linux-kernel - Re: [PATCH] pwm-backlight: allow for non-increasing brightness levels

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Date:	Thu, 19 Sep 2013 13:56:53 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Mike Dunn <mikedunn@...sguy.com>
Cc:	Richard Purdie <rpurdie@...ys.net>,
	Jingoo Han <jg1.han@...sung.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Marek Vasut <marex@...x.de>
Subject: Re: [PATCH] pwm-backlight: allow for non-increasing brightness levels

On Thu, Sep 12, 2013 at 11:35:52AM -0700, Mike Dunn wrote:
> Currently the driver assumes that the values specified in the brightness-levels
> device tree property increase as they are parsed from left to right.  But boards
> that invert the signal between the PWM output and the backlight will need to
> specify decreasing brightness-levels.  This patch removes the assumption that
> the last element of the array is the max value, and instead searches the array
> for the max value and uses that as the normalizing value when determining the
> duty cycle.

"maximum value", "... and uses that as the scale to normalize the duty
cycle"?

Also please wrap commit messages at 72 characters.

> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 1fea627..d66aaa0 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -27,6 +27,7 @@ struct pwm_bl_data {
>  	unsigned int		period;
>  	unsigned int		lth_brightness;
>  	unsigned int		*levels;
> +	unsigned int		max_level;

Perhaps call this "scale"? Otherwise there some potential to mix it up
with max_brightness.

> @@ -195,7 +196,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	}
>  
>  	if (data->levels) {
> -		max = data->levels[data->max_brightness];
> +		int i, max_value = 0, max_idx = 0;

i should be unsigned int to match the type of data->max_brightness.

> +		for (i = 0; i <= data->max_brightness; i++) {

There should be a blank line above this one to increase readability.

> +			if (data->levels[i] > max_value) {
> +				max_value = data->levels[i];
> +				max_idx = i;
> +			}
> +		}
> +		pb->max_level = max_idx;

Some here.

Also I suggest to just drop the max_ prefix from the local variables.
Perhaps also simplify all of it to something like:

	for (i = 0; i <= data->max_brightness; i++)
		if (data->levels[i] > pb->scale)
			pb->scale = data->levels[i];

And get rid of the index altogether. That way you can use pb->scale
directly during the computation of the duty cycle and don't have to
index the levels array over and over again.

Thierry

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