drm/amd/display: Optimize custom brightness curve

JIRA: https://issues.redhat.com/browse/RHEL-75958

commit d5df87e3fccecde67866f587e187e67b503a0be9
Author: Mario Limonciello <mario.limonciello@amd.com>
Date:   Mon Mar 24 12:57:25 2025 -0500

    drm/amd/display: Optimize custom brightness curve

    commit 03b979e upstream.

    [Why]
    When BIOS includes a lot of custom brightness data points, walking
    the entire list can be time consuming.  This is most noticed when
    dragging a power slider.  The "higher" values are "slower" to drag
    around.

    [How]
    Move custom brightness calculation loop into a static function. Before
    starting the loop check the "half way" data point to see how it compares
    to the input.  If greater than the half way data point use that as the
    starting point instead.

    Reviewed-by: Alex Hung <alex.hung@amd.com>
    Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
    Signed-off-by: Roman Li <roman.li@amd.com>
    Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: José Expósito <jexposit@redhat.com>

Author: jexposit

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

@@ -4655,41 +4655,54 @@ static int get_brightness_range(const struct amdgpu_dm_backlight_caps *caps,
 	return 1;
 }
 
-static u32 convert_brightness_from_user(const struct amdgpu_dm_backlight_caps *caps,
-					uint32_t brightness)
+static void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *caps,
+				      uint32_t *brightness)
 {
-	unsigned int min, max;
 	u8 prev_signal = 0, prev_lum = 0;
+	int i = 0;
 
-	if (!get_brightness_range(caps, &min, &max))
-		return brightness;
-
-	for (int i = 0; i < caps->data_points; i++) {
-		u8 signal, lum;
+	if (amdgpu_dc_debug_mask & DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE)
+		return;
 
-		if (amdgpu_dc_debug_mask & DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE)
-			break;
+	if (!caps->data_points)
+		return;
 
-		signal = caps->luminance_data[i].input_signal;
-		lum = caps->luminance_data[i].luminance;
+	/* 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;
 
 		/*
 		 * brightness == signal: luminance is percent numerator
 		 * brightness < signal: interpolate between previous and current luminance numerator
 		 * brightness > signal: find next data point
 		 */
-		if (brightness < signal)
-			lum = prev_lum + DIV_ROUND_CLOSEST((lum - prev_lum) *
-							   (brightness - prev_signal),
-							   signal - prev_signal);
-		else if (brightness > signal) {
+		if (*brightness > signal) {
 			prev_signal = signal;
 			prev_lum = lum;
+			i++;
 			continue;
 		}
-		brightness = DIV_ROUND_CLOSEST(lum * brightness, 101);
-		break;
-	}
+		if (*brightness < signal)
+			lum = prev_lum + DIV_ROUND_CLOSEST((lum - prev_lum) *
+							   (*brightness - prev_signal),
+							   signal - prev_signal);
+		*brightness = DIV_ROUND_CLOSEST(lum * *brightness, 101);
+		return;
+	} while (i < caps->data_points);
+}
+
+static u32 convert_brightness_from_user(const struct amdgpu_dm_backlight_caps *caps,
+					uint32_t brightness)
+{
+	unsigned int min, max;
+
+	if (!get_brightness_range(caps, &min, &max))
+		return brightness;
+
+	convert_custom_brightness(caps, &brightness);
 
 	// Rescale 0..255 to min..max
 	return min + DIV_ROUND_CLOSEST((max - min) * brightness,