Transcript Embedded Linux Design and Programming

Introduction to
Embedded Systems
Dr. Jerry Shiao, Silicon Valley University
Spring 2014
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Section 11 Storage Devices
 Embedded System Solid-State Storage Devices
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Uses Solid-State Storage Devices ( i.e. ROM, NOR / NAND flash
chips ).
How to effectively use each type of device?
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Each device has own capabilities, particularities, and limitations.
Tools specific to the device type.
Used as Block Device?
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Block Device layer can emulate a disk file system on Flash Device, but suboptimal.
Erasing blocks:
 Flash memory blocks have to be erased before they can be used.
 Significant overhead, should be done when device is idle.
Random access:
 Disk file systems optimized to avoid disk seeks.
 Flash memory has no seek latency.
Wear leveling:
 Flash memory device wear out when a single block is repeatedly
overwritten.
 Flash File Systems spreads out writes evenly.
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Section 11 Storage Devices
 Embedded System Solid-State Storage Devices
 Memory Technology Device ( MTD ) Subsystem
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Provide common capabilities among various technologies.
Controller:
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Perform wear leveling, bad block recovery, power loss recovery, garbage
collection and error correction.
Used in removable flash memory cards and USB flash drives.
Flash File System:
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Perform wear leveling by spreading writes over the media.
Deals with long erase times of NAND flash blocks.
Update flash blocks by:
 Write a new copy of changed data to a fresh flash block.
 Remap the file pointers.
 Erase the old flash block, when the Flash File System is idle.
Used in embedded flash memories ( no controller ).
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Section 11 Storage Devices
 Embedded System Solid-State Storage Devices
 Memory Technology Device ( MTD ) Subsystem
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Flash File System Registered as Linux device.
Linux enables processes to communicate with the device driver
and hardware as file-like objects.
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C Library I/O Operations (i.e. open / read / write / close).
MTD User Modules
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Software modules in Kernel that access the low-level MTD chip drivers.
Chip drivers: DiskOnChip, NAND, RAM, ROM, CFI-Compliant Flash.
Provides recognizable interfaces and abstractions.
MTD /dev entries corresponds to MTD User Modules.
 mtdN:
 Each entry is a separate MTD device or partition. Each MTD
partition acts as a separate MTD device.
 mtdrN:
 Each entry is the read-only equivalent of the matching /dev/mtdN.
 mtdblockN
 Each entry is the block device equivalent of the matching
/dev/mtdN.
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Section 11 Storage Devices
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MTD Subsystem Architecture
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Section 11 Storage Devices
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MTD Subsystem Architecture
MTD User Modules
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JFFS2:
 Journaling file system having all the properties for a flash file system.
YAFFS:
 Journaling file system designed specifically to work within the constraints of
and exploit features of NAND flash.
Char Device:
 “Raw” device character access offering basic read and write functionality.
 Ioctl access to the erase function and other flash functionality.
Block Device:
 “Raw” device block access offereing basic read and write functionality.
 Ioctl access to the erase function and other flash functionality.
Flash Translation Layer ( FTL ):
 Not File System Interface.
 Used for compatibility with existing devices in the field.
 Allows flash memory devices to be used as standard Block Device.
 Provides wear levelling, powerfail resilience, and mapping out bad blocks.
 Adapts a fully functional file system to the constraints and restrictions
imposed by flash memory devices.
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Section 11 Storage Devices
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MTD Subsystem Architecture
MTD Chip Drivers
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DiskOnChip (DoC) Flash:
 Flash Translation Layer providing a disk interface. Internally, a DoC module
contained a controller that implemented ECC, bad block re-mapping and
wear leveling functions that were used to implement a file system.
 CFI-Compliant Flash:
 Common Flash Memory Interface ( CFI ) is an open industry standard.
Flash chip will have information identifying : memory size, byte/word
configuration, block configuration, voltages and timings.
 Common solution, using JFFS2.
 JEDEC-Compliant Flash:
 Standardization body developing standards for semiconductor devices and
has adapted open industry standard CFI.
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Section 11 Storage Devices
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MTD Subsystem Architecture
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Section 11 Storage Devices
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MTD Subsystem Configuration
Kernel Configuration Options.
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make menuconfig: Enter MTD Submenu.
 CONFIG_MTD:
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CONFIG_MTD_CONCAT:
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Enables NAND Flash Translation Layer in the Kernel.
CONFIG_NFTL_RW:
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Enables Flash Translation Layer User Module in the Kernel.
