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Subject[PATCH v2 1/3] backlight: pwm_bl: Fix interpolation
Date
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@chromium.org>
---

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
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