Filesystems/ubifs

UBI (Unsorted Block Images) filesystem is a filesystem for flash memory devices. Unlike traditional filesystem, UBI filesystem does not work on top of block devices such as /dev/mtdblock0. It was designed to work on top of raw flash device like /dev/mtd0. UBI filesystem doesn't access raw flash devices directly, rather there is an additional layer called UBI between UBI filesystem and raw flash devices. UBI is a volume management system which manages multiple logical volumes on a single physical flash device and spreads the I/O across whole flash chip (wear-leveling).

Source Codes

UBI (Unsorted Block Images): drivers/mtd/ubi

I/O

High level I/O: ubi_read() and ubi_write()

Low level I/O: ubi_io_read() and ubi_io_write(). Thse functions make use of read and write method provided by MTD character device layer.

In include/linux/mtd/ubi.h

static inline int ubi_read(struct ubi_volume_desc *desc, int lnum, char *buf,
                           int offset, int len)
{
        return ubi_leb_read(desc, lnum, buf, offset, len, 0);
}

static inline int ubi_write(struct ubi_volume_desc *desc, int lnum,
                            const void *buf, int offset, int len)
{
        return ubi_leb_write(desc, lnum, buf, offset, len, UBI_UNKNOWN);
}

In drivers/mtd/ubi/kapi.c

int ubi_leb_read(struct ubi_volume_desc *desc, int lnum, char *buf, int offset,
                 int len, int check)
{
        .......
        err = ubi_eba_read_leb(ubi, vol, lnum, buf, offset, len, check);
        .......
}

int ubi_leb_write(struct ubi_volume_desc *desc, int lnum, const void *buf,
                  int offset, int len, int dtype)
{
        .......
        return ubi_eba_write_leb(ubi, vol, lnum, buf, offset, len, dtype);
        .......
}

In drivers/mtd/ubi/eba.c

int ubi_eba_read_leb(struct ubi_device *ubi, struct ubi_volume *vol, int lnum,
                     void *buf, int offset, int len, int check)
{
        .......
                err = ubi_io_read_vid_hdr(ubi, pnum, vid_hdr, 1);

        .......
        err = ubi_io_read_data(ubi, buf, pnum, offset, len);
        .......
}

int ubi_eba_write_leb(struct ubi_device *ubi, struct ubi_volume *vol, int lnum,
                      const void *buf, int offset, int len, int dtype)
{
        .......
                err = ubi_io_write_data(ubi, buf, pnum, offset, len);
        .......
        err = ubi_io_write_vid_hdr(ubi, pnum, vid_hdr);
        .......
}

In drivers/mtd/ubi/ubi.h

static inline int ubi_io_read_data(const struct ubi_device *ubi, void *buf,
                                   int pnum, int offset, int len)
{
        ubi_assert(offset >= 0);
        return ubi_io_read(ubi, buf, pnum, offset + ubi->leb_start, len);
}

static inline int ubi_io_write_data(struct ubi_device *ubi, const void *buf,
                                    int pnum, int offset, int len)
{
        ubi_assert(offset >= 0);
        return ubi_io_write(ubi, buf, pnum, offset + ubi->leb_start, len);
}

In drivers/mtd/ubi/io.c

int ubi_io_read_vid_hdr(struct ubi_device *ubi, int pnum,
                        struct ubi_vid_hdr *vid_hdr, int verbose)
{
        .......
        read_err = ubi_io_read(ubi, p, pnum, ubi->vid_hdr_aloffset,
                          ubi->vid_hdr_alsize);
        .......
}

int ubi_io_write_vid_hdr(struct ubi_device *ubi, int pnum,
                         struct ubi_vid_hdr *vid_hdr)
{
        .......
        err = ubi_io_write(ubi, p, pnum, ubi->vid_hdr_aloffset,
                           ubi->vid_hdr_alsize);
        .......
}

int ubi_io_read_ec_hdr(struct ubi_device *ubi, int pnum,
                       struct ubi_ec_hdr *ec_hdr, int verbose)
{
        .......
        read_err = ubi_io_read(ubi, ec_hdr, pnum, 0, UBI_EC_HDR_SIZE);
        .......
}

int ubi_io_write_ec_hdr(struct ubi_device *ubi, int pnum,
                        struct ubi_ec_hdr *ec_hdr)
{
        .......
        err = ubi_io_write(ubi, ec_hdr, pnum, 0, ubi->ec_hdr_alsize);
        .......
}

Eventually all I/O requests are processed by following functions in drivers/mtd/ubi/io.c.

int ubi_io_read(const struct ubi_device *ubi, void *buf, int pnum, int offset,
                int len)
{
        .......
        err = ubi->mtd->read(ubi->mtd, addr, len, &read, buf);
        .......
}

int ubi_io_write(struct ubi_device *ubi, const void *buf, int pnum, int offset,
                 int len)
{
        .......
        err = ubi->mtd->write(ubi->mtd, addr, len, &written, buf);
        .......
}

Test

# flash_eraseall /dev/mtd0
# modprobe ubi mtd=0
# ubimkvol /dev/ubi0 -N test -s 100MiB
# mount -t ubifs ubi0:test /mnt/ubifs

References

• UBI Filesystem documents by linux-mtd
• UBI documents by linux-mtd