CONFIG_NFTL:
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Enables read-only block device MTD User Module.
CONFIG_FTL:
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Enables block device MTD User Module.
CONFIG_MTD_BLOCK_RO:
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Enables char device MTD User Module.
CONFIG_MTD_BLOCK:
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Divide MTD devices into separate partitions.
CONFIG_MTD_CHAR:
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Combine multiple MTD devices or partitions into a single logical device (i.e. combine space
from two or more separate devices into a single filesystem).
CONFIG_MTD_PARTITIONS:
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Include core MTD Subsystem support.
Enables write to NFTL-formatted devices.
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Section 11 Storage Devices
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MTD Subsystem Configuration
Kernel Configuration Options.
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make menuconfig: RAM/ROM/Flash Chip Drivers Submenu.
 Detect flash chips by CFI probe.
 make menuconfig: Mapping Drivers for Chip Access Submenu.
 The CFI flash device-mapping driver for the board.
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Section 11 Storage Devices
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MTD Subsystem
MTD Partitioning
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CFI Flash cannot be partitioned with fdisk utility ( for block devices only ).
 Partition Information is hardcoded in the mapping driver or board’s device tree.
 Mapping driver initialization will register partition with MTD subsystem.
static struct mtd_partition tqm8xxl_partitions [ ] = {
{
name: “ppcboot”,
/* PPCBoot Firmware */
offset: 0x00000000,
size: 0x00040000,
},
{
name: “kernel”,
/* Default Kernel Image. */
offset: 0x00040000,
size: 0x000C0000,
},
{
name: “user”
offset: 0x00100000,
size: 0x00100000,
},
{
name: “initrd”,
/* Ramdisk. */
offset: 0x00200000,
size: 0x00200000,
}
};
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Section 11 Storage Devices
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MTD Subsystem
Linux Kernel
~ # ls /dev
X0R
loop3
audio
loop4
console loop5
core
loop6
cuam0 loop7
cuam1 mem
dsp
mixer
fd
mtd0
kmem mtd1
log
mtd2
loop0 mtd3
loop1 mtd4
loop2 mtd5
~#
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mtd6
mtd7
mtdblock0
mtdblock1
mtdblock2
mtdblock3
mtdblock4
mtdblock5
mtdblock6
mtdblock7
mtdr0
mtdr1
mtdr2
mtdr3
mtdr4
mtdr5
mtdr6
mtdr7
nfsd
null
ptmx
pts
ptyp0
ptyp1
ptyp2
ptyp3
ptyp4
ptyp5
ptyp6
ptyp7
ram
ram0
ram1
ram2
ram3
ramdisk
random
rawctl
rtc
rtc0
sda
sda1
sda2
sda3
sdb
sdb1
sdb2
sdb3
stderr
stdin
stdout
tty
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ttyAM0
ttyAM1
ttyp0
ttyp1
ttyp2
ttyp3
ttyp4
ttyp5
ttyp6
ttyp7
urandom
zero
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Section 11 Storage Devices
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MTD partitioning is exposed to user space as
separate MTD devices ( mtd0, mtd1, etc ).
MTD Subsystem
Linux Kernel
~ # cat /proc/mtd
dev: size
erasesize name
mtd0: 00040000 00020000 "RedBoot"
mtd1: 00200000 00020000 "ramdisk"
mtd2: 00180000 00020000 "zImage"
mtd3: 00001000 00020000 "RedBoot config"
mtd4: 00020000 00020000 "FIS directory"
~#
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Bootloader: Redboot
RedBoot> fis list
Name
FLASH addr
RedBoot
0x60000000
RedBoot config 0x603C0000
FIS directory
0x603E0000
ramdisk
0x60040000
zImage
0x60240000
RedBoot>
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Mem addr
0x60000000
0x603C0000
0x603E0000
0x00800000
0x00080000
Length
0x00040000
0x00001000
0x00020000
0x00200000
0x00180000
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Entry point
0x00000000
0x00000000
0x00000000
0x00800000
0x00080000
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Section 11 Storage Devices
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MTD Subsystem
MTD Utilities: Used for all types of MTD devices (/dev/mtdN)
flash_info device: Provides information regarding device’s erase region.
 flash_erase device start_address number_of_blocks: Erase a certain number
of blocks from a device starting at a given address.
 flash_eraseall [options] device: Erase the entire device.
 flash_unlock device: Unlocks all the sectors of a device.
 flash_lock device offset number_of_blocks: Locks a certain number of blocks
on a device.
 flashcp [options] filename flash_device: Copies a file to a flash device.
 mtd_debug operation [operation_parameteres]: MTD Debugging options.
