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 85E81C4361B for ; Mon, 7 Dec 2020 18:19:36 +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 45B5323888 for ; Mon, 7 Dec 2020 18:19:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 45B5323888 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=QfMOxSzUKPG+WZ13XG2owoelxxaXzUsUiwDV6EljQMk=; b=ISUR6Zmil11Jl45QcyvGM530l PHSXiuYoMk7wx9XM6KaQFabjThle1r1ZwGmyxf7rZOQelZY6lamqbBewQfEgXf+SGyI2NgbufKpjJ bUTcQ2UYSGNPZAo5M3q10vGIHtgnRQJ8AL0hZWyUTQP/c0E3+9ZSckSNu7y5X7DKD45mTN78Aoajt v9p6Wgg6I3J1Ji8oi0yA+e+mDIGmYz7y8BmRoXEER2PE3yXpn7LBDhNZZlFvE90NsacnGHk5CCd9S eNVhSS2pZJL5kyp2+IlMPiCnVBeBPI2JqQcMldhZDK/YGWvBA+D3nsvSwkwqqdFzOkrHzlmcflXS/ YKgfP7Jsw==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kmL5O-0001KE-6H; Mon, 07 Dec 2020 18:18:10 +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 1kmL5L-0001JQ-Bo; Mon, 07 Dec 2020 18:18:09 +0000 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?= 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-Disposition: inline In-Reply-To: <20201207135209.htp7plyotjxp37q2@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_131807_758221_37E6C6B6 X-CRM114-Status: GOOD ( 42.07 ) 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 Hi Uwe, On Mon, Dec 07, 2020 at 02:52:09PM +0100, Uwe Kleine-K=F6nig 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-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 adm= it > > > > > 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 fa= ster > > > > > than expected. > > > > = > > > > I am reading here that for driving motors, only the duty cycle matt= ers, > > > > 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 does= n't > > > > > matter that much. > > > > = > > > > So, the use-cases you have are driving motor, fan, backlight, and l= ed. > > > > 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 rou= nd-down > > > > be needed, then this is more difficult to implement if the driver a= lways > > > > does a round-closest. But, since there is no reason to have round-d= own, > > > > 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 ro= und > > > 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 f= ar > > > (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 possi= ble > > > (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 wo= uld > > 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel