How to create an embedded Linux File System

1. What is a file system)

A file system is a directory structure that includes a disk (including a CD, floppy disk, flash disk, and other storage devices) or a partition. An applicable disk device can contain one or more file systems; to access a file system, you must first mount the file system. To mount the file system, you must specify a mount point.

　　Ii. Mainly embedded file systems

* In Linux, rootfs is essential. Ramdisk and direct mounting of the root file system on HD (hard disk) are mainly implemented on the PC. in embedded systems, it is generally started from flash instead of HD, the simplest method is to load rootfs to ram's ramdisk. The more complex method is to directly read cramfs from flash, and more complicated is to partition on flash and build file systems such as jffs2.

* Ramdisk compresses the created rootfs and writes it to flash. during startup, the bootloader loads it to ram, decompress it, And then mounts it /. This method is simple, but the file system in Ram is not compressed, so it needs to occupy a lot of rare resources in embedded systems Ram.

Ramdisk uses memory space to simulate hard disk partitions. ramdisk usually uses the compression of the disk file system and stores it in flash. during system initialization, It is decompressed to SDRAM and mounted to the root file system, in Linux, there are two types of ramdisk. One is to format and load ramdisk, which is supported in Linux kernel 2.0/2.2. The disadvantage is that the size is fixed; the other is supported by the 2.4 kernel. It is implemented through ramfs and cannot be formatted, but it is easy to use. Its size increases or decreases as needed, ramdisk is a common ramdisk Technology in Linux.

* Initrd is the ramdisk format. Before kernel 2.4 was image-initrd, kernel 2.5 introduced cpio-initrd, which greatly simplified the Linux Startup Process and attached the basic philosophy of Linux: keep it simple, stupid (kiss ). however, cpio-initrd is not widely tested as a new format, and image-initrd is mainly used in embedded Linux.

* Cramfs is a simple file system written by Linus. It can be run directly from flash without being loaded to ram, thus saving Ram. However, cramfs is read-only and inconvenient for directories (such as/etc,/var,/tmp) that need to be modified during runtime. Therefore, these directories are generally made into writable FS such as ramfs.

* Squashfs is an enhancement to cramfs. It breaks through some cramfs restrictions and also has advantages in flash and RAM usage. However, according to developers, the performance may be inferior to that of cramfs. This is also a new method. More tests are required before embedded systems are used.

　　3. Create a directory containing all files

1. Create a directory rootfs to install the File System

2. Mkdir bin Dev etc lib proc sbin tmp usr VaR

3. ln-Fs bin/busybox linuxrc (using busybox)

4. to the system/dev, pack all the devices and copy them to Dev (the most convenient way). Or, to use mknod to build the desired device, I use the following:

CrW-RW-1 Root 5, 1 2006-02-24 13:12 Console

CrW-RW-1 Root 5, 64 cua0

CrW-RW-1 Root 63, 0 2006-02-24 13:12 dk0

CrW-RW-1 Root 63, 1 2006-02-24 13:12 DK1

Drwxr-XR-x 2 root Root 4096 flash

BRW-RW-1 Root 3, 0 2006-02-24 13:12 hda

CrW-RW-1 Root 36, 10 2006-02-24 13:12 IPSec

CrW-RW-1 Root 241, 0 2006-02-24 13:12 ixnpe

CrW-RW-1 Root 1, 2 2006-02-24 13:12 kmem

CrW-RW-1 Root 126, 0 2006-02-24 13:12 ledman

Lrwxrwxrwx 1 Root 16 log->/tmp/var/log

CrW-RW-1 Root 1, 1 2006-02-24 13:12 mem

CrW-RW-1 Root 90, 0 2006-02-24 13:12 mtd0

BRW-RW-1 Root 31, 0 2006-02-24 13:12 mtdblock0

BRW-RW-1 Root 31, 1 2006-02-24 13:12 mtdblock1

BRW-RW-1 Root 31, 2 mtdblock2

BRW-RW-1 Root 31, 3 2006-02-24 13:12 mtdblock3

BRW-RW-1 Root 31, 4 mtdblock4

BRW-RW-1 Root 31, 5 mtdblock5

BRW-RW-1 Root 31, 6 mtdblock6

CrW-RW-1 Root 90, 1 2006-02-24 13:12 mtdr0

CrW-RW-1 Root 1, 3 2006-02-24 13:12 null

CrW-RW-1 Root 108, 0 2006-02-24 13:12 PPP

CrW-r -- 1 Root 5, 2 ptmx

Drwxr-XR-x 2 root Root 4096 PTS

CrW-RW-1 Root 2, 0 2006-02-24 13:12 ptyp0

BRW-RW-1 Root 1, 0 2006-02-24 13:12 ram0

CrW-RW-1 Root 1, 8 random

CrW-RW-1 Root 5, 0 2006-02-24 13:12 tty

CrW-RW-1 Root 4, 0 2006-02-24 13:12 tty0

CrW-RW-1 Root 3, 0 2006-02-24 13:12 ttyp0

CrW-RW-1 Root 4, 64 2006-02-24 13:12 ttys0

CrW-RW-1 Root 1, 9 urandom

CrW-RW-1 Root 1, 5 zero

Example: mknod console C 5 1 CrW-RW-1 Root 5, 1 Console

5. Copy the compiled busybox to/bin. All other commands except busybox are their links.

Ash chgrp clear dd echo fgrep gzip IP ls modprobe MV Ping PWD sed stty tar true zcat busybox chmod CP egrep grep hostname kill mkdir more netstat ping2file RM sh sync TFTP umount cat chown date dmesg false gunzip ifconfig ln mknod Mount pidof PS rmdir sleep sysinfo touch uname

