[PATCH v1] drm/i915/bdb: Fix version check

[Lukasz Majczak <lma@semihalf.com>]

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 fixes it by comparing a total size of all fileds from
the structure (present before the change) with the value gathered from BDB.
Tested on Chromebook Pixelbook (Nocturne) (reports bdb->version = 221)

Cc: <stable@vger.kernel.org> # 5.4+
Tested-by: Lukasz Majczak <lma@semihalf.com>
Signed-off-by: Lukasz Majczak <lma@semihalf.com>
---
 drivers/gpu/drm/i915/display/intel_bios.c     | 4 +++-
 drivers/gpu/drm/i915/display/intel_vbt_defs.h | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_bios.c b/drivers/gpu/drm/i915/display/intel_bios.c
index 3c25926092de..052a19b455d1 100644
--- a/drivers/gpu/drm/i915/display/intel_bios.c
+++ b/drivers/gpu/drm/i915/display/intel_bios.c
@@ -452,7 +452,9 @@ 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)) {
+	    get_blocksize(backlight_data) >= (sizeof(backlight_data->entry_size) +
+					      sizeof(backlight_data->data) +
+					      sizeof(backlight_data->level))) {
 		const struct lfp_backlight_control_method *method;
 
 		method = &backlight_data->backlight_control[panel_type];
diff --git a/drivers/gpu/drm/i915/display/intel_vbt_defs.h b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
index 330077c2e588..fff456bf8783 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;
 
+/*
+ * Changing struct bdb_lfp_backlight_data might affect its
+ * size comparation to the value hold in BDB.
+ * (e.g. in parse_lfp_backlight())
+ */
 struct bdb_lfp_backlight_data {
 	u8 entry_size;
 	struct lfp_backlight_data_entry data[16];
-- 
2.33.0.464.g1972c5931b-goog