linux-kernel - Re: [PATCH 1/2] PWM: let of_xlate handlers check args count

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Date:	Fri, 24 Jan 2014 06:42:54 +0100
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	Mark Rutland <mark.rutland@....com>, linux-pwm@...r.kernel.org,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Rob Landley <rob@...dley.net>,
	Kumar Gala <galak@...eaurora.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] PWM: let of_xlate handlers check args count

Hi,

Russell King - ARM Linux wrote:
> On Thu, Jan 23, 2014 at 04:53:50PM +0000, Russell King - ARM Linux wrote:
> > On Thu, Jan 23, 2014 at 12:04:44PM +0100, Sascha Hauer wrote:
> > > On Thu, Jan 23, 2014 at 11:56:32AM +0100, Lothar Waßmann wrote:
> > > > Hi,
> > > > 
> > > > Sascha Hauer wrote:
> > > > > of_pwm_n_cells for the of_xlate handler is stored in struct pwm_chip,
> > > > > but it is only ever used by the of_xlate handler itsel. Remove
> > > > > of_pwm_n_cells from struct pwm_chip and let the handler do the argument
> > > > > count checking to simplify the code.
> > > > > 
> > > > This still does not make the PWM_POLARITY flag in the pwms node
> > > > optional as was the goal because of_parse_phandle_with_args() requires
> > > > at least #pwm-cells arguments in the node.
> > > > 
> > > > So, with a DT configuration like:
> > > > pwm0: pwm@0 {
> > > > 	#pwm-cells = <3>;
> > > > };
> > > > backlight {
> > > > 	pwms = <&pwm0 0 100000>;
> > > > };
> > > 
> > > We misunderstood each other. My goal was to allow the driver to also
> > > work with old devicetrees which specify #pwm-cells = <2>, not to allow
> > > inconsistent devicetrees like the snippet above.
> > 
> > In which case, the patch I've posted seems to do that job too... I'm
> > just about to test out the three-cell version.
> 
> 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.


Lothar Waßmann
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