1. Embedded Storage
Management
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2. What to Expect?
W's of Memory Technology Devices
MTD Subsystem
MTD related Drivers
Flash File Systems
Flash Tools
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3. Storage Timeline
Traditionally (for most of past 20+ years)
ROM – For read-only storage
NVRAM – For read-write storage
Equating with today's technology
Low-density
Costlier
Latest trend is flash technology devices
More commonly referred as MTD
Comes as NOR, NAND, some exotic varieties
Best suited for Embedded Systems
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4. What are
Memory Technology Devices?
Embedded Flash Memory
Contains Large Erase Blocks
32KB to 128KB
Maintains three main Operations
Read, Write, Erase from Erase Block
Bad Erase Blocks to be dealt by software
Erase Blocks get worn out after some
erase cycles
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5. NOR Flash
Reading like RAM
Random Access
Execute Code (XIP)
Could be replacement for ROM (but slower)
Manufacturers
Intel, AMD, Fujitsu and Toshiba
Capacity
Typically from a few KBs to 64MB
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6. NAND Flash
Reading like Hard Disk
Multiples of 512 bytes at a time
No XIP
Higher Density & Cheaper than NOR
Highly prone to errors at the bit level
Needs software to handle bad blocks
Manufacturers: Samsung and Toshiba
Capacity: Typically from 8MB to 1024MB. Today,
even more
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7. Writing into Flash
Same for both NOR and NAND Flash
Sequence of Steps needed
Almost always involve an erase cycle
As Writing of Data means
Flipping a bit to "0" or leaving it as a "1"
Flipping a bit to "1" needs an erase cycle
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8. MTD Subsystem
File I/O Raw I/O, MTD Utils
User Space
VFS Layer Kernel Space
Individual File Systems
mtdblock, FTL, NFTL mtdchar JFFS2 YAFFS2
MTD Core MTD Layer
NAND Chip
Drivers
NOR Chip Drivers
Map Drivers
(CFI, JEDEC)
Probe
Hardware Space
I/O
NAND NOR
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9. Map Drivers
Drivers to Map
Flash Memory Range for CPU access
Mark the Storage Partitions in the Kernel
As no partition table on flash
Shows up as /dev/mtd*
Example
Browse the drivers/mtd/maps/ folder
Data Structures
struct mtd_partition (Header: <linux/mtd/partitions.h>)
struct map_info (Header: <linux/mtd/map.h>)
Header: <linux/mtd/mtd.h>
APIs
int add_mtd_partitions(struct mtd_info *, const struct mtd_partition *, int);
int del_mtd_partitions(struct mtd_info *);
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10. NOR Chip Drivers
NOR Chip Drivers
CFI specification
JEDEC specification
CFI Command Sets
0x01 – Intel & Sharp flash chips
0x02 – AMD & Fujitsu flash chips
0x03 – ST flash chips
All three are configurable in kernel
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11. NAND Chip Drivers
Drivers for NAND Controller to access NAND
Sources: drivers/mtd/nand/
A NAND Drive should do the following
Add an entry to nand_flash_ids[] in drivers/mtd/nand/nand_ids.c
Entries being
Id Name, Device Id, Page Size, Erase Block Size, Chip Size, Options like
Bus Width, …
As, no automatic configuration support
Specify its out-of-band (OOB) spare area layout through the
“struct nand_ecclayout” (Header: <mtd/mtd-abi.h>)
For ECCs, to implement error correction & detection
Enable Error Management, in case NAND Controller doesn't do it
Software ECC is implemented in drivers/mtd/nand/nand_ecc.c
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12. Kernel Configurations for MTD
CONFIG_MTD should be enabled
And whatever required under it, should be
enabled, e.g.
Required NAND Driver, Or
Required NOR Protocol
etc
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13. File Systems for Flash
Typical Requirements
Flash wear-levelling
Less Writes / Erases
Protect Data during Erase Cycles
Bad Block Management (esp for NAND)
Possibly compression (though preferred on read-only
file systems only)
File Systems handling these
JFFS2 (log-structured design & garbage collection)
YAFFS2 (specifically designed for NAND constraints)
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14. Flash Tools
Useful tools to work with MTD layer
flash_erase, flash_eraseall
flash_lock, flash_unlock
nanddump, nandwrite
sumtool
...
Download from ftp://ftp.infradead.org/pub/mtd-utils
Tool to construct a JFFS2 file system image
mkfs.jffs2
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15. What all have we learnt?
W's of Memory Technology Devices
About NOR & NAND
MTD Subsystem
MTD related Drivers
Map, NOR Chip, NAND Chip
Flash File Systems
Flash Tools
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16. Any Queries?
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