Messages in this thread | | | Date | Mon, 12 Aug 2019 22:50:01 +0200 | From | Paul Cercueil <> | Subject | Re: [PATCH 6/7] pwm: jz4740: Make PWM start with the active part |
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Le lun. 12 août 2019 à 7:55, Uwe =?iso-8859-1?q?Kleine-K=F6nig?= <u.kleine-koenig@pengutronix.de> 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?= >> <u.kleine-koenig@pengutronix.de> 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? Then I guess I can just disable the PWM output in the driver when configured with duty == 0.
>> 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. >> >> >> > > >> > > Signed-off-by: Paul Cercueil <paul@crapouillou.net> >> > > --- >> > > 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.
> Why not add the comment above the line that already used to set the > duty > in hardware?
I thought it made sense to have the two parts of the trick closer together in the code, below the comment, so that it's clearer what it does.
>> > > /* Set polarity */ >> > > - switch (state->polarity) { >> > > - case PWM_POLARITY_NORMAL: >> > > + polarity_inversed = state->polarity == PWM_POLARITY_INVERSED; >> > > + if (!polarity_inversed ^ state->enabled) >> > >> > Why does state->enabled suddenly matter here? >> >> The pin stay inactive when the PWM is disabled, but the level of the >> inactive state depends on the polarity of the pin. So we need to >> switch >> the polarity only when the PWM is enabled. > > After some thought I got that. When knowing this, this is already > mentioned in the comment you introduced as you write about enabled > PWMs > only. Maybe it's just me, but mentioning that case more explicit would > have helped me. Something like: > > /* > * The hardware always starts a period with the inactive part. > * So invert polarity and duty cycle to yield the output that is > * expected by the PWM framework and its users. This inversion > * must not be done for a disabled PWM however because otherwise > * it outputs a constant active level. > */
Ok.
> > Best regards > Uwe > > -- > Pengutronix e.K. | Uwe Kleine-König > | > Industrial Linux Solutions | > http://www.pengutronix.de/ |
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