MTD Filesystem Creation:
 mkfs.jjfs2 [options] -r directory -o output_file: Builds a JFFS2 filesystem.
 jffs2dump [options] image: Dumps the contents of a binarty JFFS2 image.
NTFL Tools:
 nftl_format device [start_address[size]]: Formats a DisckOnChip device.
FTL Tools:
 ftl_format [options] device: Formats a NOR flash device with FTL.
NAND Tools:
 nandwrite device input_file_start_address: Writes content of a file to a NAND
chip.
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Section 11 Storage Devices
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Root Filesystem
Setup the Root Filesystem:
1) Select Filesystem type.
2) Pack the Filesystem’s content (files and directories).
3) Write the Filesystem to a storage device on the target.
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Characterizing Filesystems
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Online Write Support while in-use.
 Most filesystems support online updates: adding, deleting, and altering files.
 Persistence.
 Contents of filesystem must be preserved in-between system reboot.
 Compression.
 Auto compress some or all of the data and metadata before written to
storage device.
 Auto decompress when read from storage device.
 Compress/decompress at block level, not at file.
 Overhead of decompress occurs at the block, once cached in Linux buffer
cache.
 Filesystems Wear Leveling algorithm more reliable than FTL, NFTL.
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Section 11 Storage Devices
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Root Filesystem
Characterizing Filesystems
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Power-down Reliability.
 Power failure does not corrupt Filesystem information and metadata.
 Journaling Filesystem:
 Maintains log of the modifications done as part of each transaction in
the Filesystem.
 Information on the log is deleted, when all of transaction’s operation are
completed.
 Physical Journal: Contains copies of blocks that are to be written to the
storage device. Two copies made, once to journal, again to the storage
device location.
 Logical Journal: Compact information about the changes. Double
writes, but not to the extent of Physical Journaling.
 Log-Structured Filesystem: All data and metadata kept in the
Filesystem are stored in a log structure that spans the storage device.
 Write are transactions to the log, and log entries can be
invalidated.
 Similar to Physical Journal, without double-write penalty.
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Section 11 Storage Devices
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Root Filesystem
Filesystem Types
Ext2 ( Second Extended Filesystem )
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Built for Block Devices.
 Fast, supports 16 or 32 Tbytes of storage.
 Used for:
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Limitations:
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RAMDisk Read/Write mode.
CompactFlash Device in IDE mode.
NAND or NOR Flash in Read-Only mode. MTD Subsystem Raw Block
emulation or FTL/NFTL Translation Layer.
Does not have power-down reliability.
mke2fs command
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dd if=/dev/zero of=initrd.img bs=1k count=4096
/sbin/mke2fs -F -v -m0 initrd.img
mkdir initrd.dir
mount -o loop initrd.img initrd.dir
(cd initrd; tar -cf - .) | (cd initrd.dir ; tar -xvf -)
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Section 11 Storage Devices
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Root Filesystem
Filesystem Types
Ext3 ( Third Extended Filesystem )
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Adds Reliability to Ext2 via Journaling.
 Journals only Filesystem Metadata (i.e. File Attributes or inodes).
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Used with:
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File updates does not go through Journaling.
Mount option “data=journal” to journal file data updates also.
CompactFlash devices in IDE mode.
NAND or NOR flashes via FTL/NFTL Translation Layer.
mount -t ext3 /dev/hda1 /media/cf -o noatime,data=journal
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Options specify journaling data updates and to not update inode access time
for each access.
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Section 11 Storage Devices
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Root Filesystem
Filesystem Types
Cramfs
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Simplistic , Compressed, Read-Only Filesystem for Embedded Systems.
Max size of a file is 16 Mbytes.
Used for:
 Read-Only on NOR and NAND flash via “raw” MTD Block Device Emulation.
 NAND or NOR via FTL or NFTL Translation Layers.
 RAMDisk Image.
Limitations:
 Max size of a file is 16 Mbytes.
 Kernel Page size must be 4096 Bytes.
 Group-ID field is 8 bits, or 255 max groups.
$ mkcramfs rootfs/ images/cramfs.img
 Compresses the Filesystem (typically 50%).
# dd if=rootfs/cramfs.img of=/dev/mtd4 bs=1024
 Write to the Storage Devce.
