drm/dp: Modify drm_edp_probe_state

Modify drm_edp_probe_state to read current level from
DP_EDP_PANEL_TARGET_LUMINANCE_VALUE. We divide it by
1000 since the value in this register is in millinits.

--v2
-Add comment on the unit sent back [Arun]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://lore.kernel.org/r/20250620063445.3603086-7-suraj.kandpal@intel.com

Author: surajk8

drivers/gpu/drm/display/drm_dp_helper.c

@@ -4193,7 +4193,7 @@ drm_edp_backlight_probe_state(struct drm_dp_aux *aux, struct drm_edp_backlight_i
 			      u8 *current_mode)
 {
 	int ret;
-	u8 buf[2];
+	u8 buf[3];
 	u8 mode_reg;
 
 	ret = drm_dp_dpcd_read_byte(aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER, &mode_reg);
@@ -4210,17 +4210,37 @@ drm_edp_backlight_probe_state(struct drm_dp_aux *aux, struct drm_edp_backlight_i
 	if (*current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
 		int size = 1 + bl->lsb_reg_used;
 
-		ret = drm_dp_dpcd_read_data(aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB, buf, size);
-		if (ret < 0) {
-			drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight level: %d\n",
-				    aux->name, ret);
-			return ret;
+		if (bl->luminance_set) {
+			ret = drm_dp_dpcd_read_data(aux, DP_EDP_PANEL_TARGET_LUMINANCE_VALUE,
+						    buf, sizeof(buf));
+			if (ret < 0) {
+				drm_dbg_kms(aux->drm_dev,
+					    "%s: Failed to read backlight level: %d\n",
+					    aux->name, ret);
+				return ret;
 		}
 
-		if (bl->lsb_reg_used)
-			return (buf[0] << 8) | buf[1];
-		else
-			return buf[0];
+		/*
+		 * Incase luminance is set we want to send the value back in nits but since
+		 * DP_EDP_PANEL_TARGET_LUMINANCE stores values in millinits we need to divide
+		 * by 1000.
+		 */
+		return (buf[0] | buf[1] << 8 | buf[2] << 16) / 1000;
+		} else {
+			ret = drm_dp_dpcd_read_data(aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
+						    buf, size);
+			if (ret < 0) {
+				drm_dbg_kms(aux->drm_dev,
+					    "%s: Failed to read backlight level: %d\n",
+					    aux->name, ret);
+				return ret;
+			}
+
+			if (bl->lsb_reg_used)
+				return (buf[0] << 8) | buf[1];
+			else
+				return buf[0];
+		}
 	}
 
 	/*