[PATCH v2 1/3] backlight: pwm_bl: Fix interpolation

[Alexandru Stan]

Whenever num-interpolated-steps was larger than the distance
between 2 consecutive brightness levels the table would get really
discontinuous. The slope of the interpolation would stick with
integers only and if it was 0 the whole line segment would get skipped.

Example settings:
	brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
	num-interpolated-steps = <16>;

The distances between 1 2 4 and 8 would be 1, and only starting with 16
it would start to interpolate properly.

Let's change it so there's always interpolation happening, even if
there's no enough points available (read: values in the table would
appear more than once). This should match the expected behavior much
more closely.

Signed-off-by: Alexandru Stan <amstan@xxxxxxxxxxxx>
---

 drivers/video/backlight/pwm_bl.c | 70 ++++++++++++++------------------
 1 file changed, 31 insertions(+), 39 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index dfc760830eb9..3e77f6b73fd9 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -230,8 +230,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
 				  struct platform_pwm_backlight_data *data)
 {
 	struct device_node *node = dev->of_node;
-	unsigned int num_levels = 0;
-	unsigned int levels_count;
+	unsigned int num_levels;
 	unsigned int num_steps = 0;
 	struct property *prop;
 	unsigned int *table;
@@ -260,12 +259,11 @@ static int pwm_backlight_parse_dt(struct device *dev,
 	if (!prop)
 		return 0;
 
-	data->max_brightness = length / sizeof(u32);
+	num_levels = length / sizeof(u32);
 
 	/* read brightness levels from DT property */
-	if (data->max_brightness > 0) {
-		size_t size = sizeof(*data->levels) * data->max_brightness;
-		unsigned int i, j, n = 0;
+	if (num_levels > 0) {
+		size_t size = sizeof(*data->levels) * num_levels;
 
 		data->levels = devm_kzalloc(dev, size, GFP_KERNEL);
 		if (!data->levels)
@@ -273,7 +271,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
 
 		ret = of_property_read_u32_array(node, "brightness-levels",
 						 data->levels,
-						 data->max_brightness);
+						 num_levels);
 		if (ret < 0)
 			return ret;
 
@@ -298,7 +296,13 @@ static int pwm_backlight_parse_dt(struct device *dev,
 		 * between two points.
 		 */
 		if (num_steps) {
-			if (data->max_brightness < 2) {
+			unsigned int num_input_levels = num_levels;
+			unsigned int i;
+			u32 x1, x2, x, dx;
+			u32 y1, y2;
+			s64 dy;
+
+			if (num_input_levels < 2) {
 				dev_err(dev, "can't interpolate\n");
 				return -EINVAL;
 			}
@@ -308,14 +312,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
 			 * taking in consideration the number of interpolated
 			 * steps between two levels.
 			 */
-			for (i = 0; i < data->max_brightness - 1; i++) {
-				if ((data->levels[i + 1] - data->levels[i]) /
-				   num_steps)
-					num_levels += num_steps;
-				else
-					num_levels++;
-			}
-			num_levels++;
+			num_levels = (num_input_levels - 1) * num_steps + 1;
 			dev_dbg(dev, "new number of brightness levels: %d\n",
 				num_levels);
 
@@ -327,24 +324,25 @@ static int pwm_backlight_parse_dt(struct device *dev,
 			table = devm_kzalloc(dev, size, GFP_KERNEL);
 			if (!table)
 				return -ENOMEM;
-
-			/* Fill the interpolated table. */
-			levels_count = 0;
-			for (i = 0; i < data->max_brightness - 1; i++) {
-				value = data->levels[i];
-				n = (data->levels[i + 1] - value) / num_steps;
-				if (n > 0) {
-					for (j = 0; j < num_steps; j++) {
-						table[levels_count] = value;
-						value += n;
-						levels_count++;
-					}
-				} else {
-					table[levels_count] = data->levels[i];
-					levels_count++;
+			/*
+			 * Fill the interpolated table[x] = y
+			 * by draw lines between each (x1, y1) to (x2, y2).
+			 */
+			dx = num_steps;
+			for (i = 0; i < num_input_levels - 1; i++) {
+				x1 = i * dx;
+				x2 = x1 + dx;
+				y1 = data->levels[i];
+				y2 = data->levels[i + 1];
+				dy = (s64)y2 - y1;
+
+				for (x = x1; x < x2; x++) {
+					table[x] = y1 +
+						div_s64(dy * ((s64)x - x1), dx);
 				}
 			}
-			table[levels_count] = data->levels[i];
+			/* Fill in the last point, since no line starts here. */
+			table[x2] = y2;
 
 			/*
 			 * As we use interpolation lets remove current
@@ -353,15 +351,9 @@ static int pwm_backlight_parse_dt(struct device *dev,
 			 */
 			devm_kfree(dev, data->levels);
 			data->levels = table;
-
-			/*
-			 * Reassign max_brightness value to the new total number
-			 * of brightness levels.
-			 */
-			data->max_brightness = num_levels;
 		}
 
-		data->max_brightness--;
+		data->max_brightness = num_levels - 1;
 	}
 
 	return 0;
-- 
2.28.0