Message-ID: <20160610145449.GA7351@dell>
Date:	Fri, 10 Jun 2016 15:54:49 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Lothar Waßmann <LW@...O-electronics.de>
Cc:	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pwm@...r.kernel.org,
	Marcel Ziswiler <marcel.ziswiler@...adex.com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCHv2] backlight: pwm_bl: disable PWM when 'duty_cycle' is
 zero

On Fri, 10 Jun 2016, Lothar Waßmann wrote:
> On Fri, 10 Jun 2016 08:44:49 +0100 Lee Jones wrote:
> > On Fri, 10 Jun 2016, Lothar Waßmann wrote:
> > 
> > > Hi,
> > > 
> > > On Thu, 9 Jun 2016 14:51:25 +0100 Lee Jones wrote:
> > > > On Tue, 07 Jun 2016, Lothar Waßmann wrote:
> > > > 
> > > > > 'brightness' is usually an index into a table of duty_cycle values,
> > > > > where the value at index 0 may well be non-zero
> > > > > (tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life
> > > > > examples).
> > > > > Thus brightness == 0 does not necessarily mean that the PWM output
> > > > > will be inactive.
> > > > > Check for 'duty_cycle == 0' rather than 'brightness == 0' to decide
> > > > > whether to disable the PWM.
> > > > > 
> > > > > Signed-off-by: Lothar Waßmann <LW@...O-electronics.de>
> > > > > ---
> > > > > Changes wrt. v1:
> > > > >   - update binding docs to reflect the change
> > > > > 
> > > > >  .../devicetree/bindings/leds/backlight/pwm-backlight.txt         | 9 ++++++---
> > > > >  drivers/video/backlight/pwm_bl.c                                 | 4 ++--
> > > > >  2 files changed, 8 insertions(+), 5 deletions(-)
> > > > > 
> > > > > diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> > > > > index 764db86..95fa8a9 100644
> > > > > --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> > > > > +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> > > > > @@ -4,10 +4,13 @@ Required properties:
> > > > >    - compatible: "pwm-backlight"
> > > > >    - pwms: OF device-tree PWM specification (see PWM binding[0])
> > > > >    - brightness-levels: Array of distinct brightness levels. Typically these
> > > > > -      are in the range from 0 to 255, but any range starting at 0 will do.
> > > > > +      are in the range from 0 to 255, but any range will do.
> > > > >        The actual brightness level (PWM duty cycle) will be interpolated
> > > > > -      from these values. 0 means a 0% duty cycle (darkest/off), while the
> > > > > -      last value in the array represents a 100% duty cycle (brightest).
> > > > > +      from these values. 0 means a 0% duty cycle, while the highest value in
> > > > > +      the array represents a 100% duty cycle.
> > > > > +      The range may be in reverse order (starting with the maximum duty cycle
> > > > > +      value) to create a PWM signal with the 100% duty cycle representing
> > > > > +      minimum and 0% duty cycle maximum brigthness.
> > > > >    - default-brightness-level: the default brightness level (index into the
> > > > >        array defined by the "brightness-levels" property)
> > > > >    - power-supply: regulator for supply voltage
> > > > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> > > > > index b2b366b..80b2b52 100644
> > > > > --- a/drivers/video/backlight/pwm_bl.c
> > > > > +++ b/drivers/video/backlight/pwm_bl.c
> > > > > @@ -103,8 +103,8 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
> > > > >  	if (pb->notify)
> > > > >  		brightness = pb->notify(pb->dev, brightness);
> > > > >  
> > > > > -	if (brightness > 0) {
> > > > > -		duty_cycle = compute_duty_cycle(pb, brightness);
> > > > > +	duty_cycle = compute_duty_cycle(pb, brightness);
> > > > > +	if (duty_cycle > 0) {
> > > > 
> > > > How does this work in the aforementioned:
> > > > 
> > > >   "The range may be in reverse order"
> > > > 
> > > > ... case?  Surely when duty_cycle is when the screen should be at it's
> > > > brightest?  Wouldn't it confuse the user if they turn their brightness
> > > > *up* and the screen goes *off*?
> > > > 
> > > Assuming that the PWM output is inactive (LOW) when the duty_cycle is
> > > set to zero, there will be no difference between operating the PWM at
> > > duty_cycle 0 or disabling it.
> > > 
> > > Currently, the screen will go bright when it should be off in this
> > > case.
> > 
> > It sounds like we need something that lets the framework know if
> > duty_cycle = MAX is the brightest or if duty_cycle = 0 is.  Either way
> > someone is going to get screwed by this logic.
> > 
> The backlight framework does not (and does not need to) know anything
> about PWM duty cycles. Its 'brightness' values are consistently 0 ==
> dark, max == brightest in either case.

What I'm getting at is; by the look of the documentation, the
brightest setting can either be a duty cycle of 0 or 255.  So what
happens with your new semantics when the duty cycle of 0 represents
the brightest setting and you reach 0?  Didn't you just turn the
backlight off?

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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