MTD

The Memory Technology Devices (MTD) subsystem provides a common interface to many types of raw flash storage devices. It includes facilities for partioning flash, erasing flash, and managing bad blocks.

Devices and partitions

The mtdinfo command, provided as part of the mtd-utils distribution, displays all available MTD devices and partitions:

$ mtdinfo
Count of MTD devices:           5
Present MTD devices:            mtd0, mtd1, mtd2, mtd3, mtd4
Sysfs interface supported:      yes

Given a device argument (or --all), mtdinfo prints detailed device information:

$ mtdinfo /dev/mtd0
mtd0
Name:                           NAND 128MiB 1,8V 8-bit
Type:                           nand
Eraseblock size:                16384 bytes, 16.0 KiB
Amount of eraseblocks:          8192 (134217728 bytes, 128.0 MiB)
Minimum input/output unit size: 512 bytes
Sub-page size:                  256 bytes
OOB size:                       16 bytes
Character device major/minor:   90:0
Bad blocks are allowed:         true
Device is writable:             true

Like many such userspace tools, mtdinfo is a thin wrapper around the MTD sysfs interface in /sys/class/mtd.

Devices

MTD provides a pair of character devices for each flash device and partition:

  • /dev/mtdX (read-write)
  • /dev/mtdXro (read-only)

Most mtd-utils commands take a /dev/mtdX device node as an argument.

You can read/write directly from/to /dev/mtdX devices, but there are complications:

  • reads can be any size, but writes must be a multiple of the device's minimum write size (as reported by /sys/class/mtd/mtdX/writesize)
  • blocks must be erased before they can be written
  • on NAND devices, reading from or writing to a bad block may produce unexpected results

For these reasons, the [mtd-utils utilities](#MTD Utilities) are often used for flash I/O.

Partitions

MTD allows flash devices to be carved into partitions. Several on-flash partition table formats are supported, as well as partitions defined on the kernel command line or in the device tree.

NAND simulator

MTD includes a NAND simulator module (nandsim) that attaches a virtual MTD device:

$ modprobe nandsim

$ mtdinfo -a
Count of MTD devices:           1
Present MTD devices:            mtd0
Sysfs interface supported:      yes

mtd0
Name:                           NAND simulator partition 0
Type:                           nand
Eraseblock size:                16384 bytes, 16.0 KiB
Amount of eraseblocks:          8192 (134217728 bytes, 128.0 MiB)
Minimum input/output unit size: 512 bytes
Sub-page size:                  256 bytes
OOB size:                       16 bytes
Character device major/minor:   90:0
Bad blocks are allowed:         true
Device is writable:             true

MTD Utilities

MTD includes a suite of userspace tools, distributed as mtd-utils.

flash_erase

The flash_erase utility erases the data and out-of-band areas of a device, partition, or range of blocks:

$ flash_erase /dev/mtd0 0 0
Erasing 16 Kibyte @ 7ffc000 -- 100 % complete 

Reading an erased flash returns all-ones:

$ dd if=/dev/mtd0 bs=512 count=1 | hd
00000000  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|
*
00000200

nanddump

The nanddump utility dumps the contents of an MTD device, skipping any bad blocks that it encounters:

$ nanddump /dev/mtd0 -f nand.img
ECC failed: 0
ECC corrected: 0
Number of bad blocks: 0
Number of bbt blocks: 0
Block size 16384, page size 512, OOB size 16
Dumping data starting at 0x00000000 and ending at 0x08000000...

