drm/amd/display: Export full brightness range to userspace

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

commit 889906e6eb5fab990c9b6b5fe8f1122b2416fc22
Author: Mario Limonciello <mario.limonciello@amd.com>
Date:   Thu May 29 09:46:32 2025 -0500

    drm/amd/display: Export full brightness range to userspace

    commit 16dc8bc upstream.

    [WHY]
    Userspace currently is offered a range from 0-0xFF but the PWM is
    programmed from 0-0xFFFF.  This can be limiting to some software
    that wants to apply greater granularity.

    [HOW]
    Convert internally to firmware values only when mapping custom
    brightness curves because these are in 0-0xFF range. Advertise full
    PWM range to userspace.

    Cc: Mario Limonciello <mario.limonciello@amd.com>
    Cc: Alex Deucher <alexander.deucher@amd.com>
    Reviewed-by: Roman Li <roman.li@amd.com>
    Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
    Signed-off-by: Alex Hung <alex.hung@amd.com>
    Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    (cherry picked from commit 8dbd72c)
    Cc: stable@vger.kernel.org
    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

@@ -4655,9 +4655,23 @@ static int get_brightness_range(const struct amdgpu_dm_backlight_caps *caps,
 	return 1;
 }
 
+/* Rescale from [min..max] to [0..AMDGPU_MAX_BL_LEVEL] */
+static inline u32 scale_input_to_fw(int min, int max, u64 input)
+{
+	return DIV_ROUND_CLOSEST_ULL(input * AMDGPU_MAX_BL_LEVEL, max - min);
+}
+
+/* Rescale from [0..AMDGPU_MAX_BL_LEVEL] to [min..max] */
+static inline u32 scale_fw_to_input(int min, int max, u64 input)
+{
+	return min + DIV_ROUND_CLOSEST_ULL(input * (max - min), AMDGPU_MAX_BL_LEVEL);
+}
+
 static void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *caps,
-				      uint32_t *brightness)
+				      unsigned int min, unsigned int max,
+				      uint32_t *user_brightness)
 {
+	u32 brightness = scale_input_to_fw(min, max, *user_brightness);
 	u8 prev_signal = 0, prev_lum = 0;
 	int i = 0;
 
@@ -4668,7 +4682,7 @@ static void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *cap
 		return;
 
 	/* choose start to run less interpolation steps */
-	if (caps->luminance_data[caps->data_points/2].input_signal > *brightness)
+	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;
@@ -4679,17 +4693,18 @@ static void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *cap
 		 * brightness < signal: interpolate between previous and current luminance numerator
 		 * brightness > signal: find next data point
 		 */
-		if (*brightness > signal) {
+		if (brightness > signal) {
 			prev_signal = signal;
 			prev_lum = lum;
 			i++;
 			continue;
 		}
-		if (*brightness < signal)
+		if (brightness < signal)
 			lum = prev_lum + DIV_ROUND_CLOSEST((lum - prev_lum) *
-							   (*brightness - prev_signal),
+							   (brightness - prev_signal),
 							   signal - prev_signal);
-		*brightness = DIV_ROUND_CLOSEST(lum * *brightness, 101);
+		*user_brightness = scale_fw_to_input(min, max,
+						     DIV_ROUND_CLOSEST(lum * brightness, 101));
 		return;
 	} while (i < caps->data_points);
 }
@@ -4702,11 +4717,10 @@ static u32 convert_brightness_from_user(const struct amdgpu_dm_backlight_caps *c
 	if (!get_brightness_range(caps, &min, &max))
 		return brightness;
 
-	convert_custom_brightness(caps, &brightness);
+	convert_custom_brightness(caps, min, max, &brightness);
 
-	// Rescale 0..255 to min..max
-	return min + DIV_ROUND_CLOSEST((max - min) * brightness,
-				       AMDGPU_MAX_BL_LEVEL);
+	// Rescale 0..max to min..max
+	return min + DIV_ROUND_CLOSEST_ULL((u64)(max - min) * brightness, max);
 }
 
 static u32 convert_brightness_to_user(const struct amdgpu_dm_backlight_caps *caps,
@@ -4719,8 +4733,8 @@ static u32 convert_brightness_to_user(const struct amdgpu_dm_backlight_caps *cap
 
 	if (brightness < min)
 		return 0;
-	// Rescale min..max to 0..255
-	return DIV_ROUND_CLOSEST(AMDGPU_MAX_BL_LEVEL * (brightness - min),
+	// Rescale min..max to 0..max
+	return DIV_ROUND_CLOSEST_ULL((u64)max * (brightness - min),
 				 max - min);
 }
 
@@ -4870,11 +4884,10 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 		drm_dbg(drm, "Backlight caps: min: %d, max: %d, ac %d, dc %d\n", min, max,
 			caps->ac_level, caps->dc_level);
 	} else
-		props.brightness = AMDGPU_MAX_BL_LEVEL;
+		props.brightness = props.max_brightness = AMDGPU_MAX_BL_LEVEL;
 
 	if (caps->data_points && !(amdgpu_dc_debug_mask & DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE))
 		drm_info(drm, "Using custom brightness curve\n");
-	props.max_brightness = AMDGPU_MAX_BL_LEVEL;
 	props.type = BACKLIGHT_RAW;
 
 	snprintf(bl_name, sizeof(bl_name), "amdgpu_bl%d",