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[23.128.96.18]) by mx.google.com with ESMTP id kd26si6539155ejc.672.2021.05.31.10.29.27; Mon, 31 May 2021 10:29:49 -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; dkim=pass header.i=@pqgruber.com header.s=mail header.b=XhJKpe1m; 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; dmarc=pass (p=REJECT sp=NONE dis=NONE) header.from=pqgruber.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233828AbhEaRaJ (ORCPT + 99 others); Mon, 31 May 2021 13:30:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55040 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233597AbhEaR3q (ORCPT ); Mon, 31 May 2021 13:29:46 -0400 Received: from mail.pqgruber.com (mail.pqgruber.com [IPv6:2a05:d014:575:f70b:4f2c:8f1d:40c4:b13e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 00508C074B07; Mon, 31 May 2021 09:12:38 -0700 (PDT) Received: from workstation.tuxnet (213-47-165-233.cable.dynamic.surfer.at [213.47.165.233]) by mail.pqgruber.com (Postfix) with ESMTPSA id 6A115C725BF; Mon, 31 May 2021 18:12:35 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=pqgruber.com; s=mail; t=1622477555; bh=EFaky8IOQy5PwHRMrB7IOme+U42GCsDFYetfe6i+Tmw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=XhJKpe1mG8dT/UZNpPy+BDDlUoPvaRRYvVfTumw9Q8J/zrmkAZsPBhVnna6LqZgiv ZsWQ15q7ysWvw7wbnrCDvTCaO0ZZvfgYWjmZRtuNIMfpWNAEMOiLN5i4U969KcmxKZ HnGCqZrsmFjkvKAVzwjjfcTXBXZnTx9ddGS+/9eE= Date: Mon, 31 May 2021 18:12:32 +0200 From: Clemens Gruber To: Thierry Reding Cc: linux-pwm@vger.kernel.org, Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= , linux-kernel@vger.kernel.org, kernel@pengutronix.de, Sven Van Asbroeck Subject: Re: [PATCH 1/4] pwm: core: Support new usage_power setting in PWM state Message-ID: References: <20210507131845.37605-1-clemens.gruber@pqgruber.com> <20210507150317.bnluhqrqepde4xjm@pengutronix.de> <20210507231831.zmvyspcq7xhm25y4@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210507231831.zmvyspcq7xhm25y4@pengutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 08, 2021 at 01:18:31AM +0200, Uwe Kleine-K?nig wrote: > Hello Clemens, > > On Fri, May 07, 2021 at 05:47:58PM +0200, Clemens Gruber wrote: > > On Fri, May 07, 2021 at 05:03:17PM +0200, Uwe Kleine-K?nig wrote: > > > On Fri, May 07, 2021 at 03:18:42PM +0200, Clemens Gruber wrote: > > > > diff --git a/include/linux/pwm.h b/include/linux/pwm.h > > > > index 5bb90af4997e..5a73251d28e3 100644 > > > > --- a/include/linux/pwm.h > > > > +++ b/include/linux/pwm.h > > > > @@ -54,12 +54,17 @@ enum { > > > > * @duty_cycle: PWM duty cycle (in nanoseconds) > > > > * @polarity: PWM polarity > > > > * @enabled: PWM enabled status > > > > + * @usage_power: If set, the PWM driver is only required to maintain the power > > > > + * output but has more freedom regarding signal form. > > > > + * If supported, the signal can be optimized, for example to > > > > + * improve EMI by phase shifting individual channels. > > > > */ > > > > struct pwm_state { > > > > u64 period; > > > > u64 duty_cycle; > > > > enum pwm_polarity polarity; > > > > bool enabled; > > > > + bool usage_power; > > > > }; > > > > > > > > /** > > > > > > If we really want to support this usecase, I would prefer to not have it > > > in pwm_state because this concept is not a property of the wave form. So > > > for a driver it doesn't really make sense to set this flag in > > > .get_state(). > > > > It is related to the wave form in so far as it allows the driver to > > change the wave form as long as the power output remains the same. > > Yes, the thing I wanted to express is: usage_power is a software thing. > Just from reading the hardware registers you can never know if > usage_power is set or not. So it is conceptually slightly different than > the other members of pwm_state which all are represented 1:1 in > hardware. > > > > Maybe it makes more sense to put this in a separate argument for a > > > variant of pwm_apply_state? something like: > > > > > > int pwm_apply_state_transition(struct pwm_device *pwm, const struct pwm_state *state, const struct pwm_state_transition *transition); > > > > > > and pwm_state_transition can then contain something like this usage > > > power concept and maybe also a sync flag that requests that the call > > > should only return when the new setting is active and maybe also a > > > complete_period flag that requests that the currently running period > > > must be completed