You can use make menuconfig to increase or decrease all commands in busybox.

6. Similarly, the link of busybox is under/sbin.

Halt ifconfig init insmod klogd losetup lsmod mkswap modprobe reboot rmmod route swapoff Swapon

7. The link of busybox is also under/usr/bin.

Basename dirname env free ID logger reset tail tr tty uptime which xargs

Awk cut du expr head killall mkfifo sort test traceroute uniq WC whoami Yes

The above is almost the most complete link, and each viewer can delete it as appropriate, but the link does not occupy any space!

8. Similarly, all the compiled executable files are placed under/usr/sbin.

9. It is very important to pay attention to/lib.

Find the target directory of your compiling environment, and first use strip to compress the required lib file (except the Strip provided by the compiling environment under the non-target directory). First, set the most basic libc, LD and so on. The link must be the same as the link in target/lib.

Then add the corresponding lib according to the specific application. Do not add unnecessary Lib. Lib occupies space.

10. Add the required configuration file under/etc, and the most important thing is the RFM.

#! /Bin/sh

Export Path =/sbin:/bin:/usr/sbin:/usr/bin:/usr/sbin/scripts

UTC = Yes

Mount-n-t proc/proc

Mount-n-t ramfs/tmp

Mount-n-t devpts/dev/PTS

# Build var Directories

/Bin/mkdir-M 0777/tmp/var

/Bin/mkdir-M 0777/var/lock

/Bin/mkdir-M 0777/var/log

/Bin/mkdir-M 0777/var/run

/Bin/mkdir-M 0777/var/tmp

#/Bin/mkdir-M 0777/tmp/etc

#/Bin/CP-A/usr/etc/*/etc

#/Bin/CP-A/usr/dev/*/dev/

# Loads the NPE Ethernet modules into the kernel.

Insmod/lib/modules/2.6.13.2/Intel/ixp400.ko

# Firmware code for NPE engine.

CAT/etc/ixnpemicrocode. dat>/dev/ixnpe

Insmod/lib/modules/2.6.13.2/Intel/ixp400_eth.ko netdev_max_backlog = 500

Insmod/lib/modules/led. Ko

Insmod/lib/modules/push_button.ko

The specific application below is not added as an example. This is required by the system init.

In other cases, the target of a lunksys GPL project is provided below.

　　4. Generate a ramdisk

#! /Bin/sh

Module_name = ramdisk

Rampath = 'pwd'

Tmppath =$ {rampath}/tmp

Source =$ {rampath}/../Target

If [! -D $ {tmppath}]

Then

Mkdir $ {tmppath}

Fi

If ['whoam '! = 'Root']

Then {

Echo "You shoshould run the shell as root, please rerun as a root ."

Echo "aborting ."

Exit 1

}

Fi

# Clear in TMP path

Rm-RF $ {tmppath}/tmpmnt

Rm-RF $ {tmppath}/ramrootfs

Mkdir $ {tmppath}/tmpmnt

# Clear the old ramdisk

Rm-F $ {rampath}/$ module_name

# Make a temp file which size is suitable

Dd If =/dev/Zero of =$ {tmppath}/ramrootfs BS = 1 k count = 6144

# Create a ext2 filesystem

Mke2fs-F-M 0-I 2000 $ {tmppath}/ramrootfs

# Mount it to tmpmnt/

Mount-o loop-T ext2 $ {tmppath}/ramrootfs $ {tmppath}/tmpmnt

# Copy everything from kernel to this.

CD $ {tmppath}/tmpmnt

Echo $ {source}

CP-AV $ {source }/*.

CD $ {tmppath}

# Unmount it the ext2 filesystem

Umount $ {tmppath}/tmpmnt

Cat $ {tmppath}/ramrootfs | gzip-9>/$ {rampath}/ramdisk

Echo copying ramdisk image to $ {rampath}

Sync

Here is an automatic generation script!

　　5. Generate a cramfs

Find the cramfs toolchain. /Mkcramfs-r $ (fs1_dir) $ (fs_name). 1

　　6. Generate a mksquashfs

Find the toolchain of squashfs. /Mksquashfs $ (fs_dir) $ (fs_name)-noappend-be-lzma-no-fragments-Noi

The most difficult and possible problems with file systems are the/lib library and/dev. Please pay attention to these two aspects.