sunxi_nand_spl: Be smarter about where to look for backup u-boot.bin

We know when u-boot is written to its own partition, in this case the
layout always is:

eb 0 spl
eb 1 spl-backup
eb 2 u-boot
eb 3 u-boot-backup

eb: erase-block

So if we cannot load u-boot from its primary offset we know exactly where
to look for it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>

drivers/mtd/nand/sunxi_nand_spl.c

@@ -356,18 +356,32 @@ static int nand_read_buffer(uint32_t offs, unsigned int size, void *dest,
 
 int nand_spl_load_image(uint32_t offs, unsigned int size, void *dest)
 {
+#if CONFIG_SYS_NAND_U_BOOT_OFFS == CONFIG_SPL_PAD_TO
+	/*
+	 * u-boot-dtb.bin appended to SPL, use syndrome (like the BROM does)
+	 * and try different erase block sizes to find the backup.
+	 */
 	const uint32_t boot_offsets[] = {
 		0 * 1024 * 1024 + CONFIG_SYS_NAND_U_BOOT_OFFS,
 		1 * 1024 * 1024 + CONFIG_SYS_NAND_U_BOOT_OFFS,
 		2 * 1024 * 1024 + CONFIG_SYS_NAND_U_BOOT_OFFS,
 		4 * 1024 * 1024 + CONFIG_SYS_NAND_U_BOOT_OFFS,
 	};
-	int i, syndrome;
-
-	if (CONFIG_SYS_NAND_U_BOOT_OFFS == CONFIG_SPL_PAD_TO)
-		syndrome = 1; /* u-boot-dtb.bin appended to SPL */
-	else
-		syndrome = 0; /* u-boot-dtb.bin on its own partition */
+	const int syndrome = 1;
+#else
+	/*
+	 * u-boot-dtb.bin on its own partition, do not use syndrome, u-boot
+	 * partition sits after 2 eraseblocks (spl, spl-backup), look for
+	 * backup u-boot 1 erase block further.
+	 */
+	const uint32_t eraseblock_size = CONFIG_SYS_NAND_U_BOOT_OFFS / 2;
+	const uint32_t boot_offsets[] = {
+		CONFIG_SYS_NAND_U_BOOT_OFFS,
+		CONFIG_SYS_NAND_U_BOOT_OFFS + eraseblock_size,
+	};
+	const int syndrome = 0;
+#endif
+	int i;
 
 	if (offs == CONFIG_SYS_NAND_U_BOOT_OFFS) {
 		for (i = 0; i < ARRAY_SIZE(boot_offsets); i++) {