linux-kernel - RE: [PATCH] backlight: add low threshold to pwm backlight

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Message-ID: <F45880696056844FA6A73F415B568C69532D594BA2@EXDCVYMBSTM006.EQ1STM.local>
Date:	Tue, 14 Sep 2010 09:05:55 +0200
From:	Arun MURTHY <arun.murthy@...ricsson.com>
To:	Arun MURTHY <arun.murthy@...ricsson.com>,
	"rpurdie@...ys.net" <rpurdie@...ys.net>,
	"linux@...tec.co.uk" <linux@...tec.co.uk>,
	"ben@...tec.co.uk" <ben@...tec.co.uk>,
	"tj@...nel.org" <tj@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	STEricsson_nomadik_linux <STEricsson_nomadik_linux@...t.st.com>
Subject: RE: [PATCH] backlight: add low threshold to pwm backlight

Any comments?

> -----Original Message-----
> From: Arun MURTHY
> Sent: Thursday, September 09, 2010 5:00 PM
> To: rpurdie@...ys.net; linux@...tec.co.uk; ben@...tec.co.uk;
> tj@...nel.org
> Cc: linux-kernel@...r.kernel.org; STEricsson_nomadik_linux; Arun MURTHY
> Subject: [PATCH] backlight: add low threshold to pwm backlight
> 
> The intensity of the backlight can be varied from a range of
> max_brightness to zero. Though most, if not all the pwm based backlight
> devices start flickering at lower brightness value. And also for each
> device there exists a brightness value below which the backlight
> appears
> to be turned off though the value is not equal to zero.
> 
> If the range of brightness for a device is from zero to
> max_brightness. A graph is plotted for brightness Vs intensity fo the
> pwm based backlight device has to be a linear graph.
> 
> intensity
> 	  |   /
> 	  |  /
> 	  | /
> 	  |/
> 	  ---------
> 	 0	max_brightness
> But pratically on measuring the above we note that the intensity of
> backlight goes to zero(OFF) when the value in not zero almost nearing
> to
> zero(some x%). so the graph looks like
> 
> intensity
> 	  |    /
> 	  |   /
> 	  |  /
> 	  |  |
> 	  ------------
> 	 0   x	 max_brightness
> 
> In order to overcome this drawback knowing this x% i.e nothing but the
> low threshold beyond which the backlight is off and will have no
> effect,
> the brightness value is being offset by the low threshold
> value(retaining the linearity of the graph). Now the graph becomes
> 
> intensity
> 	  |     /
> 	  |    /
> 	  |   /
> 	  |  /
> 	  -------------
> 	   0	  max_brightness
> With this for each and every digit increment in the brightness from
> zero
> there is a change in the intensity of backlight.
> Devices having this behaviour can set the low threshold
> brightness(lth_brightness) and pass the same as platform data else can
> have it as zero.
> 
> Signed-off-by: Arun Murthy <arun.murthy@...ricsson.com>
> Acked-by: Linus Walleij <linus.walleij@...ricsson.com>
> ---
>  drivers/video/backlight/pwm_bl.c |    7 ++++++-
>  include/linux/pwm_backlight.h    |    1 +
>  2 files changed, 7 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c
> b/drivers/video/backlight/pwm_bl.c
> index 5504435..1f958ce 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -25,6 +25,7 @@ struct pwm_bl_data {
>  	struct pwm_device	*pwm;
>  	struct device		*dev;
>  	unsigned int		period;
> +	unsigned int		lth_brightness;
>  	int			(*notify)(struct device *,
>  					  int brightness);
>  };
> @@ -48,7 +49,9 @@ static int pwm_backlight_update_status(struct
> backlight_device *bl)
>  		pwm_config(pb->pwm, 0, pb->period);
>  		pwm_disable(pb->pwm);
>  	} else {
> -		pwm_config(pb->pwm, brightness * pb->period / max, pb-
> >period);
> +		brightness = pb->lth_brightness +
> +			(brightness * (pb->period - pb->lth_brightness)/
> max);
> +		pwm_config(pb->pwm, brightness, pb->period);
>  		pwm_enable(pb->pwm);
>  	}
>  	return 0;
> @@ -92,6 +95,8 @@ static int pwm_backlight_probe(struct platform_device
> *pdev)
> 
>  	pb->period = data->pwm_period_ns;
>  	pb->notify = data->notify;
> +	pb->lth_brightness = data->lth_brightness *
> +		(data->pwm_period_ns / data->max_brightness);
>  	pb->dev = &pdev->dev;
> 
>  	pb->pwm = pwm_request(data->pwm_id, "backlight");
> diff --git a/include/linux/pwm_backlight.h
> b/include/linux/pwm_backlight.h
> index 01b3d75..e031e1a 100644
> --- a/include/linux/pwm_backlight.h
> +++ b/include/linux/pwm_backlight.h
> @@ -8,6 +8,7 @@ struct platform_pwm_backlight_data {
>  	int pwm_id;
>  	unsigned int max_brightness;
>  	unsigned int dft_brightness;
> +	unsigned int lth_brightness;
>  	unsigned int pwm_period_ns;
>  	int (*init)(struct device *dev);
>  	int (*notify)(struct device *dev, int brightness);
> --
> 1.7.2.dirty

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