:44:03 -0500 Received: from gofer.mess.org ([88.97.38.141]:45065 "EHLO gofer.mess.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726041AbgLGJoC (ORCPT ); Mon, 7 Dec 2020 04:44:02 -0500 Received: by gofer.mess.org (Postfix, from userid 1000) id 3F95FC63E5; Mon, 7 Dec 2020 09:43:20 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mess.org; s=2020; t=1607334200; bh=DZlMLoq4qsBnYniUp6Q77rn7ISx5VdIDUqFrvh/AOEM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=C+xHZ+8qF17KHekNtmRqS2tbivtDM9fWuBXML8c90oFI1R3OhmOl06zuZnP5dtimq mc2xtpGxcjOFVeBFBU8vvm5oGnYb/XgXL+br2Zgo04nYiVvsKta64Xb6qMHodaq8oE dULjdVMELCLHys+VpxUJj99GUhMrg+NLTgW2Qsao8nAw+ICop+BQWTKEFEn5zYw0eg USwlhTI1zPwyLeqYABhGjrY4hdS8VBgmfXH0ydYG3ZmpwyAaVcURYMge9kgszs84vU EKulVtGWYjdKgLuke56HXNPknrRAVXnD50bQeWLPpgJaprX2bJlXLLbAWjxpRQIlho NOSnPBMfVBdKw== Date: Mon, 7 Dec 2020 09:43:20 +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: <20201207094320.GA10460@gofer.mess.org> References: <20201129181050.p6rkif5vjoumvafm@pengutronix.de> <4683237c-7b40-11ab-b3c0-f94a5dd39b4d@gmx.de> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201207081628.tm3yg7az5k5sbivu@pengutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Uwe, Thank you for taking the time to explain your thinking. 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? > > > > 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. > - 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. Please let me know when/if this new API exists for pwm so that pwm-ir-tx can select the right rounding. > [1] Consider a PWM with a parent frequency of 66 MHz, to select the > period you can pick an integer divider "div" resulting in the period > 4096 / (pclk * d). So the obvious implementation for round-nearest > would be: > > pclk = clk_get_rate(myclk); > div = DIV_ROUND_CLOSEST(NSEC_PER_SEC * 4096, targetperiod * pclk); Note NSEC_PER_SEC * 4096 >> 2^32 so this would need to be DIV_ROUND_CLOSEST_ULL. > , right? > > With targetperiod = 2641 ns this picks div = 23 and so a period of > 2698.2872200263505 ns (delta = 57.2872200263505 ns). > The optimal divider however is div = 24. (implemented period = > 2585.8585858585857 ns, delta = 55.14141414141448 ns) > > For round-down the correct implementation is: > > pclk = clk_get_rate(myclk); > div = DIV_ROUND_UP(NSEC_PER_SEC * 4096, targetperiod * pclk); > > Exercise for the reader: Come up with a correct implementation for > "round-nearest" and compare its complexity to the round-down code. To be fair, I haven't been been able to come up with a solution without control flow. Thank you for an interesting conversation about this. Sean