From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thierry Reding Subject: Re: [PATCH v3 4/5] pwm: omap-dmtimer: Do not disable pwm before changing period/duty_cycle Date: Mon, 30 Mar 2020 16:14:36 +0200 Message-ID: <20200330141436.GG2431644@ulmo> References: <20200312042210.17344-1-lokeshvutla@ti.com> <20200312042210.17344-5-lokeshvutla@ti.com> <20200312064042.p7himm3odxjyzroi@pengutronix.de> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="NY6JkbSqL3W9mApi" Return-path: Content-Disposition: inline In-Reply-To: <20200312064042.p7himm3odxjyzroi@pengutronix.de> Sender: linux-omap-owner@vger.kernel.org To: Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= Cc: Lokesh Vutla , Tony Lindgren , Linux OMAP Mailing List , linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org, Sekhar Nori , Vignesh R List-Id: linux-pwm@vger.kernel.org --NY6JkbSqL3W9mApi Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Mar 12, 2020 at 07:40:42AM +0100, Uwe Kleine-K=C3=B6nig wrote: > On Thu, Mar 12, 2020 at 09:52:09AM +0530, Lokesh Vutla wrote: > > Only the Timer control register(TCLR) cannot be updated when the timer > > is running. Registers like Counter register(TCRR), loader register(TLDR= ), > > match register(TMAR) can be updated when the counter is running. Since > > TCLR is not updated in pwm_omap_dmtimer_config(), do not stop the > > timer for period/duty_cycle update. >=20 > I'm not sure what is sensible here. Stopping the PWM for a short period > is bad, but maybe emitting a wrong period isn't better. You can however > optimise it if only one of period or duty_cycle changes. >=20 > @Thierry, what is your position here? I tend to say a short stop is > preferable. It's not clear to me from the above description how exactly the device behaves, but I suspect that it may latch the values in those registers and only update the actual signal output once a period has finished. I know of a couple of other devices that do that, so it wouldn't be surprising. Even if that was not the case, I think this is just the kind of thing that we have to live with. Sometimes it just isn't possible to have all supported devices adhere strictly to an API. So I think the best we can do is have an API that loosely defines what's supposed to happen and make a best effort to implement those semantics. If a device deviates slightly from those expectations, we can always cross fingers and hope that things still work. And it looks like they are. So I think if Lokesh and Tony agree that this is the right thing to do and have verified that things still work after this, that's about as good as it's going to get. I know this is perhaps cheating a little, or turning a blind eye, but I don't know what the alternative would be. Do we want to tell people that a given PWM controller can't be used if it doesn't work according to our expectations? That's hard to argue if that controller works just fine for all known use-cases. Thierry --NY6JkbSqL3W9mApi Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCAAdFiEEiOrDCAFJzPfAjcif3SOs138+s6EFAl6B/soACgkQ3SOs138+ s6F2WxAAp3bIGtRCKqt67F7d3JG708w1+creHUOXONHOapSQCaWuU1SSJgI73p2P AHdNHXR/YURCBKVTVrLizfkqAR6rQ1EE3ovlSuu5ZkaCMx1brw92l6MoVIHxlLQ1 XchiOycQgdI4BM03ENpihJi73/HHm30/SeAJ/pTgFMr3S0pr7iNvxI+AzLfiD0c/ 7Y8tNjcDtMOesdLym/jMNelCxYovlQyB1gDYGPm65xrFb4cqWHuzuro1s71OqJzX l3J9qYjcKF7XPNHze3Q75BP/wZc0+aztBja4N6ZqoR1nvkKLPAAuDBDPI87RD5LR j4xBM/J7QSUhlh/anUfdbPxOx9YJswvfCswcifzIEwmzb7EOwpq2Yqh3BVyp4DoS 4027Z5g3sRwdR4OiNkTYHezpwp/qz1Wxrh9LWP8wqpoounrCYqTX5QDhiHSu9cr+ jfAaQGi0EgILeQjsAv3BqjoHnVjgMSVAtNV720U40J0MxkZKSf73Ak2/Jduk1dQv j1jTJL+WYGZtFBBojS73HOkSHRk4sFNvab5iotVYaro8JaCTYGNWLohJvhG+bw5X AfLthkohTXarCOkcHET4YI6PcDtzhU++HiCSfRZ+TBAiE5W2ZF5rKjFYCcBQuPCo dnAbijxeDHJ33fRP905WbK6B+GhvcgyWIDI9mJxm3FhZf+1onT0= =T9wd -----END PGP SIGNATURE----- --NY6JkbSqL3W9mApi--