diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 105f199a656dddfe0663582f96f5fd02d5a3ec57..9ee4c1b735b28727b7e27f2732b6328eb1e81910 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -143,6 +143,107 @@ static const struct backlight_ops pwm_backlight_ops = {
};
#ifdef CONFIG_OF
+#define PWM_LUMINANCE_SCALE 10000 /* luminance scale */
+
+/* An integer based power function */
+static u64 int_pow(u64 base, int exp)
+{
+ u64 result = 1;
+
+ while (exp) {
+ if (exp & 1)
+ result *= base;
+ exp >>= 1;
+ base *= base;
+ }
+
+ return result;
+}
+
+/*
+ * CIE lightness to PWM conversion.
+ *
+ * The CIE 1931 lightness formula is what actually describes how we perceive
+ * light:
+ * Y = (L* / 902.3) if L* ≤ 0.08856
+ * Y = ((L* + 16) / 116)^3 if L* > 0.08856
+ *
+ * Where Y is the luminance, the amount of light coming out of the screen, and
+ * is a number between 0.0 and 1.0; and L* is the lightness, how bright a human
+ * perceives the screen to be, and is a number between 0 and 100.
+ *
+ * The following function does the fixed point maths needed to implement the
+ * above formula.
+ */
+static u64 cie1931(unsigned int lightness, unsigned int scale)
+{
+ u64 retval;
+
+ lightness *= 100;
+ if (lightness <= (8 * scale)) {
+ retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9023);
+ } else {
+ retval = int_pow((lightness + (16 * scale)) / 116, 3);
+ retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
+ }
+
+ return retval;
+}
+
+/*
+ * Create a default correction table for PWM values to create linear brightness
+ * for LED based backlights using the CIE1931 algorithm.
+ */
+static
+int pwm_backlight_brightness_default(struct device *dev,
+ struct platform_pwm_backlight_data *data,
+ unsigned int period)
+{
+ unsigned int counter = 0;
+ unsigned int i, n;
+ u64 retval;
+
+ /*
+ * Count the number of bits needed to represent the period number. The
+ * number of bits is used to calculate the number of levels used for the
+ * brightness-levels table, the purpose of this calculation is have a
+ * pre-computed table with enough levels to get linear brightness
+ * perception. The period is divided by the number of bits so for a
+ * 8-bit PWM we have 255 / 8 = 32 brightness levels or for a 16-bit PWM
+ * we have 65535 / 16 = 4096 brightness levels.
+ *
+ * Note that this method is based on empirical testing on different
+ * devices with PWM of 8 and 16 bits of resolution.
+ */
+ n = period;
+ while (n) {
+ counter += n % 2;
+ n >>= 1;
+ }
+
+ data->max_brightness = DIV_ROUND_UP(period, counter);
+ data->levels = devm_kcalloc(dev, data->max_brightness,
+ sizeof(*data->levels), GFP_KERNEL);
+ if (!data->levels)
+ return -ENOMEM;
+
+ /* Fill the table using the cie1931 algorithm */
+ for (i = 0; i < data->max_brightness; i++) {
+ retval = cie1931((i * PWM_LUMINANCE_SCALE) /
+ data->max_brightness, PWM_LUMINANCE_SCALE) *
+ period;
+ retval = DIV_ROUND_CLOSEST_ULL(retval, PWM_LUMINANCE_SCALE);
+ if (retval > UINT_MAX)
+ return -EINVAL;
+ data->levels[i] = (unsigned int)retval;
+ }
+
+ data->dft_brightness = data->max_brightness / 2;
+ data->max_brightness--;
+
+ return 0;
+}
+
static int pwm_backlight_parse_dt(struct device *dev,
struct platform_pwm_backlight_data *data)
{
@@ -161,10 +262,13 @@ static int pwm_backlight_parse_dt(struct device *dev,
memset(data, 0, sizeof(*data));
- /* determine the number of brightness levels */
+ /*
+ * Determine the number of brightness levels, if this property is not
+ * set a default table of brightness levels will be used.
+ */
prop = of_find_property(node, "brightness-levels", &length);
if (!prop)
- return -EINVAL;
+ return 0;
data->max_brightness = length / sizeof(u32);
@@ -294,6 +398,14 @@ static int pwm_backlight_parse_dt(struct device *dev,
{
return -ENODEV;
}
+
+static
+int pwm_backlight_brightness_default(struct device *dev,
+ struct platform_pwm_backlight_data *data,
+ unsigned int period)
+{
+ return -ENODEV;
+}
#endif
static int pwm_backlight_initial_power_state(const struct pwm_bl_data *pb)
@@ -334,7 +446,9 @@ static int pwm_backlight_probe(struct platform_device *pdev)
struct backlight_device *bl;
struct device_node *node = pdev->dev.of_node;
struct pwm_bl_data *pb;
+ struct pwm_state state;
struct pwm_args pargs;
+ unsigned int i;
int ret;
if (!data) {
@@ -359,17 +473,6 @@ static int pwm_backlight_probe(struct platform_device *pdev)
goto err_alloc;
}
- if (data->levels) {
- 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
- pb->scale = data->max_brightness;
-
pb->notify = data->notify;
pb->notify_after = data->notify_after;
pb->check_fb = data->check_fb;
@@ -436,6 +539,26 @@ static int pwm_backlight_probe(struct platform_device *pdev)
dev_dbg(&pdev->dev, "got pwm for backlight\n");
+ if (!data->levels) {
+ /* Get the PWM period (in nanoseconds) */
+ pwm_get_state(pb->pwm, &state);
+
+ ret = pwm_backlight_brightness_default(&pdev->dev, data,
+ state.period);
+ if (ret < 0) {
+ dev_err(&pdev->dev,
+ "failed to setup default brightness table\n");
+ goto err_alloc;
+ }
+ }
+
+ for (i = 0; i <= data->max_brightness; i++) {
+ if (data->levels[i] > pb->scale)
+ pb->scale = data->levels[i];
+
+ pb->levels = data->levels;
+ }
+
/*
* FIXME: pwm_apply_args() should be removed when switching to
* the atomic PWM API.