It can optionally dump out-of-band data and bad blocks.

nandwrite

The nandwrite utility copies a source file to an MTD device, skipping any bad blocks that it encounters:

$ sudo nandwrite /dev/mtd0 nand.img
Writing data to block 0 at offset 0x0
Writing data to block 1 at offset 0x4000
Writing data to block 2 at offset 0x8000
Writing data to block 3 at offset 0xc000
...

mtdpart

The mtdpart utility adds and deletes MTD partitions (but does not update the on-flash partition table):

$ mtdinfo
Count of MTD devices:           2
Present MTD devices:            mtd0, mtd1
Sysfs interface supported:      yes

$ sudo mtdpart del /dev/mtd0 1

$ mtdinfo
Count of MTD devices:           1
Present MTD devices:            mtd0
Sysfs interface supported:      yes

$ sudo mtdpart add /dev/mtd0 ubi 0 1048576

$ mtdinfo
Count of MTD devices:           2
Present MTD devices:            mtd0, mtd1
Sysfs interface supported:      yes

$ mtdinfo /dev/mtd1
mtd1
Name:                           ubi
Type:                           nand
Eraseblock size:                16384 bytes, 16.0 KiB
Amount of eraseblocks:          64 (1048576 bytes, 1024.0 KiB)
Minimum input/output unit size: 512 bytes
Sub-page size:                  256 bytes
OOB size:                       16 bytes
Character device major/minor:   90:2
Bad blocks are allowed:         true
Device is writable:             true

Programming Interfaces

ioctl ABI

Behind the scenes, the mtd-utils commands interact with the MTD subsystem via an ioctl interface:

/* Get basic MTD characteristics info (better to use sysfs) */
#define MEMGETINFO              _IOR('M', 1, struct mtd_info_user)

/* Erase segment of MTD */
#define MEMERASE                _IOW('M', 2, struct erase_info_user)

/* Write out-of-band data from MTD */
#define MEMWRITEOOB             _IOWR('M', 3, struct mtd_oob_buf)

/* Read out-of-band data from MTD */
#define MEMREADOOB              _IOWR('M', 4, struct mtd_oob_buf)

/* Lock a chip (for MTD that supports it) */
#define MEMLOCK                 _IOW('M', 5, struct erase_info_user)

/* Unlock a chip (for MTD that supports it) */
#define MEMUNLOCK               _IOW('M', 6, struct erase_info_user)

/* Check if chip is locked (for MTD that supports it) */
#define MEMISLOCKED             _IOR('M', 23, struct erase_info_user)

/* Check if an eraseblock is bad */
#define MEMGETBADBLOCK          _IOW('M', 11, __kernel_loff_t)

/* Mark an eraseblock as bad */
#define MEMSETBADBLOCK          _IOW('M', 12, __kernel_loff_t)

/*
 * Most generic write interface; can write in-band and/or out-of-band in various
 * modes (see "struct mtd_write_req")
 */
#define MEMWRITE                _IOWR('M', 24, struct mtd_write_req)

libmtd

The mtd-utils source builds a static library (libmtd.a) that wraps the MTD ioctls in a tidy API, e.g., mtd_mark_bad():

/**
 * mtd_mark_bad - mark an eraseblock as bad.
 * @mtd: MTD device description object
 * @fd: MTD device node file descriptor
 * @eb: eraseblock to mark as bad
 *
 * This function marks eraseblock @eb as bad. Returns %0 in case of success and
 * %-1 in case of failure.
 */
int mtd_mark_bad(const struct mtd_dev_info *mtd, int fd, int eb);

Sadly, libmtd.a is not included in the mtd-utils binary distribution.

Kernel API

Higher layer kernel subsystems (flash filesytems, UBI) interact with raw flash via the MTD API defined in include/linux/mtd/mtd.h, including:

int mtd_read(struct mtd_info *mtd, loff_t from, size_t len, size_t *retlen, u_char *buf);
int mtd_write(struct mtd_info *mtd, loff_t to, size_t len, size_t *retlen, const u_char *buf);
int mtd_erase(struct mtd_info *mtd, struct erase_info *instr);
int mtd_read_oob(struct mtd_info *mtd, loff_t from, struct mtd_oob_ops *ops);
int mtd_write_oob(struct mtd_info *mtd, loff_t to, struct mtd_oob_ops *ops);

Device drivers register MTD devices with the MTD core via mtd_device_register().

Reading

Copyright © 2018 Clay McClure. Made with void.