linux-kernel - Re: [PATCH v4 4/7] pwm: ntxec: Add driver for PWM function in Netronix EC

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Date:   Sun, 29 Nov 2020 18:48:43 +0100
From:   Jonathan Neuschäfer <j.neuschaefer@....net>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Jonathan Neuschäfer <j.neuschaefer@....net>,
        Mark Brown <broonie@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Heiko Stuebner <heiko@...ech.de>, devicetree@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Sam Ravnborg <sam@...nborg.org>, linux-rtc@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Fabio Estevam <festevam@...il.com>,
        Daniel Palmer <daniel@...f.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Andreas Kemnade <andreas@...nade.info>,
        NXP Linux Team <linux-imx@....com>,
        linux-pwm@...r.kernel.org, Stephan Gerhold <stephan@...hold.net>,
        allen <allen.chen@....com.tw>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Lubomir Rintel <lkundrak@...sk>,
        Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-kernel@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Heiko Stuebner <heiko.stuebner@...obroma-systems.com>,
        Josua Mayer <josua.mayer@....eu>,
        Shawn Guo <shawnguo@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v4 4/7] pwm: ntxec: Add driver for PWM function in
 Netronix EC

On Fri, Nov 27, 2020 at 08:11:05AM +0100, Uwe Kleine-König wrote:
> Hello Jonathan,
> 
> On Fri, Nov 27, 2020 at 12:19:31AM +0100, Jonathan Neuschäfer wrote:
> > On Tue, Nov 24, 2020 at 09:20:19AM +0100, Uwe Kleine-König wrote:
[...]
> > > state->duty_cycle and state->period are u64, so you're losing
> > > information here. Consider state->duty_cycle = 0x100000001 and
> > > state->period = 0x200000001.
> > 
> > Oh, good point, I didn't notice the truncation.
> > 
> > The reason I picked unsigned int was to avoid a 64-bit division;
> > I suppose I can do something like this:
> > 
> >     period = (u32)period / TIME_BASE_NS;
> >     duty = (u32)duty / TIME_BASE_NS;
> 
> You can do that after you checked period > MAX_PERIOD_NS below, yes.
> Something like:
> 
> 	if (state->polarity != PWM_POLARITY_NORMAL)
> 		return -EINVAL;
> 
> 	if (state->period > MAX_PERIOD_NS) {
> 		period = MAX_PERIOD_NS;
> 	else
> 		period = state->period;
> 
> 	if (state->duty_cycle > period)
> 		duty_cycle = period;
> 	else
> 		duty_cycle = state->duty_cycle;
> 
> should work with even keeping the local variables as unsigned int.

With the min_t() macro, this becomes nice and short:

	 period = min_t(u64, state->period, MAX_PERIOD_NS);
	 duty   = min_t(u64, state->duty_cycle, period);

	 period /= TIME_BASE_NS;
	 duty   /= TIME_BASE_NS;


> > > I think I already asked, but I don't remember the reply: What happens to
> > > the output between these writes? A comment here about this would be
> > > suitable.
> > 
> > I will add something like the following:
> > 
> > /*
> >  * Changes to the period and duty cycle take effect as soon as the
> >  * corresponding low byte is written, so the hardware may be configured
> >  * to an inconsistent state after the period is written and before the
> >  * duty cycle is fully written. If, in such a case, the old duty cycle
> >  * is longer than the new period, the EC will output 100% for a moment.
> >  */
> 
> Is the value pair taken over by hardware atomically? That is, is it
> really "will" in your last line, or only "might". (E.g. when changing
> from duty_cycle, period = 1000, 2000 to 500, 800 and a new cycle begins
> after reducing period, the new duty_cycle is probably written before the
> counter reaches 500. Do we get a 100% cycle here?)

I am not sure when exactly a new period or duty cycle value is applied,
and I don't have the test equipment to measure it. I'll change the text
to "may output 100%".

> Other than that the info is fine. Make sure to point this out in the
> Limitations paragraph at the top of the driver please, too.

Okay.


> > > /*
> > >  * The current state cannot be read out, so there is no .get_state
> > >  * callback.
> > >  */
> > > 
> > > Hmm, at least you could provice a .get_state() callback that reports the
> > > setting that was actually implemented for in the last call to .apply()?
> > 
> > Yes... I see two options:
> > 
> > 1. Caching the state in the driver's private struct. I'm not completely
> >    convinced of the value, given that the information is mostly
> >    available in the PWM core already (except for the adjustments that
> >    the driver makes).
> > 
> > 2. Writing the adjusted state back into pwm_dev->state (via pwm_set_*).
> >    This seems a bit dirty.
> 
> 2. isn't a good option. Maybe regmap caches this stuff anyhow for 1. (or
> can be made doing that)?

With regmap caching, I'd be concerned that a read operation may slip
through and reach the device, producing a bogus result. Not sure if
write-only/write-through caching can be configured in regmap.


Thanks,
Jonathan

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