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, 20 insertions, 0 deletions

diff --git a/drivers/mtd/ubi/cdev.c b/drivers/mtd/ubi/cdev.c
index 8ca49f2043e4..39d377489c29 100644
--- a/drivers/mtd/ubi/cdev.c
+++ b/drivers/mtd/ubi/cdev.c
@@ -561,6 +561,26 @@ static long vol_cdev_ioctl(struct file *file, unsigned int cmd,
 		break;
 	}
 
+	/* Attach a block device to an UBI volume */
+	case UBI_IOCVOLATTBLK:
+	{
+		struct ubi_volume_info vi;
+
+		ubi_get_volume_info(desc, &vi);
+		err = ubiblock_add(&vi);
+		break;
+	}
+
+	/* Dettach a block device from an UBI volume */
+	case UBI_IOCVOLDETBLK:
+	{
+		struct ubi_volume_info vi;
+
+		ubi_get_volume_info(desc, &vi);
+		err = ubiblock_del(&vi);
+		break;
+	}
+
 	default:
 		err = -ENOTTY;
 		break;