drm/amd/display: Optimize custom brightness curve interpolation

[Why]
Custom brightness curve works by walking through all data points one
by one.  When the brightness value is at either extreme this is a lot
of data points to walk.  This is especially noticeable when moving a
brightness slider around how it can lag.

[How]
Bisect the data points to find the closest for interpolation.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: superm1

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4817,41 +4817,53 @@ static void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *cap
 				      uint32_t *user_brightness)
 {
 	u32 brightness = scale_input_to_fw(min, max, *user_brightness);
-	u8 prev_signal = 0, prev_lum = 0;
-	int i = 0;
+	u8 lower_signal, upper_signal, upper_lum, lower_lum, lum;
+	int left, right;
 
 	if (amdgpu_dc_debug_mask & DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE)
 		return;
 
 	if (!caps->data_points)
 		return;
 
-	/* choose start to run less interpolation steps */
-	if (caps->luminance_data[caps->data_points/2].input_signal > brightness)
-		i = caps->data_points/2;
-	do {
-		u8 signal = caps->luminance_data[i].input_signal;
-		u8 lum = caps->luminance_data[i].luminance;
+	left = 0;
+	right = caps->data_points - 1;
+	while (left <= right) {
+		int mid = left + (right - left) / 2;
+		u8 signal = caps->luminance_data[mid].input_signal;
 
-		/*
-		 * brightness == signal: luminance is percent numerator
-		 * brightness < signal: interpolate between previous and current luminance numerator
-		 * brightness > signal: find next data point
-		 */
-		if (brightness > signal) {
-			prev_signal = signal;
-			prev_lum = lum;
-			i++;
-			continue;
+		/* Exact match found */
+		if (signal == brightness) {
+			lum = caps->luminance_data[mid].luminance;
+			goto scale;
 		}
-		if (brightness < signal)
-			lum = prev_lum + DIV_ROUND_CLOSEST((lum - prev_lum) *
-							   (brightness - prev_signal),
-							   signal - prev_signal);
-		*user_brightness = scale_fw_to_input(min, max,
-						     DIV_ROUND_CLOSEST(lum * brightness, 101));
-		return;
-	} while (i < caps->data_points);
+
+		if (signal < brightness)
+			left = mid + 1;
+		else
+			right = mid - 1;
+	}
+
+	/* verify bound */
+	if (left >= caps->data_points)
+		left = caps->data_points - 1;
+
+	/* At this point, left > right */
+	lower_signal = caps->luminance_data[right].input_signal;
+	upper_signal = caps->luminance_data[left].input_signal;
+	lower_lum = caps->luminance_data[right].luminance;
+	upper_lum = caps->luminance_data[left].luminance;
+
+	/* interpolate */
+	if (right == left || !lower_lum)
+		lum = upper_lum;
+	else
+		lum = lower_lum + DIV_ROUND_CLOSEST((upper_lum - lower_lum) *
+						    (brightness - lower_signal),
+						    upper_signal - lower_signal);
+scale:
+	*user_brightness = scale_fw_to_input(min, max,
+					     DIV_ROUND_CLOSEST(lum * brightness, 101));
 }
 
 static u32 convert_brightness_from_user(const struct amdgpu_dm_backlight_caps *caps,