Merge tag 'backlight-next-4.19' of git://git.kernel.org/pub/scm/linux/kernel/git/lee/backlight

Pull backlight updates from Lee Jones:
 "Core Framework:
   - Remove unused/obsolete code/comments

  New Functionality:
   - Allow less granular brightness specification for high-res PWMs; pwm_bl
   - Align brightness {inc,dec}rements with that perceived by the human-eye; pwm_bl

  Fix-ups:
   - Prepare for the introduction of -Wimplicit-fall-through; adp8860_bl

  Bug Fixes:
   - Fix uninitialised variable; pwm_bl"

* tag 'backlight-next-4.19' of git://git.kernel.org/pub/scm/linux/kernel/git/lee/backlight:
  backlight: pwm_bl: Fix uninitialized variable
  backlight: adp8860: Mark expected switch fall-through
  backlight: Remove obsolete comment for ->state
  dt-bindings: pwm-backlight: Move brightness-levels to optional
  backlight: pwm_bl: Compute brightness of LED linearly to human eye
  dt-bindings: pwm-backlight: Add a num-interpolation-steps property
  backlight: pwm_bl: Linear interpolation between brightness-levels

2 files changed, 220 insertions, 13 deletions

diff --git a/drivers/video/backlight/adp8860_bl.c b/drivers/video/backlight/adp8860_bl.c
index 16119bde9750..f1dc41cf19e3 100644
--- a/drivers/video/backlight/adp8860_bl.c
+++ b/drivers/video/backlight/adp8860_bl.c
@@ -690,6 +690,7 @@ static int adp8860_probe(struct i2c_client *client,
 	switch (ADP8860_MANID(reg_val)) {
 	case ADP8863_MANUFID:
 		data->gdwn_dis = !!pdata->gdwn_dis;
+		/* fall through */
 	case ADP8860_MANUFID:
 		data->en_ambl_sens = !!pdata->en_ambl_sens;
 		break;
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 44ac5bde4e9d..bdfcc0a71db1 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -143,11 +143,116 @@ 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)
 {
 	struct device_node *node = dev->of_node;
+	unsigned int num_levels = 0;
+	unsigned int levels_count;
+	unsigned int num_steps = 0;
 	struct property *prop;
+	unsigned int *table;
 	int length;
 	u32 value;
 	int ret;
@@ -157,16 +262,20 @@ 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);
 
 	/* 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 +293,84 @@ 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--;
 	}
 
@@ -211,6 +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)
@@ -251,7 +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) {
@@ -276,17 +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;
@@ -353,6 +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.