before the requested setting is implemented. > > > > > > OTOH the information "I only care about the relative duty cycle" is > > > relevant longer than during the transition to a new setting. (So if > > > there are two consumers and one specified to be only interested in the > > > relative duty cycle, the other might be allowed to change the common > > > period.) > > > > As you said, usage_power does not only apply to one state transition. > > > > > Having said that, I don't like the proposed names. Neither "usage_power" > > > nor "pwm_apply_state_transition". > > > > > > In a non-representative survey among two hardware engineers and one > > > software engineer who already contributed to PWM (!= me) I found out > > > that these three have an intuitive right understanding of > > > "allow-phase-shift" but have no idea what "usage-power" could mean. > > > > One advantage of usage_power is that it is not limited to phase > > shifting. Drivers could do other optimizations as long as the power > > output remains the same. > > Freedom you give to the lowlevel driver might be a burden to the > consumer. I think it makes sense to split the concept into: > > PWM_ALLOW_PHASE_SHIFT 1 > PWM_SET_RELATIVE_DUTY 2 > PWM_SET_POWER (PWM_ALLOW_PHASE_SHIFT | PWM_SET_RELATIVE_DUTY) > > This way a consumer (e.g. a clock driver) who doesn't care about the > phase shift but wants a fixed period can specify that, and if a consumer > really only cares about the emitted power that's possible, too. > > And given that your driver actually only implements a phase shift I > suggest not to generalize more than necessary here; also because the > semantic of usage-power isn't well defined. So this is something where I > agree to Thierry: Let's not solve a problem we don't have. (Though he > comes to a different conclusion here.) > > > > On a side note: The hardware guys noted that it might make sense to > > > align the shifted phases. i.e. instead of shifting channel i by i * > > > period/16, it might be better to let the 2nd channel get active when the > > > first gets inactive. (i.e. try to keep the count of active channels > > > constant). > > > > I am not sure what you mean exactly, because the duty cycles of the > > 16 outputs are not necessarily the same and can all be active at the > > same time. The idea is to spread the edges out as evenly as possible. > > Shifting them by period/16 is the best way to smoothen the current > > spikes in my opinion and the implementation is also very simple. > > Yes, the algorithm needed to satisfy this wish is more complicated. And > maybe it even isn't possible to implement it in a sane way, I didn't > think about it much. I just believed them that if you have two channels > that run at 50% it's better to have a phase shift of 50% between them > than 6.25%. Maybe it makes sense to align the start of channel #i+1 (if > allowed) to the end of channel #i to already get a better behaviour on > average than the fixed offset depending on the channel number. > > > > And as already mentioned earlier I still think we should define the > > > concept of "usage power" better. It should be clearly defined for a > > > driver author which setting they should pick for a given request. This > > > removes surprises for consumers and guessing for lowlevel driver > > > authors. Also a uniform policy brings conflicts better to light. > > > (Something like driver A does the right thing for consumer C and driver > > > B makes it right for D. But once D tries to use A things break. (And > > > then maybe A is changed to fit D, and C doesn't object because they > > > don't read the pwm list resulting in a breakage for C later.)) > > > > I added documentation and comments to the header file as a first step > > but we can always improve them. > > In my book the documentation is inadequate because it doesn't define how > a driver should behave and so doesn't define what the consumer can > expect. With your description all settings I suggested in > https://lore.kernel.org/r/20210413175631.pwbynvwmnn7oog4m@pengutronix.de > are allowed. I don't think this is a good idea. > > And given that all people I talked to about "usage-power" were unable to > correctly guess its semantic, I'm convinced that we need a better name. > This is something you cannot outweigh with documentation -- most people > won't read it anyway. Thierry: Would be great to get your input on this. Thanks, Clemens