Re: [PATCH v7 06/13] pwm: add support for sl28cpld PWM controller - Uwe Kleine-König

From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Michael Walle <michael@walle.cc>
Cc: linux-gpio@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-hwmon@vger.kernel.org,
	linux-pwm@vger.kernel.org, linux-watchdog@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Linus Walleij <linus.walleij@linaro.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Rob Herring <robh+dt@kernel.org>,
	Jean Delvare <jdelvare@suse.com>,
	Guenter Roeck <linux@roeck-us.net>,
	Lee Jones <lee.jones@linaro.org>,
	Thierry Reding <thierry.reding@gmail.com>,
	Wim Van Sebroeck <wim@linux-watchdog.org>,
	Shawn Guo <shawnguo@kernel.org>, Li Yang <leoyang.li@nxp.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jason Cooper <jason@lakedaemon.net>,
	Marc Zyngier <maz@kernel.org>, Mark Brown <broonie@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Pavel Machek <pavel@ucw.cz>
Subject: Re: [PATCH v7 06/13] pwm: add support for sl28cpld PWM controller
Date: Fri, 7 Aug 2020 12:24:41 +0200	[thread overview]
Message-ID: <20200807102441.qcshhsc36nzj7bpn@pengutronix.de> (raw)
In-Reply-To: <92116be9aa56250becc4019c6c7a1538@walle.cc>

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Hi Michael,

On Fri, Aug 07, 2020 at 09:55:19AM +0200, Michael Walle wrote:
> Am 2020-08-07 09:45, schrieb Uwe Kleine-König:
> > On Fri, Aug 07, 2020 at 09:28:31AM +0200, Michael Walle wrote:
> > > Am 2020-08-06 10:40, schrieb Uwe Kleine-König:
> > > > On Mon, Aug 03, 2020 at 11:35:52AM +0200, Michael Walle wrote:
> > > > > +static void sl28cpld_pwm_get_state(struct pwm_chip *chip,
> > > > > +				   struct pwm_device *pwm,
> > > > > +				   struct pwm_state *state)
> > > > > +{
> > > > > +	struct sl28cpld_pwm *priv = dev_get_drvdata(chip->dev);
> > > > > +	unsigned int reg;
> > > > > +	int prescaler;
> > > > > +
> > > > > +	sl28cpld_pwm_read(priv, SL28CPLD_PWM_CTRL, &reg);
> > > > > +
> > > > > +	state->enabled = reg & SL28CPLD_PWM_CTRL_ENABLE;
> > > > > +
> > > > > +	prescaler = FIELD_GET(SL28CPLD_PWM_CTRL_PRESCALER_MASK, reg);
> > > > > +	state->period = SL28CPLD_PWM_PERIOD(prescaler);
> > > > > +
> > > > > +	sl28cpld_pwm_read(priv, SL28CPLD_PWM_CYCLE, &reg);
> > > > > +	state->duty_cycle = SL28CPLD_PWM_TO_DUTY_CYCLE(reg);
> > > >
> > > > Should reg be masked to SL28CPLD_PWM_CYCLE_MAX, or is it guaranteed that
> > > > the upper bits are zero?
> > > 
> > > Mh, the hardware guarantees that bit7 is zero. So masking with
> > > SL28CPLD_PWM_CYCLE_MAX won't buy us much. But what I could think
> > > could go wrong is this: someone set the prescaler to != 0 and the
> > > duty cycle to a value greater than the max value for this particular
> > > prescaler mode. For the above calculations this would result in a
> > > duty_cycle greater than the period, if I'm not mistaken.
> > > 
> > > The behavior of the hardware is undefined in that case (at the moment
> > > it will be always on, I guess). So this isn't a valid setting.
> > > Nevertheless it might happen. So what about the following:
> > > 
> > > state->duty_cycle = min(state->duty_cycle, state->period);
> > 
> > If you care about this: This can also happen (at least shortly) in
> > sl28cpld_pwm_apply() as you write SL28CPLD_PWM_CTRL before
> > SL28CPLD_PWM_CYCLE there.
> 
> It could also happen if it was the other way around, couldn't it?
> Changing modes might glitch.

If you want to prevent this, you have to order the writes depending on
prescaler increasing or decreasing.

Best regards
Uwe

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

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