On Mon, Aug 12, 2019 at 10:50:01PM +0200, Paul Cercueil wrote: > > > Le lun. 12 août 2019 à 7:55, Uwe =?iso-8859-1?q?Kleine-K=F6nig?= > <[email protected]> a écrit : > > On Fri, Aug 09, 2019 at 07:33:24PM +0200, Paul Cercueil wrote: > > > > > > > > > Le ven. 9 août 2019 à 19:10, Uwe =?iso-8859-1?q?Kleine-K=F6nig?= > > > <[email protected]> a écrit : > > > > On Fri, Aug 09, 2019 at 02:30:30PM +0200, Paul Cercueil wrote: > > > > > The PWM will always start with the inactive part. To counter > > > that, > > > > > when PWM is enabled we switch the configured polarity, and use > > > > > 'period - duty + 1' as the real duty. > > > > > > > > Where does the + 1 come from? This looks wrong. (So if duty=0 is > > > > requested you use duty = period + 1?) > > > > > > You'd never request duty == 0, would you? > > > > > > Your duty must always be in the inclusive range [1, period] > > > (hardware values, not ns). A duty of 0 is a hardware fault > > > (on the jz4740 it is). > > > > From the PWM framework's POV duty cycle = 0 is perfectly valid. Similar > > to duty == period. Not supporting dutz cycle 0 is another limitation of > > your PWM that should be documented. > > > > For actual use cases of duty cycle = 0 see drivers/hwmon/pwm-fan.c or > > drivers/leds/leds-pwm.c. > > Perfectly valid for the PWM framework, maybe; but what is the expected > output then? A constant inactive state?
Yes, a constant inactive state is expected. This is consistent and in a similar way when using duty == period an constant active output is expected. > Then I guess I can just disable the PWM output in the driver when > configured with duty == 0. Some time ago I argued with Thierry that we could drop the concept of enabled/disabled for a PWM because a disabled PWM is supposed to behave identically to duty=0. This is however only nearly true because with duty=0 the time the PWM is inactive still is a multiple of the period. I tend to agree that disabling the PWM when duty=0 is requested is better than to fail the request (or configure for duty=1 $whateverunit). I'm looking forward to what Thierry's opinion is here. > > > If you request duty == 1 (the minimum), then the new duty is equal > > > to (period - 1 + 1) == period, which is the maximum of your range. > > > > > > If you request duty == period (the maximum), then the new duty > > > calculated is equal to (period - period + 1) == 1, which is the > > > minimum of your range. Note that the wrong border (because duty=0 is impossible for your hardware) shifts the whole space. The right inverse of duty = period - 1 is duty = 1, isn't it? > > > > > Signed-off-by: Paul Cercueil <[email protected]> > > > > > --- > > > > > drivers/pwm/pwm-jz4740.c | 22 +++++++++++++--------- > > > > > 1 file changed, 13 insertions(+), 9 deletions(-) > > > > > > > > > > diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c > > > > > index 85e2110aae4f..8df898429d47 100644 > > > > > --- a/drivers/pwm/pwm-jz4740.c > > > > > +++ b/drivers/pwm/pwm-jz4740.c > > > > > @@ -121,6 +121,7 @@ static int jz4740_pwm_apply(struct pwm_chip > > > > > *chip, struct pwm_device *pwm, > > > > > *parent_clk = clk_get_parent(clk); > > > > > unsigned long rate, parent_rate, period, duty; > > > > > unsigned long long tmp; > > > > > + bool polarity_inversed; > > > > > int ret; > > > > > > > > > > parent_rate = clk_get_rate(parent_clk); > > > > > @@ -183,24 +184,27 @@ static int jz4740_pwm_apply(struct pwm_chip > > > > > *chip, struct pwm_device *pwm, > > > > > /* Reset counter to 0 */ > > > > > regmap_write(jz4740->map, TCU_REG_TCNTc(pwm->hwpwm), 0); > > > > > > > > > > - /* Set duty */ > > > > > - regmap_write(jz4740->map, TCU_REG_TDHRc(pwm->hwpwm), duty); > > > > > - > > > > > /* Set period */ > > > > > regmap_write(jz4740->map, TCU_REG_TDFRc(pwm->hwpwm), period); > > > > > > > > > > + /* > > > > > + * The PWM will always start with the inactive part. To counter > > > > > that, > > > > > + * when PWM is enabled we switch the configured polarity, and > > > > > use > > > > > + * 'period - duty + 1' as the real duty. > > > > > + */ > > > > > + > > > > > + /* Set duty */ > > > > > + regmap_write(jz4740->map, TCU_REG_TDHRc(pwm->hwpwm), period - > > > > > duty + 1); > > > > > + > > > > > > > > Before you set duty first, then period, now you do it the other way > > > > round. Is there a good reason? > > > > > > To move it below the comment that explains why we use 'period - duty + > > > 1'. > > > > > > We modify that line anyway, so it's not like it makes the patch much more > > > verbose. > > > > It doesn't make it more verbose, but that's not the background of my > > question. For most(?) PWM implementation the order of hardware accesses > > matters and introducing such a difference as an unneeded side effect > > isn't optimal. > > There's no side effect. The PWM is disabled when reconfigured. Then please mention it in the commit log. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |
