[PATCH 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye.

[Enric Balletbo i Serra]

When you want to change the brightness using a PWM signal, one thing you
need to consider is how human perceive the brightness. Human perceive
the brightness change non-linearly, we have better sensitivity at low
luminance than high luminance, so to achieve perceived linear dimming,
the brightness must be matches to the way our eyes behave. The CIE 1931
lightness formula is what actually describes how we perceive light.

This patch computes a default table with the brightness levels filled
with the numbers provided by the CIE 1931 algorithm, the number of the
brightness levels is calculated based on the PWM resolution.

The calculation of the table using the CIE 1931 algorithm is enabled by
default when you do not define the 'brightness-levels' propriety in your
device tree.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
---
Changes since RFCv2:
- Pre-compute the table at boot using the cie 1931 algorithm, this
  introduced again the fixed point calculations that needs to be
  reviewed.
- Calculate the number of needed steps based on the number of bits of
  the PWM. 
- Improve some code documentation.

Changes since RFCv1:
- Get rid of fixed point calculations and use a table instead.

 drivers/video/backlight/pwm_bl.c | 149 +++++++++++++++++++++++++++++++++++----
 1 file changed, 136 insertions(+), 13 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index eabe0a4462af..9398516db0ce 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* â 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);
 
@@ -299,6 +403,14 @@ 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)
@@ -339,7 +451,9 @@ 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) {
@@ -364,17 +478,6 @@ 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;
@@ -441,6 +544,26 @@ 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.
-- 
2.15.1