vbnv: Do not silently reset cache in read_vbnv

Currently, read_vbnv performs a reset of the vbnv cache if it is not
valid. However, this information is not passed up to the vboot layer,
thus resulting in missed write-back of vbnv cache to storage if vboot
does not update the cache itself.

Update read_vbnv to return a value depending upon whether it wants a
write-back to be performed when save is called.
Return value:
0 = No write-back required
1 = Write-back of VBNV cache is required.

Change-Id: I239939d5f9731d89a9d53fe662321b93fc1ab113
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/15457
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/vendorcode/google/chromeos/vbnv.c

@@ -85,8 +85,12 @@ int verify_vbnv(uint8_t *vbnv_copy)
 		(crc8_vbnv(vbnv_copy, CRC_OFFSET) == vbnv_copy[CRC_OFFSET]);
 }
 
-/* Read VBNV data from configured storage backend. */
-void read_vbnv(uint8_t *vbnv_copy)
+/*
+ * Read VBNV data from configured storage backend.
+ * If VBNV verification fails, reset the vbnv copy.
+ * Returns 1 if write-back of vbnv copy is required. Else, returns 0.
+ */
+int read_vbnv(uint8_t *vbnv_copy)
 {
 	if (IS_ENABLED(CONFIG_CHROMEOS_VBNV_CMOS))
 		read_vbnv_cmos(vbnv_copy);
@@ -96,8 +100,11 @@ void read_vbnv(uint8_t *vbnv_copy)
 		read_vbnv_flash(vbnv_copy);
 
 	/* Check data for consistency */
-	if (!verify_vbnv(vbnv_copy))
-		reset_vbnv(vbnv_copy);
+	if (verify_vbnv(vbnv_copy))
+		return 0;
+
+	reset_vbnv(vbnv_copy);
+	return 1;
 }
 
 /*

src/vendorcode/google/chromeos/vbnv.h

@@ -19,7 +19,12 @@
 #include <types.h>
 
 /* Generic functions */
-void read_vbnv(uint8_t *vbnv_copy);
+/*
+ * Return value for read_vbnv:
+ * 1 = write-back of vbnv copy is required.
+ * 0 = otherwise
+ */
+int read_vbnv(uint8_t *vbnv_copy);
 void save_vbnv(const uint8_t *vbnv_copy);
 int verify_vbnv(uint8_t *vbnv_copy);
 int get_recovery_mode_from_vbnv(void);

src/vendorcode/google/chromeos/vboot2/vboot_logic.c

@@ -301,8 +301,12 @@ void verstage_main(void)
 	/* Set up context and work buffer */
 	vb2_init_work_context(&ctx);
 
-	/* Read nvdata from a non-volatile storage */
-	read_vbnv(ctx.nvdata);
+	/*
+	 * Read nvdata from a non-volatile storage and mark data as changed
+	 * if instructed.
+	 */
+	if (read_vbnv(ctx.nvdata))
+		ctx.flags |= VB2_CONTEXT_NVDATA_CHANGED;
 
 	/* Set S3 resume flag if vboot should behave differently when selecting
 	 * which slot to boot.  This is only relevant to vboot if the platform