[RFC v2 1/2] backlight: pwm_bl: linear interpolation between values of brightness-levels

Enric Balletbo i Serra
2017-11-16T14:12:28Z

Setting use-linear-interpolation in the dts will allow you to have linear
interpolation between values of brightness-levels.

There are now 256 between each of the values of brightness-levels. If
something is requested halfway between 2 values, we'll use linear
interpolation.

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 (eg: "brightness-levels = <0 2 4 8 16 32>;").

Patch based on the Alexandru M Stan work done for ChromeOS kernels.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
---
.../bindings/leds/backlight/pwm-backlight.txt | 2 +
drivers/video/backlight/pwm_bl.c | 55 +++++++++++++++++-----
include/linux/pwm_backlight.h | 2 +
3 files changed, 47 insertions(+), 12 deletions(-)

diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
index 764db86..7c48f20 100644
--- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
+++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
@@ -17,6 +17,8 @@ Optional properties:
"pwms" property (see PWM binding[0])
- enable-gpios: contains a single GPIO specifier for the GPIO which enables
and disables the backlight (see GPIO binding[1])
+ - use-linear-interpolation: set this propriety to enable linear interpolation
+ between each of the values of brightness-levels.

[0]: Documentation/devicetree/bindings/pwm/pwm.txt
[1]: Documentation/devicetree/bindings/gpio/gpio.txt
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 9bd1768..59b1bfb 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -24,6 +24,8 @@
#include <linux/regulator/consumer.h>
#include <linux/slab.h>

+#define NSTEPS 256
+
struct pwm_bl_data {
struct pwm_device *pwm;
struct device *dev;
@@ -35,6 +37,7 @@ struct pwm_bl_data {
struct gpio_desc *enable_gpio;
unsigned int scale;
bool legacy;
+ bool piecewise;
int (*notify)(struct device *,
int brightness);
void (*notify_after)(struct device *,
@@ -76,17 +79,36 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)
pb->enabled = false;
}

-static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness)
+static int scale(struct pwm_bl_data *pb, int x)
{
unsigned int lth = pb->lth_brightness;
+
+ return (x * (pb->period - lth) / pb->scale) + lth;
+}
+
+static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness)
+{
+ int coarse = brightness / NSTEPS;
+ int fine = brightness % NSTEPS;
int duty_cycle;

- if (pb->levels)
- duty_cycle = pb->levels[brightness];
- else
- duty_cycle = brightness;
+ if (pb->levels) {
+ if (pb->piecewise) {
+ duty_cycle = scale(pb, pb->levels[coarse]);
+ if (fine > 0)
+ duty_cycle += (scale(pb, pb->levels[coarse + 1])
+ - scale(pb, pb->levels[coarse]))
+ * fine / NSTEPS;
+ dev_dbg(pb->dev, "brightness=%d coarse=%d fine=%d\n",
+ brightness, coarse, fine);
+ } else {
+ duty_cycle = scale(pb, pb->levels[brightness]);
+ }
+ } else {
+ duty_cycle = scale(pb, brightness);
+ }

- return (duty_cycle * (pb->period - lth) / pb->scale) + lth;
+ return duty_cycle;
}

static int pwm_backlight_update_status(struct backlight_device *bl)
@@ -149,11 +171,11 @@ static int pwm_backlight_parse_dt(struct device *dev,
if (!prop)
return -EINVAL;

- data->max_brightness = length / sizeof(u32);
+ data->levels_count = length / sizeof(u32);

/* read brightness levels from DT property */
- if (data->max_brightness > 0) {
- size_t size = sizeof(*data->levels) * data->max_brightness;
+ if (data->levels_count > 0) {
+ size_t size = sizeof(*data->levels) * data->levels_count;

data->levels = devm_kzalloc(dev, size, GFP_KERNEL);
if (!data->levels)
@@ -161,7 +183,7 @@ static int pwm_backlight_parse_dt(struct device *dev,

ret = of_property_read_u32_array(node, "brightness-levels",
data->levels,
- data->max_brightness);
+ data->levels_count);
if (ret < 0)
return ret;

@@ -170,10 +192,18 @@ static int pwm_backlight_parse_dt(struct device *dev,
if (ret < 0)
return ret;

+ data->piecewise = of_property_read_bool(node,
+ "use-linear-interpolation");
+
data->dft_brightness = value;
- data->max_brightness--;
+ data->levels_count--;
}

+ if (data->piecewise)
+ data->max_brightness = data->levels_count * NSTEPS;
+ else
+ data->max_brightness = data->levels_count;
+
data->enable_gpio = -EINVAL;
return 0;
}
@@ -258,7 +288,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
if (data->levels) {
unsigned int i;

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

@@ -272,6 +302,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
pb->exit = data->exit;
pb->dev = &pdev->dev;
pb->enabled = false;
+ pb->piecewise = data->piecewise;

pb->enable_gpio = devm_gpiod_get_optional(&pdev->dev, "enable",
GPIOD_ASIS);
diff --git a/include/linux/pwm_backlight.h b/include/linux/pwm_backlight.h
index e8afbd7..444a91b 100644
--- a/include/linux/pwm_backlight.h
+++ b/include/linux/pwm_backlight.h
@@ -14,6 +14,8 @@ struct platform_pwm_backlight_data {
unsigned int lth_brightness;
unsigned int pwm_period_ns;
unsigned int *levels;
+ unsigned int levels_count;
+ bool piecewise;
/* TODO remove once all users are switched to gpiod_* API */
int enable_gpio;
int (*init)(struct device *dev);
--
2.9.3