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>,
	linux-kernel@vger.kernel.org,
	"Alexandre Belloni" <alexandre.belloni@bootlin.com>,
	"Heiko Stuebner" <heiko@sntech.de>,
	linux-pwm@vger.kernel.org,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Fabio Estevam" <festevam@gmail.com>,
	linux-rtc@vger.kernel.org, "Arnd Bergmann" <arnd@arndb.de>,
	"Mauro Carvalho Chehab" <mchehab+huawei@kernel.org>,
	"Sam Ravnborg" <sam@ravnborg.org>,
	"Daniel Palmer" <daniel@0x0f.com>,
	"Andy Shevchenko" <andy.shevchenko@gmail.com>,
	"Andreas Kemnade" <andreas@kemnade.info>,
	"NXP Linux Team" <linux-imx@nxp.com>,
	devicetree@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>,
	"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: Sun, 27 Sep 2020 23:10:44 +0200
Message-ID: <20200927211044.GC2510@latitude> (raw)
In-Reply-To: <20200925063037.fcrmqvpe5noi3ef4@pengutronix.de>


[-- Attachment #1: Type: text/plain, Size: 5793 bytes --]

On Fri, Sep 25, 2020 at 08:30:37AM +0200, Uwe Kleine-König wrote:
> Hello Jonathan,
[...]
> > +config PWM_NTXEC
> > +	tristate "Netronix embedded controller PWM support"
> > +	depends on MFD_NTXEC
> > +	help
> > +	  Say yes here if you want to support the PWM output of the embedded
> > +	  controller found in certain e-book readers designed by the ODM
> > +	  Netronix.
> 
> Is it only me who had to look up what ODM means? If not, maybe spell it
> out?

I'm sure other readers will have the same problem. I'll spell it out.

> > +/*
> > + * The maximum input value (in nanoseconds) is determined by the time base and
> > + * the range of the hardware registers that hold the converted value.
> > + * It fits into 32 bits, so we can do our calculations in 32 bits as well.
> > + */
> > +#define MAX_PERIOD_NS (TIME_BASE_NS * 0x10000 - 1)
> 
> The maximal configurable period length is 0xffff, so I would have
> expected MAX_PERIOD_NS to be 0xffff * TIME_BASE_NS?

Due to the division rounding down, TIME_BASE_NS * 0x10000 - 1 would be
the highest input that results in a representable value after the
division, but I'm not sure it otherwise makes sense.

> 
> > +static int ntxec_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm_dev,
> > +			   const struct pwm_state *state)
> > +{
> > +	struct ntxec_pwm *pwm = pwmchip_to_pwm(pwm_dev->chip);
> > +	unsigned int duty = state->duty_cycle;
> > +	unsigned int period = state->period;
> > +	int res = 0;
> > +
> 
> I assume your device only supports normal polarity? If so, please check
> for it here and point out this limitation in the header (in the format
> that is for example used in pwm-sifive.c to make it easy to grep for
> that).

I haven't seen any indication that it supports inverted polarity. I'll
point it out in the header comment, and add a check.

> 
> > +	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?

> 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

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.

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.

> > +}
> > +
> > +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?

> > +	.owner = THIS_MODULE,
> > +};
> > +
> > +static int ntxec_pwm_probe(struct platform_device *pdev)
> > +{
> > +	struct ntxec *ec = dev_get_drvdata(pdev->dev.parent);
> > +	struct ntxec_pwm *pwm;
> 
> Please don't call this variable pwm. I would expect that a variable with
> this name is of type pwm_device. I would have called it "ddata" (and the
> type would be named ntxec_pwm_ddata for me); another usual name is "priv".

Ok, I'll rename it.

> > +	chip->npwm = 1;
> > +
> > +	res = pwmchip_add(chip);
> > +	if (res < 0)
> > +		return res;
> > +
> > +	platform_set_drvdata(pdev, pwm);
> 
> If you do the platform_set_drvdata earlier you can just do
> 
> 	return pwmchip_add(chip);

Good idea, I'll do that.


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 [this message]
2020-09-28  8:35       ` Uwe Kleine-König
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

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=20200927211044.GC2510@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