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From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>,
	Heiko Stuebner <heiko@sntech.de>,
	devicetree@vger.kernel.org,
	Linus Walleij <linus.walleij@linaro.org>,
	Thierry Reding <thierry.reding@gmail.com>,
	Sam Ravnborg <sam@ravnborg.org>,
	linux-rtc@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
	Fabio Estevam <festevam@gmail.com>,
	Daniel Palmer <daniel@0x0f.com>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	Andreas Kemnade <andreas@kemnade.info>,
	NXP Linux Team <linux-imx@nxp.com>,
	linux-pwm@vger.kernel.org, Stephan Gerhold <stephan@gerhold.net>,
	allen <allen.chen@ite.com.tw>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Lubomir Rintel <lkundrak@v3.sk>, Rob Herring <robh+dt@kernel.org>,
	Lee Jones <lee.jones@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	Alessandro Zummo <a.zummo@towertech.it>,
	linux-kernel@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	Heiko Stuebner <heiko.stuebner@theobroma-systems.com>,
	Josua Mayer <josua.mayer@jm0.eu>, Shawn Guo <shawnguo@kernel.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v3 4/7] pwm: ntxec: Add driver for PWM function in Netronix EC
Date: Mon, 28 Sep 2020 10:35:46 +0200
Message-ID: <20200928083546.gwu7ucx7os5yc5bn@pengutronix.de> (raw)
In-Reply-To: <20200927211044.GC2510@latitude>


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Hello Jonathan,

On Sun, Sep 27, 2020 at 11:10:44PM +0200, Jonathan Neuschäfer wrote:
> On Fri, Sep 25, 2020 at 08:30:37AM +0200, Uwe Kleine-König wrote:
> > > +	if (period > MAX_PERIOD_NS) {
> > > +		dev_warn(pwm->dev,
> > > +			 "Period is not representable in 16 bits after division by %u: %u\n",
> > > +			 TIME_BASE_NS, period);
> > 
> > No error messages in .apply() please; this might spam the kernel log.
> >
> > Also the expectation when a too big period is requested is to configure
> > for the biggest possible period. So just do:
> > 
> > 	if (period > MAX_PERIOD_NS) {
> > 		period = MAX_PERIOD_NS;
> > 
> > 		if (duty > period)
> > 			duty = period;
> > 	}
> > 
> > (or something equivalent).
> 
> Okay, I'll adjust it.
> 
> > > +	/*
> > > +	 * Writing a duty cycle of zone puts the device into a state where
> > 
> > What is "zone"? A mixture of zero and one and so approximately 0.5?
> 
> Oops, that's a typo. I just meant "zero".
> 
> > > +	 * writing a higher duty cycle doesn't result in the brightness that it
> > > +	 * usually results in. This can be fixed by cycling the ENABLE register.
> > > +	 *
> > > +	 * As a workaround, write ENABLE=0 when the duty cycle is zero.
> > > +	 */
> > > +	if (state->enabled && duty != 0) {
> > > +		res = regmap_write(pwm->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(1));
> > > +		if (res)
> > > +			return res;
> > > +
> > > +		/* Disable the auto-off timer */
> > > +		res = regmap_write(pwm->ec->regmap, NTXEC_REG_AUTO_OFF_HI, ntxec_reg8(0xff));
> > > +		if (res)
> > > +			return res;
> > > +
> > > +		return regmap_write(pwm->ec->regmap, NTXEC_REG_AUTO_OFF_LO, ntxec_reg8(0xff));
> > > +	} else {
> > > +		return regmap_write(pwm->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(0));
> > > +	}
> > 
> > This code is wrong for state->enabled = false.
> 
> Why?

