bcache: use a separate data structure for the on-disk super block

Split out an on-disk version struct cache_sb with the proper endianness
annotations.  This fixes a fair chunk of sparse warnings, but there are
some left due to the way the checksum is defined.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -63,14 +63,14 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
 			      struct page **res)
 {
 	const char *err;
-	struct cache_sb *s;
+	struct cache_sb_disk *s;
 	struct buffer_head *bh = __bread(bdev, 1, SB_SIZE);
 	unsigned int i;
 
 	if (!bh)
 		return "IO error";
 
-	s = (struct cache_sb *) bh->b_data;
+	s = (struct cache_sb_disk *)bh->b_data;
 
 	sb->offset		= le64_to_cpu(s->offset);
 	sb->version		= le64_to_cpu(s->version);
@@ -209,7 +209,7 @@ static void write_bdev_super_endio(struct bio *bio)
 
 static void __write_super(struct cache_sb *sb, struct bio *bio)
 {
-	struct cache_sb *out = page_address(bio_first_page_all(bio));
+	struct cache_sb_disk *out = page_address(bio_first_page_all(bio));
 	unsigned int i;
 
 	bio->bi_iter.bi_sector	= SB_SECTOR;

include/uapi/linux/bcache.h

@@ -156,6 +156,57 @@ static inline struct bkey *bkey_idx(const struct bkey *k, unsigned int nr_keys)
 
 #define BDEV_DATA_START_DEFAULT		16	/* sectors */
 
+struct cache_sb_disk {
+	__le64			csum;
+	__le64			offset;	/* sector where this sb was written */
+	__le64			version;
+
+	__u8			magic[16];
+
+	__u8			uuid[16];
+	union {
+		__u8		set_uuid[16];
+		__le64		set_magic;
+	};
+	__u8			label[SB_LABEL_SIZE];
+
+	__le64			flags;
+	__le64			seq;
+	__le64			pad[8];
+
+	union {
+	struct {
+		/* Cache devices */
+		__le64		nbuckets;	/* device size */
+
+		__le16		block_size;	/* sectors */
+		__le16		bucket_size;	/* sectors */
+
+		__le16		nr_in_set;
+		__le16		nr_this_dev;
+	};
+	struct {
+		/* Backing devices */
+		__le64		data_offset;
+
+		/*
+		 * block_size from the cache device section is still used by
+		 * backing devices, so don't add anything here until we fix
+		 * things to not need it for backing devices anymore
+		 */
+	};
+	};
+
+	__le32			last_mount;	/* time overflow in y2106 */
+
+	__le16			first_bucket;
+	union {
+		__le16		njournal_buckets;
+		__le16		keys;
+	};
+	__le64			d[SB_JOURNAL_BUCKETS];	/* journal buckets */
+};
+
 struct cache_sb {
 	__u64			csum;
 	__u64			offset;	/* sector where this sb was written */