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

Lothar Waßmann
2016-06-10T05:24:01Z

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.

Lothar Waßmann