[PATCH v3] drm/i915/bdb: Fix version check

[Lukasz Majczak]

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)

Tested-by: Lukasz Majczak <lma at semihalf.com>
Signed-off-by: Lukasz Majczak <lma at semihalf.com>
---
 drivers/gpu/drm/i915/display/intel_bios.c     | 22 ++++++++++++++-----
 drivers/gpu/drm/i915/display/intel_vbt_defs.h |  5 +++++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_bios.c b/drivers/gpu/drm/i915/display/intel_bios.c
index 3c25926092de..f9776ca85de3 100644
--- a/drivers/gpu/drm/i915/display/intel_bios.c
+++ b/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;
diff --git a/drivers/gpu/drm/i915/display/intel_vbt_defs.h b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
index 330077c2e588..a2108a8f544d 100644
--- a/drivers/gpu/drm/i915/display/intel_vbt_defs.h
+++ b/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];
-- 
2.33.0.685.g46640cef36-goog