LKML Archive on lore.kernel.org
 help / color / Atom feed
From: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>,
	"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: Sat, 3 Oct 2020 01:36:30 +0200
Message-ID: <20201002233630.GB500800@latitude> (raw)
In-Reply-To: <20200928083546.gwu7ucx7os5yc5bn@pengutronix.de>


[-- Attachment #1: Type: text/plain, Size: 3680 bytes --]

On Mon, Sep 28, 2020 at 10:35:46AM +0200, Uwe Kleine-König wrote:
> Hello Jonathan,
> 
> On Sun, Sep 27, 2020 at 11:10:44PM +0200, Jonathan Neuschäfer wrote:
[...]
> > > > +	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?

Just based on thinking about the code, the register writes setting this
intermediate state would always happen, but I don't know if the state
changes are applied in the middle of a running period, or when the new
period starts, because I can't measure the signal in good enough detail
at the 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.

The longest configurable period is about 8ms, so it's not long enough to
see anything. However, after writing enable=0, it can take about a
second for the PWM signal to turn off... this hardware is a bit weird.

> > > > +}
> > > > +
> > > > +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.

Okay.


Thanks,
Jonathan Neuschäfer

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

  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
2020-10-02 23:36         ` Jonathan Neuschäfer [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201002233630.GB500800@latitude \
    --to=j.neuschaefer@gmx.net \
    --cc=a.zummo@towertech.it \
    --cc=alexandre.belloni@bootlin.com \
    --cc=allen.chen@ite.com.tw \
    --cc=andreas@kemnade.info \
    --cc=andy.shevchenko@gmail.com \
    --cc=arnd@arndb.de \
    --cc=broonie@kernel.org \
    --cc=daniel@0x0f.com \
    --cc=davem@davemloft.net \
    --cc=devicetree@vger.kernel.org \
    --cc=festevam@gmail.com \
    --cc=heiko.stuebner@theobroma-systems.com \
    --cc=heiko@sntech.de \
    --cc=josua.mayer@jm0.eu \
    --cc=kernel@pengutronix.de \
    --cc=lee.jones@linaro.org \
    --cc=linus.walleij@linaro.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-imx@nxp.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-pwm@vger.kernel.org \
    --cc=linux-rtc@vger.kernel.org \
    --cc=lkundrak@v3.sk \
    --cc=mchehab+huawei@kernel.org \
    --cc=robh+dt@kernel.org \
    --cc=s.hauer@pengutronix.de \
    --cc=sam@ravnborg.org \
    --cc=shawnguo@kernel.org \
    --cc=stephan@gerhold.net \
    --cc=thierry.reding@gmail.com \
    --cc=u.kleine-koenig@pengutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git