linux-kernel - Re: [PATCHv2] backlight: pwm_bl: disable PWM when 'duty

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Date:	Thu, 9 Jun 2016 14:51:25 +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 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*?

>  		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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