Hm, I wonder the same. Probably I just misunderstood the code, sorry :-\

> > How does the PWM behave when .apply is called? Does it complete the
> > currently running period? Can it happen that when you switch from say
> > 
> > 	.duty_cycle = 900 * TIME_BASE_NS (0x384)
> > 	.period = 1800 * TIME_BASE_NS (0x708)
> > 
> > to
> > 
> > 	.duty_cycle = 300 * TIME_BASE_NS (0x12c)
> > 	.period = 600 * TIME_BASE_NS (0x258)
> > 
> > that a period with
> > 
> > 	.duty_cycle = 388 * TIME_BASE_NS (0x184)
> > 	.period = 1800 * TIME_BASE_NS (0x708)
> > 	
> > (because only NTXEC_REG_PERIOD_HIGH was written when the new period
> > started) or something similar is emitted?
> 
> Changes take effect after the low byte is written, so a result like 0x184
> in the above example should not happen.
> 
> When the period and duty cycle are both changed, it temporarily results
> in an inconsistent state:
> 
>  - period = 1800ns, duty cycle = 900ns
>  - period =  600ns, duty cycle = 900ns (!)
>  - period =  600ns, duty cycle = 300ns

Does this always happen, or only if a new cycle starts at an unlucky
moment?

> The inconsistent state of duty cycle > period is handled gracefully by
> the EC and it outputs a 100% duty cycle, as far as I can tell.

OK.

> I currently don't have a logic analyzer / oscilloscope to measure
> whether we get full PWM periods, or some kind of glitch when the new
> period starts in the middle of the last one.

You can even check this with an LED using something like:

	pwm_apply(mypwm, {.enabled = true, .duty_cycle = $big, .period = $big});
	pwm_apply(mypwm, {.enabled = false, ... });

. If the period is completed the LED is on for $big ns, if not the LED
is on for a timespan that is probably hardly noticable with the human
eye.

> > > +}
> > > +
> > > +static struct pwm_ops ntxec_pwm_ops = {
> > > +	.apply = ntxec_pwm_apply,
> > 
> > Please implement a .get_state() callback. And enable PWM_DEBUG during
> > your tests.
> 
> The device doesn't support reading back the PWM state. What should a
> driver do in this case?

Document it as a limitation, please.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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  reply index

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-24 19:24 [PATCH v3 0/7] Netronix embedded controller driver for Kobo and Tolino ebook readers Jonathan Neuschäfer
2020-09-24 19:24 ` [PATCH v3 1/7] dt-bindings: Add vendor prefix for Netronix, Inc Jonathan Neuschäfer
2020-10-12  7:19   ` Uwe Kleine-König
2020-10-12 11:48     ` Lee Jones
2020-09-24 19:24 ` [PATCH v3 2/7] dt-bindings: mfd: Add binding for Netronix embedded controller Jonathan Neuschäfer
2020-09-29 18:52   ` Rob Herring
2020-09-24 19:24 ` [PATCH v3 3/7] mfd: Add base driver " Jonathan Neuschäfer
2020-09-25  9:29   ` Andy Shevchenko
2020-09-25 21:32     ` Jonathan Neuschäfer
2020-09-29 16:37       ` Andy Shevchenko
2020-10-02 22:54         ` Jonathan Neuschäfer
2020-09-24 19:24 ` [PATCH v3 4/7] pwm: ntxec: Add driver for PWM function in Netronix EC Jonathan Neuschäfer
2020-09-25  6:30   ` Uwe Kleine-König
2020-09-27 21:10     ` Jonathan Neuschäfer
2020-09-28  8:35       ` Uwe Kleine-König [this message]
2020-10-02 23:36         ` Jonathan Neuschäfer
2020-10-03  8:17           ` Andy Shevchenko
2020-09-24 19:24 ` [PATCH v3 5/7] rtc: New driver for RTC in Netronix embedded controller Jonathan Neuschäfer
2020-09-24 20:40   ` Andreas Kemnade
2020-10-02 23:41     ` Jonathan Neuschäfer
2020-09-25  5:44   ` Uwe Kleine-König
2020-10-04  1:43     ` Jonathan Neuschäfer
2020-10-04  8:42       ` Alexandre Belloni
2020-10-11 19:09         ` Jonathan Neuschäfer
2020-10-05  7:35       ` Uwe Kleine-König
2020-09-25  9:36   ` Alexandre Belloni
2020-09-25 22:00     ` Jonathan Neuschäfer
2020-09-24 19:24 ` [PATCH v3 6/7] MAINTAINERS: Add entry for " Jonathan Neuschäfer
2020-09-25  5:08 ` [PATCH v3 7/7] ARM: dts: imx50-kobo-aura: Add " Jonathan Neuschäfer
2020-10-07  7:46   ` Krzysztof Kozlowski
2020-10-11 19:42     ` Jonathan Neuschäfer

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