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=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_NEOMUTT autolearn=ham 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 12E04C169C4 for ; Wed, 6 Feb 2019 16:35:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D4BC420811 for ; Wed, 6 Feb 2019 16:35:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731203AbfBFQfw (ORCPT ); Wed, 6 Feb 2019 11:35:52 -0500 Received: from metis.ext.pengutronix.de ([85.220.165.71]:57671 "EHLO metis.ext.pengutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730678AbfBFQfw (ORCPT ); Wed, 6 Feb 2019 11:35:52 -0500 Received: from pty.hi.pengutronix.de ([2001:67c:670:100:1d::c5]) by metis.ext.pengutronix.de with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.89) (envelope-from ) id 1grQAr-00058p-3Z; Wed, 06 Feb 2019 17:35:45 +0100 Received: from ukl by pty.hi.pengutronix.de with local (Exim 4.89) (envelope-from ) id 1grQAq-0001bv-B4; Wed, 06 Feb 2019 17:35:44 +0100 Date: Wed, 6 Feb 2019 17:35:44 +0100 From: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= To: Thierry Reding Cc: Yash Shah , Palmer Dabbelt , linux-pwm@vger.kernel.org, linux-riscv@lists.infradead.org, robh+dt@kernel.org, mark.rutland@arm.com, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, Sachin Ghadi , Paul Walmsley Subject: Re: [PATCH v5 1/2] pwm: sifive: Add DT documentation for SiFive PWM Controller Message-ID: <20190206163544.rwzinr75cw262njv@pengutronix.de> References: <1548762199-7065-1-git-send-email-yash.shah@sifive.com> <1548762199-7065-2-git-send-email-yash.shah@sifive.com> <20190130081411.5rva7cpwoy2l5qjd@pengutronix.de> <20190206110730.ogqxncrnblyazgjw@pengutronix.de> <20190206124055.GF21676@ulmo> <20190206153854.52bqbwf2ty2j7rvj@pengutronix.de> <20190206161634.GB23805@ulmo> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190206161634.GB23805@ulmo> User-Agent: NeoMutt/20170113 (1.7.2) X-SA-Exim-Connect-IP: 2001:67c:670:100:1d::c5 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 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 06, 2019 at 05:16:34PM +0100, Thierry Reding wrote: > On Wed, Feb 06, 2019 at 04:38:54PM +0100, Uwe Kleine-König wrote: > > On Wed, Feb 06, 2019 at 01:40:55PM +0100, Thierry Reding wrote: > > > On Wed, Feb 06, 2019 at 12:07:30PM +0100, Uwe Kleine-König wrote: > > > > On Wed, Feb 06, 2019 at 04:18:47PM +0530, Yash Shah wrote: > > > > > On Wed, Jan 30, 2019 at 1:44 PM Uwe Kleine-König > > > > > wrote: > > > > > > > > > > > > On Tue, Jan 29, 2019 at 05:13:18PM +0530, Yash Shah wrote: > > > > > > > DT documentation for PWM controller added. > > > > > > > > > > > > > > Signed-off-by: Wesley W. Terpstra > > > > > > > [Atish: Compatible string update] > > > > > > > Signed-off-by: Atish Patra > > > > > > > Signed-off-by: Yash Shah > > > > > > > --- > > > > > > > .../devicetree/bindings/pwm/pwm-sifive.txt | 33 ++++++++++++++++++++++ > > > > > > > 1 file changed, 33 insertions(+) > > > > > > > create mode 100644 Documentation/devicetree/bindings/pwm/pwm-sifive.txt > > > > > > > > > > > > > > diff --git a/Documentation/devicetree/bindings/pwm/pwm-sifive.txt b/Documentation/devicetree/bindings/pwm/pwm-sifive.txt > > > > > > > new file mode 100644 > > > > > > > index 0000000..8dcb40d > > > > > > > --- /dev/null > > > > > > > +++ b/Documentation/devicetree/bindings/pwm/pwm-sifive.txt > > > > > > > @@ -0,0 +1,33 @@ > > > > > > > +SiFive PWM controller > > > > > > > + > > > > > > > +Unlike most other PWM controllers, the SiFive PWM controller currently only > > > > > > > +supports one period for all channels in the PWM. This is set globally in DTS. > > > > > > > > > > > > You can simply drop this if the first user can set this using the usual > > > > > > interface. Don't you like this suggestion that I already made a few > > > > > > times now? > > > > > > > > > > > > Did you consider to make the driver support only a single output with a > > > > > > more flexible period setting? > > > > > > > > > > We cannot consider supporting only single output since we have boards that > > > > > use the additional PWM channels to control individual LED brightness > > > > > of a tri-color LED. > > > > > If we go down to one channel, then we can't control the brightness of > > > > > the individual LEDs. > > > > > It will break the use case. > > > > > > > > OK. > > > > > > > > > I am considering the below approach, let me know if it's fine by you. > > > > > > > > > > - Drop the global period property and allow the only first user to change period > > > > > using the usual interface. > > > > > - A note in the binding that all PWMs need to run at the same > > > > > period. If the driver already refuses to apply incompatible periods, > > > > > the users are going to notice that they've got the DT wrong. > > > > > - In driver code, count the users using the .request and .free callbacks. > > > > > Based on this, allow changes to period iff the user count is one. > > > > > > > > Not sure you need to point this limitation in the binding doc. Other > > > > than that this is fine. > > > > > > I think it's useful to point this out in the binding documentation since > > > it's something that device tree writers will want to know. > > > > OK, we're talking about an FPGA implementation but I still think if the > > dt-author is the first to know about this limitation this is too late. > > The hardware designer must know about that, and they don't look into the > > bindind documentation. The binding documentation should IMHO only > > describe the binding and functional limitiations of the device are out > > of scope. > > > > The binding for the Freescale network interface doesn't describe that it > > only supports 480 MBit/s in its GHz mode, and that's good. The binding > > for i2c devices doesn't describe that a reboot in the middle of a > > transfer might block the bus. > > > > The target for the binding documentation is to document how a given > > device is described in a device tree. It's not there to duplicate > > information that belongs in the reference manual. > > But the device tree bindings for PWM devices describe where and how to > describe the period for each PWM that you reference. That's entirely > different from the examples that you give above. Ah right, I had not on my radar that the binding also gives a suggestion for the period. Then yes, I agree that pointing out that all pwms belonging to the same provider should have matching periods is a good idea. Is using this "typical" approach to specify the period in a pwm-list a bad idea for this device type for this reason? On the other hand diverting from the typical approach has its own cost. Not sure ... Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |