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authorEnric Balletbo i Serra <enric.balletbo@collabora.com>2018-04-09 10:33:30 +0200
committerLee Jones <lee.jones@linaro.org>2018-07-04 10:30:46 +0100
commit573fe6d1c25c81b0af856ffafb516db7e8d978c5 (patch)
tree3030df9310ebb37825159ab167a03aa97edeb475 /drivers/video/backlight
parentce397d215ccd07b8ae3f71db689aedb85d56ab40 (diff)
downloadlinux-573fe6d1c25c81b0af856ffafb516db7e8d978c5.tar.bz2
backlight: pwm_bl: Linear interpolation between brightness-levels
Setting num-interpolated-steps in the dts will allow you to have linear interpolation between values of brightness-levels. This way a high resolution pwm duty cycle can be used without having to list out every possible value in the dts. This system also allows for gamma corrected values. The most simple example is interpolate between two brightness values a number of steps, this can be done setting the following in the dts: brightness-levels = <0 65535>; num-interpolated-steps = <1024>; default-brightness-level = <512>; This will create a brightness-level table with the following values: <0 63 126 189 252 315 378 441 ... 64260 64323 64386 64449 65535> Another use case can be describe a gamma corrected curve, as we have better sensitivity at low luminance than high luminance we probably want have smaller steps for low brightness levels values and bigger steps for high brightness levels values. This can be achieved with the following in the dts: brightness-levels = <0 4096 65535>; num-interpolated-steps = <1024>; default-brightness-level = <512>; This will create a brightness-levels table with the following values: <0 4 8 12 16 20 ... 4096 4156 4216 4276 ... 65535> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com> Acked-by: Daniel Thompson <daniel.thompson@linaro.org> Signed-off-by: Lee Jones <lee.jones@linaro.org>
Diffstat (limited to 'drivers/video/backlight')
-rw-r--r--drivers/video/backlight/pwm_bl.c83
1 files changed, 83 insertions, 0 deletions
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 44ac5bde4e9d..105f199a656d 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -147,7 +147,11 @@ 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_steps;
struct property *prop;
+ unsigned int *table;
int length;
u32 value;
int ret;
@@ -167,6 +171,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
/* 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;
data->levels = devm_kzalloc(dev, size, GFP_KERNEL);
if (!data->levels)
@@ -184,6 +189,84 @@ static int pwm_backlight_parse_dt(struct device *dev,
return ret;
data->dft_brightness = value;
+
+ /*
+ * This property is optional, if is set enables linear
+ * interpolation between each of the values of brightness levels
+ * and creates a new pre-computed table.
+ */
+ of_property_read_u32(node, "num-interpolated-steps",
+ &num_steps);
+
+ /*
+ * Make sure that there is at least two entries in the
+ * brightness-levels table, otherwise we can't interpolate
+ * between two points.
+ */
+ if (num_steps) {
+ if (data->max_brightness < 2) {
+ dev_err(dev, "can't interpolate\n");
+ return -EINVAL;
+ }
+
+ /*
+ * Recalculate the number of brightness levels, now
+ * 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++;
+ dev_dbg(dev, "new number of brightness levels: %d\n",
+ num_levels);
+
+ /*
+ * Create a new table of brightness levels with all the
+ * interpolated steps.
+ */
+ size = sizeof(*table) * num_levels;
+ 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++;
+ }
+ }
+ table[levels_count] = data->levels[i];
+
+ /*
+ * As we use interpolation lets remove current
+ * brightness levels table and replace for the
+ * new interpolated table.
+ */
+ 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--;
}