drm/i915/vbt: Fix backlight parsing for VBT 234+

Child min_brightness is obsolete from VBT 234+, instead the new
min_brightness field in the main structure should be used.

This new field is 16 bits wide, so backlight_precision_bits is needed
to check if value needs to be scaled down but it is only available in
VBT 236+ so working around it by using the also new backlight_level
in the main struct.

v2:
- missed that backlight_data->level is also obsolete

v3:
- s/backlight/brightness to better match specification
- using u16 to specify brightness level instead of a u32 : 16

BSpec: 20149
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201008211932.24989-1-jose.souza@intel.com

drivers/gpu/drm/i915/display/intel_bios.c

@@ -425,6 +425,7 @@ parse_lfp_backlight(struct drm_i915_private *dev_priv,
 	const struct bdb_lfp_backlight_data *backlight_data;
 	const struct lfp_backlight_data_entry *entry;
 	int panel_type = dev_priv->vbt.panel_type;
+	u16 level;
 
 	backlight_data = find_section(bdb, BDB_LVDS_BACKLIGHT);
 	if (!backlight_data)
@@ -459,14 +460,39 @@ parse_lfp_backlight(struct drm_i915_private *dev_priv,
 
 	dev_priv->vbt.backlight.pwm_freq_hz = entry->pwm_freq_hz;
 	dev_priv->vbt.backlight.active_low_pwm = entry->active_low_pwm;
-	dev_priv->vbt.backlight.min_brightness = entry->min_brightness;
+
+	if (bdb->version >= 234) {
+		u16 min_level;
+		bool scale;
+
+		level = backlight_data->brightness_level[panel_type].level;
+		min_level = backlight_data->brightness_min_level[panel_type].level;
+
+		if (bdb->version >= 236)
+			scale = backlight_data->brightness_precision_bits[panel_type] == 16;
+		else
+			scale = level > 255;
+
+		if (scale)
+			min_level = min_level / 255;
+
+		if (min_level > 255) {
+			drm_warn(&dev_priv->drm, "Brightness min level > 255\n");
+			level = 255;
+		}
+		dev_priv->vbt.backlight.min_brightness = min_level;
+	} else {
+		level = backlight_data->level[panel_type];
+		dev_priv->vbt.backlight.min_brightness = entry->min_brightness;
+	}
+
 	drm_dbg_kms(&dev_priv->drm,
 		    "VBT backlight PWM modulation frequency %u Hz, "
 		    "active %s, min brightness %u, level %u, controller %u\n",
 		    dev_priv->vbt.backlight.pwm_freq_hz,
 		    dev_priv->vbt.backlight.active_low_pwm ? "low" : "high",
 		    dev_priv->vbt.backlight.min_brightness,
-		    backlight_data->level[panel_type],
+		    level,
 		    dev_priv->vbt.backlight.controller);
 }
 

drivers/gpu/drm/i915/display/intel_vbt_defs.h

@@ -782,7 +782,7 @@ struct lfp_backlight_data_entry {
 	u8 active_low_pwm:1;
 	u8 obsolete1:5;
 	u16 pwm_freq_hz;
-	u8 min_brightness;
+	u8 min_brightness; /* Obsolete from 234+ */
 	u8 obsolete2;
 	u8 obsolete3;
 } __packed;
@@ -792,11 +792,19 @@ struct lfp_backlight_control_method {
 	u8 controller:4;
 } __packed;
 
+struct lfp_brightness_level {
+	u16 level;
+	u16 reserved;
+} __packed;
+
 struct bdb_lfp_backlight_data {
 	u8 entry_size;
 	struct lfp_backlight_data_entry data[16];
-	u8 level[16];
+	u8 level[16]; /* Obsolete from 234+ */
 	struct lfp_backlight_control_method backlight_control[16];
+	struct lfp_brightness_level brightness_level[16];		/* 234+ */
+	struct lfp_brightness_level brightness_min_level[16];	/* 234+ */
+	u8 brightness_precision_bits[16];						/* 236+ */
 } __packed;
 
 /*