Introduction to mtd interface

Memory Technology Device (MTD) is the name of the Linux subsystem that handles most raw flash devices, such as NOR, NAND, dataflash, and SPI flash. It provides both character and block access to these devices, as well as a number of specialized filesystem.

Device support

The following devices are supported by the MTD subsystem:

• NAND Flash
• NOR Flash
• One NAND Flash
• Atmel Data Flash
• SPI Flash

The following devices are NOT handled by the MTD subsystem, even though they are commonly referred to as flash. Many of these include Flash Translation Layer (FTL) hardware that takes care of many of the concerns surrounding flash usage, such as wear-leveling and bad blocks.

• USB Sticks — handled by USB Host and SCSI subsystems
• Compact Flash — handled by the PC Card/IDE subsystems, depending on the implementation
• EEPROMs — handled by either the SPI EEPROM driver, or I2C EEPROM driver
• MMC/SD Cards — handled by the MMC subsystem

Overview

The MTD subsystem provides a number of mechanisms for interacting with raw flash chips. It consists of a number of generic drivers for classes of chips (NAND, NOR), as well drivers for individual flash controllers. It also includes support for accessing these chips as either block or character devices, as well as the ability to divide a single chip into multiple, smaller partitions. Many features of the MTD subsystem deal with flash-centric issues such as wear-leveling, bad block detection and handling, and out-of-band (OOB) data. There are a number of filesystems available specifically for the MTD subsystem.

Wear Leveling

Wear-leveling is an important consideration when dealing with flash chips. A flash chip is composed of a large number of memory cells, or gates. Each of these gates are rated for a limited number of erase/program cycles. This means that they will eventually wear down, and become “bad blocks.” Without wear leveling, some blocks will wear down much faster than others. For example, consider a file that keeps track of the total operational time of the board. If we were to update this file once a minute, and always use the same block when storing the file, that block would wear down after around 70 days on flash rated for 100,000 erase/program cycles.

Wear-leveling ensures that this burden is spread around to multiple blocks, which increases the overall life of the chip. MTD itself doesn’t perform wear-leveling, but many of the flash filesystems do.

Bad Blocks

One distinguishing feature of NAND flash is that it is allowed to contain bad blocks straight from the factory. These bad blocks are tracked on the chip so that software can skip them when necessary. This allows the manufacturers to achieve a much larger yield, and therefore decrease the costs of the chips. Most (if not all) NAND flash manufacturers will also guarantee that the first block is good, allowing you to use that space for a boot loader.

Linux MTD also has facilities for detecting and tracking bad blocks on NAND chips. Linux generates a bad block table (bbt) and stores this information in the last two good blocks of the chip. Most Linux-capable bootloaders also treat NAND in this way. When using a flash-friendly filesystem, or userspace MTD utilities, bad blocks are automatically handled.

Is an MTD device a block device or a char device?

First off, an MTD is a “Memory Technology Device”, so it’s just “MTD”. Unix traditionally only knew block devices and character devices. Character devices were things like keyboards or mice, that you could read current data from, but couldn’t be seek-ed and didn’t have a size. Block devices had a fixed size and could be seek-ed. They also happened to be organized in blocks of multiple bytes, usually 512.

Flash doesn’t match the description of either block or character devices. They behave similar to block device, but have differences. For example, block devices don’t distinguish between write and erase operations. Therefore, a special device type to match flash characteristics was created: MTD.

So MTD is neither a block nor a char device. There are translations to use them, as if they were. But those translations are nowhere near the original, just like translated Chinese poems.