Re: [PATCHv2] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero

[Lee Jones <lee.jones@linaro.org>]

On Sat, 11 Jun 2016, Lothar Waßmann wrote:
> On Fri, 10 Jun 2016 15:54:49 +0100 Lee Jones wrote:
> > 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@KARO-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?
> > 
> As mentioned earlier, disabling the PWM has generally the same result as
> setting the duty cycle to 0. The current behaviour is broken in this
> case, since setting brightness to 0 with a non-zero duty_cycle as the
> first element of brightness-levels, the PWM will be disabled rather than
> switched to the given duty cycle.
> Disabling the PWM should have the same effect as setting the duty cycle
> to 0, so it is safe to check for duty_cycle = 0 to decide whether to
> disable the PWM.

I agree with this. BUT, that's not what you're doing is it?

Look at the code you're trying to write:

duty_cycle = compute_duty_cycle(pb, brightness);
if (duty_cycle > 0) {
        pwm_config(pb->pwm, duty_cycle, pb->period);
        pwm_backlight_power_on(pb, brightness);
} else
        pwm_backlight_power_off(pb);

Let's say duty_cycle = 0.  In some cases this can mean "turn
brightness up to the *maximum*", but with your new logic you just
turned the backlight *off*.

Conversely, let's say duty_cycle = 255.  In some cases this can mean
"turn the brightness to the *lowest* setting" i.e. *off*. Well your
logic just turned the backlight *on*.

If there is something I'm missing, you're going to have to find a
better way to explain it to me.

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