drm/i915/bdb: Fix version check

With patch "drm/i915/vbt: Fix backlight parsing for VBT 234+"
the size of bdb_lfp_backlight_data structure has been increased,
causing if-statement in the parse_lfp_backlight function
that comapres this structure size to the one retrieved from BDB,
always to fail for older revisions.
This patch calculates expected size of the structure for a given
BDB version and compares it with the value gathered from BDB.
Tested on Chromebook Pixelbook (Nocturne) (reports bdb->version = 221)

Fixes: d381baad ("drm/i915/vbt: Fix backlight parsing for VBT 234+")
Tested-by: Lukasz Majczak <lma@semihalf.com>
Signed-off-by: Lukasz Majczak <lma@semihalf.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930134606.227234-1-lma@semihalf.com

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

@@ -451,13 +451,23 @@ parse_lfp_backlight(struct drm_i915_private *i915,
 	}
 
 	i915->vbt.backlight.type = INTEL_BACKLIGHT_DISPLAY_DDI;
-	if (bdb->version >= 191 &&
-	    get_blocksize(backlight_data) >= sizeof(*backlight_data)) {
-		const struct lfp_backlight_control_method *method;
+	if (bdb->version >= 191) {
+		size_t exp_size;
 
-		method = &backlight_data->backlight_control[panel_type];
-		i915->vbt.backlight.type = method->type;
-		i915->vbt.backlight.controller = method->controller;
+		if (bdb->version >= 236)
+			exp_size = sizeof(struct bdb_lfp_backlight_data);
+		else if (bdb->version >= 234)
+			exp_size = EXP_BDB_LFP_BL_DATA_SIZE_REV_234;
+		else
+			exp_size = EXP_BDB_LFP_BL_DATA_SIZE_REV_191;
+
+		if (get_blocksize(backlight_data) >= exp_size) {
+			const struct lfp_backlight_control_method *method;
+
+			method = &backlight_data->backlight_control[panel_type];
+			i915->vbt.backlight.type = method->type;
+			i915->vbt.backlight.controller = method->controller;
+		}
 	}
 
 	i915->vbt.backlight.pwm_freq_hz = entry->pwm_freq_hz;

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

@@ -814,6 +814,11 @@ struct lfp_brightness_level {
 	u16 reserved;
 } __packed;
 
+#define EXP_BDB_LFP_BL_DATA_SIZE_REV_191 \
+	offsetof(struct bdb_lfp_backlight_data, brightness_level)
+#define EXP_BDB_LFP_BL_DATA_SIZE_REV_234 \
+	offsetof(struct bdb_lfp_backlight_data, brightness_precision_bits)
+
 struct bdb_lfp_backlight_data {
 	u8 entry_size;
 	struct lfp_backlight_data_entry data[16];