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

Lee Jones
2016-06-09T13:51:25Z

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*?

> pwm_config(pb->pwm, duty_cycle, pb->period);
> pwm_backlight_power_on(pb, brightness);
> } else

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