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[23.128.96.18]) by mx.google.com with ESMTP id bo3si10676181ejb.121.2021.04.13.15.31.33; Tue, 13 Apr 2021 15:31:57 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240383AbhDMR5H (ORCPT + 99 others); Tue, 13 Apr 2021 13:57:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48018 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231336AbhDMR5G (ORCPT ); Tue, 13 Apr 2021 13:57:06 -0400 Received: from metis.ext.pengutronix.de (metis.ext.pengutronix.de [IPv6:2001:67c:670:201:290:27ff:fe1d:cc33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4A332C061574 for ; Tue, 13 Apr 2021 10:56:46 -0700 (PDT) Received: from ptx.hi.pengutronix.de ([2001:67c:670:100:1d::c0]) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1lWNH9-0001gH-CJ; Tue, 13 Apr 2021 19:56:35 +0200 Received: from ukl by ptx.hi.pengutronix.de with local (Exim 4.92) (envelope-from ) id 1lWNH8-0005v0-T8; Tue, 13 Apr 2021 19:56:34 +0200 Date: Tue, 13 Apr 2021 19:56:31 +0200 From: Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= To: Thierry Reding Cc: Clemens Gruber , linux-pwm@vger.kernel.org, Sven Van Asbroeck , devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, Rob Herring Subject: Re: [PATCH v8 4/8] dt-bindings: pwm: Support new PWM_USAGE_POWER flag Message-ID: <20210413175631.pwbynvwmnn7oog4m@pengutronix.de> References: <20210412132745.76609-1-clemens.gruber@pqgruber.com> <20210412132745.76609-4-clemens.gruber@pqgruber.com> <20210412162723.7hlhgqp6wlfbkeky@pengutronix.de> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="ky54kfraiix4vkcz" Content-Disposition: inline In-Reply-To: X-SA-Exim-Connect-IP: 2001:67c:670:100:1d::c0 X-SA-Exim-Mail-From: ukl@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-kernel@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --ky54kfraiix4vkcz Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Apr 13, 2021 at 01:51:15PM +0200, Thierry Reding wrote: > On Mon, Apr 12, 2021 at 06:27:23PM +0200, Uwe Kleine-K=F6nig wrote: > > On Mon, Apr 12, 2021 at 03:27:41PM +0200, Clemens Gruber wrote: > > > Add the flag and corresponding documentation for PWM_USAGE_POWER. > >=20 > > My concern here in the previous round was that PWM_USAGE_POWER isn't a > > name that intuitively suggests its semantic. Do you disagree? >=20 > I suggested PWM_USAGE_POWER because I think it accurately captures what > we want here. >=20 > > > Cc: Rob Herring > > > Signed-off-by: Clemens Gruber > > > --- > > > Documentation/devicetree/bindings/pwm/pwm.txt | 3 +++ > > > include/dt-bindings/pwm/pwm.h | 1 + > > > 2 files changed, 4 insertions(+) > > >=20 > > > diff --git a/Documentation/devicetree/bindings/pwm/pwm.txt b/Document= ation/devicetree/bindings/pwm/pwm.txt > > > index 084886bd721e..fe3a28f887c0 100644 > > > --- a/Documentation/devicetree/bindings/pwm/pwm.txt > > > +++ b/Documentation/devicetree/bindings/pwm/pwm.txt > > > @@ -46,6 +46,9 @@ period in nanoseconds. > > > Optionally, the pwm-specifier can encode a number of flags (defined = in > > > ) in a third cell: > > > - PWM_POLARITY_INVERTED: invert the PWM signal polarity > > > +- PWM_USAGE_POWER: Only care about the power output of the signal. T= his > > > + allows drivers (if supported) to optimize the signals, for example= to > > > + improve EMI and reduce current spikes. > >=20 > > IMHO there are too many open questions about which freedom this gives to > > the lowlevel driver. If the consumer requests .duty_cycle =3D 25ns + > > .period =3D 100ns, can the driver provide .duty_cycle =3D 25s + .period= =3D > > 100s which nominally has the same power output? Let's not introduce more > > ambiguity than there already