On Mon, Aug 12, 2019 at 11:58:53PM +0200, Uwe Kleine-König wrote: > 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.
Agreed. If in order to meet the expectations of duty == 0 you have to disable the PWM, then that's what you should do. Thierry
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