Messages in this thread Patch in this message | | | Date | Fri, 24 Jan 2014 08:41:03 +0100 | From | Sascha Hauer <> | Subject | Re: [PATCH 1/2] PWM: let of_xlate handlers check args count |
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On Fri, Jan 24, 2014 at 06:42:54AM +0100, Lothar Waßmann wrote: > Hi, > > > Okay, this works, but there's a problem with pwm-leds. > > > > When the duty cycle is set to zero (when you set the brightness to zero) > > pwm-leds decides to disable the PWM after configuring it. This causes > > the PWM output to be driven low, causing the LED to go to maximum > > brightness. > > > > So, using the inversion at PWM level doesn't work. > > > The problem is that the driver calls pwm_disable() when the duty cycle is 0. > This sets the PWM output low independent from the output polarity setting. > > > To make this work correctly, we really need pwm-leds to do the inversion > > rather than setting the inversion bit in hardware. > > > The same holds for the pwm-backlight driver. > > The easiest fix would be not to call pwm_disable() even for a zero duty > cycle.
IMO that's the right thing to do anyway due to the different PWM hardware controllers we have. I'm thinking about the following patch for some time.
Sascha
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From 9ebbc3d72c71bd97d7fc4458f60ae3ecd5876984 Mon Sep 17 00:00:00 2001 From: Sascha Hauer <s.hauer@pengutronix.de> Date: Fri, 24 Jan 2014 08:34:16 +0100 Subject: [PATCH] PWM: Document disabled PWM output as undefined
When disabled PWM hardware reacts differently. Some controllers just stop with their current value, others produce a constant high or low output, sometimes depending on the output inversion bit. Update the documentation to reflect that and request from the PWM consumer drivers to set a constant high/low value with duty cycles of 0/100% instead of disabling the PWM.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de> --- Documentation/pwm.txt | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt index 93cb979..b7e8a31 100644 --- a/Documentation/pwm.txt +++ b/Documentation/pwm.txt @@ -44,6 +44,8 @@ After being requested, a PWM has to be configured using: int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns); To start/stop toggling the PWM output use pwm_enable()/pwm_disable(). +The output of a disabled PWM is undefined. Set the duty cycle to 100% +for a constant high output and to 0 for constant low output. Using PWMs with the sysfs interface ----------------------------------- -- 1.8.5.2 -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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