MTD subsystem (stands for Memory Technology Devices) provides an abstraction layer for raw flash devices. It makes it possible to use the same API when working with different flash types and technologies, e.g. NAND, OneNAND, NOR, AG-AND, ECC'd NOR, etc.
MTD subsystem does not deal with block devices like MMC, eMMC, SD, CompactFlash, etc. These devices are not raw flashes but they have a Flash Translation layer inside, which makes them look like block devices. These devices are the subject of the Linux block subsystem, not MTD. Please, refer to this FAQ section for a short list of the main differences between block and MTD devices. And the raw flash vs. FTL devices UBIFS section discusses this in more details.
MTD subsystem has the following interfaces.
/dev/mtd1, and so on. These character devices provide I/O access to the raw flash. They support a number of
ioctlcalls for erasing eraseblocks, marking them as bad or checking if an eraseblock is bad, getting information about MTD devices, etc.
sysfsinterface is relatively newer and it provides full information about each MTD device in the system. This interface is easily extensible and developers are encouraged to use the
sysfsinterface instead of older
/proc/mtdinterfaces, when possible. The
sysfsinterface for the mtd subsystem is documentated in the kernel, and currently can be found at
/proc/mtdproc file system file provides general MTD information. This is a legacy interface and the sysfs interface provides more information.
MTD subsystem supports bare NAND flashes with software and hardware ECC, OneNAND flashes, CFI (Common Flash Interface) NOR flashes, and other flash types.
Additionally, MTD supports legacy FTL/NFTL "translation layers",
M-Systems' DiskOnChip 2000 and Millennium chips, and PCMCIA flashes
pcmciamtd driver). But the corresponding drivers are very old and
not maintained very much.
The MTD subsystem API is defined in
The methods and data structures in this file are used by higher layer kernel
code such as flash file systems to access and control the mtd devices, and also
by device driver authors to interface their device to the mtd subsystem. The
various methods by which a driver provides access to the device are defined
struct mtd_info. Prior to kernel version 3.4, higher layers
called the driver methods directly by way of a pointer to
mtd_info. As of kernel 3.4, these methods are implemented within the mtd
subsystem core code, which then calls the corresponding driver methods. Users
of kernel 3.4 and later should not call the driver methods directly, but instead
use those prototyped in
mtd.h outside of
mtd_info. These methods include
-EUCLEANin some circumstances. This return code is applicable mainly to NAND flash devices, and is used to indicate that some bit errors were corrected by the device's ECC facility. Prior to kernel version 3.4,
-EUCLEANwas returned if one or more bit errors were corrected during the read operation. As of kernel 3.4, the meaning is more nuanced, and can be broadly interpreted to mean "a dangerously high number of bit errors were corrected". The
-EUCLEANreturn code is intended to help higher layers detect degradation of erase blocks. The conditions by which
-EUCLEANcan be tuned using the
bitflip_thresholdelement of the sysfs interface. Please see the kernel documentation for the MTD sysfs interface (referenced above) before adjusting this value.
The MTD subsystem includes a set of tests which you may run to verify your
flash hardware and drivers. The tests are available in the mainline kernels
starting from kernel version
2.6.29 and they live in the
drivers/mtd/tests directory of the linux kernel source codes. You
may compile the tests as kernel modules by enabling them in the kernel
configuration menu by marking: "Device Drivers" ->
"Memory Technology Device (MTD) support" -> "MTD tests support" (or
MTD_TESTS symbol in the
If you have a pre-
2.6.29 kernel, you may find the tests
The MTD test-suite contains the following tests:
modinfo mtd_torturetest) which allow you to set the amount of eraseblocks to torture and how the torturing is done. You may limit the amount of torturing cycles using the
cycles_countmodule parameter. It may be very god idea to run this test for some time and validate your flash driver and HW, providing you have a spare device. For example, we caught rather rare and nasty DMA issues on an OMAP2 board with OneNAND flash, just by running this tests for few hours.
mtdblock driver available in the MTD is an archaic tool
which emulates block devices on top of MTD devices. It does not even have
bad eraseblock handling, so it is not really usable with NAND flashes. And it
works by caching a whole flash erase block in RAM, modifying it as requested,
then erasing the whole block and writing back the modified. This means that
mtdblock does not try to do any optimizations, and that you will
lose lots of data in case of power cuts. And last, but not least,
mtdblock does not do any wear-leveling.
Often people consider
mtdblock as general FTL layer and try to
use block-based file systems on top of bare flashes using
mtdblock. This is wrong in most cases.
In other words, please, do not use
mtdblock unless you
know exactly what you are doing.
There is also a read-only version of this driver which doesn't have the capacity to do the caching and erase/writeback, mainly for use with uCLinux where the extra RAM requirement was considered too large.
Old MTD web site and old MTD documentation is available here. Old NAND flash interface description is available here.