pwm-backlight: Allow for non-increasing brightness levels

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 maximum value, and instead searches the array for the maximum value
and uses that in the duty cycle calculation.
Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/video/backlight/pwm_bl.c

@@ -34,6 +34,7 @@ struct pwm_bl_data {
 	struct regulator	*power_supply;
 	int			enable_gpio;
 	unsigned long		enable_gpio_flags;
+	unsigned int		scale;
 	int			(*notify)(struct device *,
 					  int brightness);
 	void			(*notify_after)(struct device *,
@@ -42,23 +43,20 @@ struct pwm_bl_data {
 	void			(*exit)(struct device *);
 };
 
-static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness,
-				   int max)
+static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness)
 {
+	unsigned int lth = pb->lth_brightness;
 	int duty_cycle, err;
 
 	if (pb->enabled)
 		return;
 
-	if (pb->levels) {
+	if (pb->levels)
 		duty_cycle = pb->levels[brightness];
-		max = pb->levels[max];
-	} else {
+	else
 		duty_cycle = brightness;
-	}
 
-	duty_cycle = (duty_cycle * (pb->period - pb->lth_brightness) / max) +
-		     pb->lth_brightness;
+	duty_cycle = (duty_cycle * (pb->period - lth) / pb->scale) + lth;
 
 	pwm_config(pb->pwm, duty_cycle, pb->period);
 
@@ -100,7 +98,6 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
 {
 	struct pwm_bl_data *pb = bl_get_data(bl);
 	int brightness = bl->props.brightness;
-	int max = bl->props.max_brightness;
 
 	if (bl->props.power != FB_BLANK_UNBLANK ||
 	    bl->props.fb_blank != FB_BLANK_UNBLANK ||
@@ -111,7 +108,7 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
 		brightness = pb->notify(pb->dev, brightness);
 
 	if (brightness > 0)
-		pwm_backlight_power_on(pb, brightness, max);
+		pwm_backlight_power_on(pb, brightness);
 	else
 		pwm_backlight_power_off(pb);
 
@@ -218,7 +215,6 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	struct backlight_properties props;
 	struct backlight_device *bl;
 	struct pwm_bl_data *pb;
-	unsigned int max;
 	int ret;
 
 	if (!data) {
@@ -245,10 +241,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	}
 
 	if (data->levels) {
-		max = data->levels[data->max_brightness];
+		unsigned int i;
+
+		for (i = 0; i <= data->max_brightness; i++)
+			if (data->levels[i] > pb->scale)
+				pb->scale = data->levels[i];
+
 		pb->levels = data->levels;
 	} else
-		max = data->max_brightness;
+		pb->scale = data->max_brightness;
 
 	pb->enable_gpio = data->enable_gpio;
 	pb->enable_gpio_flags = data->enable_gpio_flags;
@@ -304,7 +305,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 		pwm_set_period(pb->pwm, data->pwm_period_ns);
 
 	pb->period = pwm_get_period(pb->pwm);
-	pb->lth_brightness = data->lth_brightness * (pb->period / max);
+	pb->lth_brightness = data->lth_brightness * (pb->period / pb->scale);
 
 	memset(&props, 0, sizeof(struct backlight_properties));
 	props.type = BACKLIGHT_RAW;