UBI: R/O block driver on top of UBI volumes

This commit introduces read-only block device emulation on top of UBI volumes.

Given UBI takes care of wear leveling and bad block management it's possible
to add a thin layer to enable block device access to UBI volumes.
This allows to use a block-oriented filesystem on a flash device.

The UBI block devices are meant to be used in conjunction with any
regular, block-oriented file system (e.g. ext4), although it's primarily
targeted at read-only file systems, such as squashfs.

Block devices are created upon user request through new ioctls:
UBI_IOCVOLATTBLK to attach and UBI_IOCVOLDETBLK to detach.
Also, a new UBI module parameter is added 'ubi.block'. This parameter is
needed in order to attach a block device on boot-up time, allowing to
mount the rootfs on a ubiblock device.
For instance, you could have these kernel parameters:

  ubi.mtd=5 ubi.block=0,0 root=/dev/ubiblock0_0

Or, if you compile ubi as a module:

  $ modprobe ubi mtd=/dev/mtd5 block=/dev/ubi0_0

Artem: amend commentaries and massage the patch a little bit.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/drivers/mtd/ubi/ubi.h b/drivers/mtd/ubi/ubi.h
index 8ea6297a208f..e76ff98440a4 100644
--- a/drivers/mtd/ubi/ubi.h
+++ b/drivers/mtd/ubi/ubi.h
@@ -864,6 +864,20 @@ int ubi_update_fastmap(struct ubi_device *ubi);
 int ubi_scan_fastmap(struct ubi_device *ubi, struct ubi_attach_info *ai,
 		     int fm_anchor);
 
+/* block.c */
+#ifdef CONFIG_MTD_UBI_BLOCK
+int ubiblock_init(void);
+void ubiblock_exit(void);
+int ubiblock_add(struct ubi_volume_info *vi);
+int ubiblock_del(struct ubi_volume_info *vi);
+#else
+static inline int ubiblock_init(void) { return 0; }
+static inline void ubiblock_exit(void) {}
+static inline int ubiblock_add(struct ubi_volume_info *vi) { return -ENOTTY; }
+static inline int ubiblock_del(struct ubi_volume_info *vi) { return -ENOTTY; }
+#endif
+
+
 /*
  * ubi_rb_for_each_entry - walk an RB-tree.
  * @rb: a pointer to type 'struct rb_node' to use as a loop counter