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=-5.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 651AEC433FE for ; Mon, 7 Dec 2020 09:44:45 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 18AA92220B for ; Mon, 7 Dec 2020 09:44:45 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 18AA92220B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=mess.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=eQgeuF6JdXD0qdznLsrBFPdj/StKxvBeGcJibOj0F50=; b=rjqrKM8bNnurDD02rXutjhTBN 6u2+jsK5BoeKC/+I/oYfywwPDfPdqmFIJg8EL11r6ItgxZi49kapkK0tM4Zd3mC+CIV/PlucvsK3a e2IoytYcclkm+Rmh+yIWdwPROY1HqJvxTjJXPXdWsmMbspLaNlcrQPnA5toWRXCKtIfs2uO1XSpWd 5vH/6e+wY76R9wy/qP/riRKReAmz4azU6ARBNOUqy43DefTVip0f8KZddSvFCeurgsLuJSgiHaA2n tFoMKoZu2dLoRi7kwVZS4Ja5VO8+lPsK5WghNPaD9AO05g0y5gWCpPVbsmgZMk3UHMQfLvkcwM5TA zLIGPC76w==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kmD3K-00075w-SV; Mon, 07 Dec 2020 09:43:30 +0000 Received: from gofer.mess.org ([88.97.38.141]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1kmD3G-000738-55; Mon, 07 Dec 2020 09:43:27 +0000 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?= 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-Disposition: inline In-Reply-To: <20201207081628.tm3yg7az5k5sbivu@pengutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201207_044326_549490_DD3B31E5 X-CRM114-Status: GOOD ( 34.23 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arm-kernel@lists.infradead.org, linux-pwm@vger.kernel.org, f.fainelli@gmail.com, sbranden@broadcom.com, rjui@broadcom.com, linux-kernel@vger.kernel.org, Lino Sanfilippo , thierry.reding@gmail.com, linux-rpi-kernel@lists.infradead.org, bcm-kernel-feedback-list@broadcom.com, lee.jones@linaro.org, nsaenzjulienne@suse.de Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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-K=F6nig 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-K=F6nig 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-d= own > > 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 =3D clk_get_rate(myclk); > div =3D 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 =3D 2641 ns this picks div =3D 23 and so a period of > 2698.2872200263505 ns (delta =3D 57.2872200263505 ns). > The optimal divider however is div =3D 24. (implemented period =3D > 2585.8585858585857 ns, delta =3D 55.14141414141448 ns) > = > For round-down the correct implementation is: > = > pclk =3D clk_get_rate(myclk); > div =3D 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel