stm32/main: Check block 0 and 1 when auto-detecting littlefs.

The superblock for littlefs is in block 0 and 1, but block 0 may be erased
or partially written, so block 1 must be checked if block 0 does not have a
valid littlefs superblock in it.

Prior to this commit, if block 0 did not contain a valid littlefs
superblock (but block 1 did) then the auto-detection would fail, mounting a
FAT filesystem would also fail, and the system would reformat the flash,
even though it may have contained a valid littlefs filesystem.  This is now
fixed.
Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/main.c

@@ -168,31 +168,35 @@ MP_NOINLINE STATIC bool init_flash_fs(uint reset_mode) {
 
     #if MICROPY_VFS_LFS1 || MICROPY_VFS_LFS2
 
-    // Try to detect the block device used for the main filesystem, based on the first block
-
-    uint8_t buf[64];
-    ret = storage_readblocks_ext(buf, 0, 0, sizeof(buf));
+    // Try to detect the block device used for the main filesystem based on the
+    // contents of the superblock, which can be the first or second block.
     mp_int_t len = -1;
+    uint8_t buf[64];
+    for (size_t block_num = 0; block_num <= 1; ++block_num) {
+        ret = storage_readblocks_ext(buf, block_num, 0, sizeof(buf));
+
+        #if MICROPY_VFS_LFS1
+        if (ret == 0 && memcmp(&buf[40], "littlefs", 8) == 0) {
+            // LFS1
+            lfs1_superblock_t *superblock = (void *)&buf[12];
+            uint32_t block_size = lfs1_fromle32(superblock->d.block_size);
+            uint32_t block_count = lfs1_fromle32(superblock->d.block_count);
+            len = block_count * block_size;
+            break;
+        }
+        #endif
 
-    #if MICROPY_VFS_LFS1
-    if (ret == 0 && memcmp(&buf[40], "littlefs", 8) == 0) {
-        // LFS1
-        lfs1_superblock_t *superblock = (void *)&buf[12];
-        uint32_t block_size = lfs1_fromle32(superblock->d.block_size);
-        uint32_t block_count = lfs1_fromle32(superblock->d.block_count);
-        len = block_count * block_size;
-    }
-    #endif
-
-    #if MICROPY_VFS_LFS2
-    if (ret == 0 && memcmp(&buf[8], "littlefs", 8) == 0) {
-        // LFS2
-        lfs2_superblock_t *superblock = (void *)&buf[20];
-        uint32_t block_size = lfs2_fromle32(superblock->block_size);
-        uint32_t block_count = lfs2_fromle32(superblock->block_count);
-        len = block_count * block_size;
+        #if MICROPY_VFS_LFS2
+        if (ret == 0 && memcmp(&buf[8], "littlefs", 8) == 0) {
+            // LFS2
+            lfs2_superblock_t *superblock = (void *)&buf[20];
+            uint32_t block_size = lfs2_fromle32(superblock->block_size);
+            uint32_t block_count = lfs2_fromle32(superblock->block_count);
+            len = block_count * block_size;
+            break;
+        }
+        #endif
     }
-    #endif
 
     if (len != -1) {
         // Detected a littlefs filesystem so create correct block device for it