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Section 11 Storage Devices
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Root Filesystem
Filesystem Types
Squashfs
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Compressed, Read-Only Filesystem for Embedded Systems.
Data, inodes, and directories are compressed.
Full UID/GID (32 bits).
Files up-to 2^64 Bytes are supported.
Filesystem up-to 2^64 Bytes are supportedl
Block sizes up-to 64KBytes. Increases the compression ratios (better than
4KByte Block size.
Used for:
 Systems with constrained disk sizes and memory.
 PowerPC, i586, Sparc, and ARM Archtectures.
Limitations:
 Not part of the Linux Kernel tree. Installed as a patch to the Kernel.
 Has to be installed from Squash archives.
mksqushfs source1 source2 … destination [ options ]
 Source1 … Pathnames of files or directories to be added to the resulting
Filesystem .
 Destination … Path of the Squashfs Filesystem.
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Section 11 Storage Devices
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Root Filesystem
Filesystem Types
JFFS2
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Compressed, Journaling, and provides Wear Leveling.
 Used for:
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Keeps all the Filesystem information in log structure that spans entire
flash storage space.
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NOR or NAND on-board flashes or Disk-on-Chip Devices.
Better compression than Cramfs.
Log is nodes, each describing a single block of data or metadata.
Updates to file involves creating new node with changed information.
Erase Block Summary (EBS) at end of each erase block.
 Updated with each write to the block.
 EBS read during mount, instead of scanning the entire block.
Keeps track of free space on Filesystem.
Threshold reached: Background garbage-collection process of stale nodes.
 mkfs.jffs2 -r rootfs/ -o images/rootfs-jffs2.img -e 128KiB
 -r: Location of directory containing the root Filesystem.
 -o: Name of output file where Filesystem is stored.
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Section 11 Storage Devices
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Root Filesystem
Filesystem Types
YAFFS2 (Yet Another Flash Filing System Version 2)
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Journaling, and provides Wear Leveling.
 Used for:
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Optimized for NAND on-board flashes.
Uses less RAM to hold its state, scales better than JJFS2.
Better garbage-collection than JFFS2.
Limitations:
Does not support compression.
 Not part of the Linux Kernel tree. Download and installed as a patch to the
Kernel.
 $ mkyaffs2image ${PROJROOT}/rootfs yaffs2.img
 Creates Filesystem image (yaffs2.img) with the content from
${PRJROOT}/rootfs directory.
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Section 11 Storage Devices
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Root Filesystem
Filesystem Types
Tmpfs
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Read/Write Filesystem stored in Virtual Memory.
 Not persistent.
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Uses Linux Page and Dentry Cache for storing file data and metadata.
Linux Page Cache is the Kernel’s internal data structure.
Linux Dentry Cache hold information about directory entries in mounted
Filesystems.
Used for:
Mount directories that do not require permanent storage (i.e. /tmp directory).
 Kernel Configuration Submenu: Enable “Virtual Memory Filesystem Support”.
 # mount -t tmpfs none /tmp -o size=4m
 Size=4m: Limit size of the Filesystem. If “Size” not configured, Filesystem
will grow in content to half of the available RAM.
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Section 11 Storage Devices
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Root Filesystem
Choosing Filesystem Type and Layout
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List type of data stored on Filesystem.
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Executable binary files and libraries.
Configuration information.
Requirements/Characteristics:
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Writeable.
Persistent.
Power-down reliable.
Compressed.
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Find combination of Filesystem type and hardware storage medium.
 Partitioning storage device.
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Different partitions on the same storage device can contain different
Filesystem type.
Applications, Libraries, and Static Data
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Load software back into RAM after reboot, RAMDisk.
Do not need to keep any changes made to the software or files in RAM.
Read-Only: Cramfs or Squashfs.
Read-Write: Ext2
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Section 11 Storage Devices
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Root Filesystem
Applications, Libraries, and Static Data
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RAMDisk:
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Read-Only:
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Load software back into RAM after reboot, RAMDisk.
Do not need to keep any changes made to the software or files in RAM.
Read-Write: Ext2
Read-Only: Cramfs
NOR or NAND.
“raw” MTD Block Device Emulation, FTL, NFTL.
Cramfs, Squashfs.
Dynamic Configuration Files and Data:
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Persistent, writable, and power-down reliable.
NOR or NAND.
 YAFFS2, JFFS2
Compact Flash or NAND with NFTL (DoC)
 Ext3
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