is. >=20 > The freedom given to the driver should be to adjust the signal within > reasonable bounds. Changing the time unit by a factor of 1000000000 is > not within reason, and I doubt anyone would interpret it that way, even > if we didn't document this at all. Please define a rule that allows to judge if any given implementation is correct or not. For the record neither "within reasonable bounds" nor "a factor of 1000000000 is not within reason" is good enough. This is not only important to be able to review drivers that implement it, but also for consumers, because they should know what to expect. > To be frank I think that quest of yours to try and rid the PWM API of > all ambiguity is futile. I consider my quest about rounding reasonable. And I think this is painful because when the PWM framework was introduced it was too much ad hoc and the APIs were not thought through enough. And because I don't want to have that repeated, I express my concerns here. > I've been trying to be lenient because you seem > motivated, but I think you're taking this too far. There are always > going to be cases that aren't completely clear-cut and where drivers > need the flexibility to cheat in order to be useful at all. If we get to > a point where everything needs to be 100% accurate, the majority of the > PWM controllers won't be usable at all. >=20 > Don't let perfect be the enemy of good. I admit here I don't have a constructive idea how to define what is needed. For example if we only care about the relative duty cycle, a consumer requests .period =3D 1045 .duty_cyle =3D 680 and the driver can provide multiples of 100 ns for both .period and =2Eduty_cycle, the candidates that might be sensible to chose from are (IMHO): - exact relative duty: .period =3D 104500 .duty_cycle =3D 68000 - round both values in the same direction, minimizing error .period =3D 1100 .duty_cycle =3D 700 (requested relative duty =3D 65.07%, implemented =3D 63.64%; when rounding both down we get 60%) - round both values mathematically:=20 .period =3D 1000 .duty_cycle =3D 700 (yielding a relative duty of 70% instead of the requested 65.07%) - Maybe .period =3D 1000 .duty_cycle =3D 600 might also be preferable for some consumers?! (60%) - Maybe .period =3D 2000 .duty_cycle =3D 1300 is a good compromise because the relative duty is nearly exactly matched and the period is only stretched by a factor < 2. In my eyes a driver author should be told which of these options should be picked. Do you consider it obvious which of these options is the objective best? If so why? Do you agree that we should tell driver authors how to implement this before we have several drivers that all implement their own ideas and getting this in a consistent state is another pain? (My bet is you are lax and don't consider consistency among drivers soo important. In this case we don't agree. I think it's important for consumer driver authors to be able to rely on some expectations independently which lowlevel driver is in use.) Best regards Uwe --=20 Pengutronix e.K. | Uwe Kleine-K=F6nig | Industrial Linux Solutions | https://www.pengutronix.de/ | --ky54kfraiix4vkcz Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAABCgAdFiEEfnIqFpAYrP8+dKQLwfwUeK3K7AkFAmB120wACgkQwfwUeK3K 7AkVXwf/Q85RA3Alm9y+oU4JSwFqQYvgpIdHmvqarGeBuncdhYUjiRnnB/UV9EKU auNZVYY4/+EtObhL/oBZti5V/tpLAO+aleSPwVr7ZmJvm6CtRjS2JTVS1FAQd195 UdSVBzCVF46uo7UViXZH9UGnohYl3+HxvAZSxL2z+AAoRyBG57zVFDZu0vbRH6Xl o8Q0Jysl4pcMpWE2yLgI6aXDnUeNxaQikJy1nT3wU+g6FRngCQQNSwbYrz3GJ7L2 fgmZ6fIu7kEusztqdSQmmAa9/Ts1qQVCAuf+Zz+NcbFaaDdHou0lgM6T+ASe5M5U 6hsKqu9Q4qzg6/o4C1I/k2+yw20bww== =YLWV -----END PGP SIGNATURE----- --ky54kfraiix4vkcz--