1. Computer System Laboratory
Lab6 Root Filesystem

2. Experimental Goal
Understand the basic process of root filesystem building
Lab6
2019/4/9

3. Environment Host System Build System Target System Software
Windows XP/Ubuntu 8.04
Build System
Ubuntu 8.04
Target System
XScale PXA270
Software
BusyBox source code
Compiled toolchain
mkfs.jffs2 & genext2fs utility
Misc.
You can download all the source codes from
RSWiki CSL Course Software
Lab6
2019/4/9

4. Introduction to Root Filesystem
A file system is a method of storing and organizing computer files and their data
A root filesystem is the file system that is contained on the same partition on which the root directory is located
The exact contents of the root filesystem will vary according to the computer, but they will include the files that are necessary for booting the system
It is the filesystem on which all the other filesystems are mounted as the system is booted up
~ from The Linux Information Project
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5. PXA270
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6. Basic Root Filesystem Structure
Root filesystem follows FHS (Filesystem Hierarchy Standard)
FHS has the guideline of your filesystem
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7. Types of File Systems Some well-known file systems
Windows : FAT, NTFS
Mac OS X : HFS Plus, UFS
Linux : ex*, XFS, JFS, ReiserFS
We will use JFFS2 & ext2 in this lab
ext2 (The second extended filesystem)
A non-journaling file system
Writable
JFFS2 (Journaling Flash File System Version 2)
A journaling file system
Writable, compressed, power-down-reliable file system
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8. Create Root Filesystem Skeleton
Step1 : create a folder for placing root filesystem content
% mkdir $HOME/<root filesystem folder’s name>
Step2 : create the essential directories
% cd <root filesystem folder’s path>
% mkdir etc lib var proc mnt dev tmp
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9. Compiled Toolchain Installation
Step1 : extract binary files
% tar –jxvf cross tar.bz2
Step2 : move files to specified location
% mkdir /usr/local/arm
% sudo mv ./ /usr/local/arm
Step3 : append installation directory to PATH
% …
Test
% arm-linux-gcc -v
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10. Build BusyBox
BusyBox contains a collection of stripped-down Unix utilities into a single executable
Why BusyBox?
You don’t have to configure and build the sources of each utility
Step1 : extract source code
% tar –zxvf busybox-1.00.tar.gz
Step2 : copy header from kernel source
% cd busybox-1.00
% cp –R <creator mt-linux path>/pxa270/linux/include/linux \
include
% wget tware/version.h
% cp version.h include/linux
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11. Build BusyBox (Cont.) Step3 : configure BusyBox
% make menuconfig
Build Options 
[*] Build BusyBox as a static binary (no shared libs)
[*] Do you want to build BusyBox with a Cross Compiler?
(arm-linux-) Cross Compiler prefix
Networking Utilities 
[ ] route
Step4 : compile BusyBox
% make -j4
Step5 : install BusyBox
% make -j4 install
Step6 : move files to your root filesystem folder
% mv _install/* <root filesystem folder’s path>
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12. Create Essential Device Files
Device files allow user programs to access hardware devices on the system through the kernel
The kernel just relies on device file’s type and major number to find which driver manages this device
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13. Create Essential Device Files (Cont.)
The official source of information for device major and minor numbers can be found in Documentation/devices.txt under the kernel source
We use a script to create device files
% wget tware/mknod.sh
% cp mknod.sh <root filesystem folder’s path>/dev
% cd <root filesystem folder’s path>/dev
% sudo sh mknod.sh
% rm mknod.sh
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14. Misc.
Add configuration file used by first user-space program (i.e. /sbin/init)
% wget tware/inittab
% cp inittab <root filesystem folder’s path>/etc
inittab describes which processes are started during bootup
The entry syntax of inittab
id : runlevels : action : process
sysinit : execute the process during system boot
askfirst : prompt “Please press Enter to activate this console.” before execution
Some other actions: respawn, ctrlaltdel
::sysinit:/etc/rcS
::askfirst:/bin/sh
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15. Misc. (Cont.) Add system initialization script Change their permission
% wget tware/rcS
% cp rcS <root filesystem folder’s path>/etc
Change their permission
% cd <root filesystem folder’s path>/etc
% chmod 777 inittab rcS
Please refer to the website below for more information
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16. Create the Root Filesystem Image
Make a JFFS2 file system image
% ./mkfs.jffs2 -v -e 0x pad=0xf r \
<root filesystem folder’s path> -o rootfs.jffs2
Make a ext2 file system image
% ./genext2fs -b d <root filesystem folder’s path> \
-e 0 rootfs.ext2
Now you can write rootfs image to board
Tip
If you choose ext2 file system format, you need to change U-Boot bootargs environment variable
$ setenv bootargs root=/dev/mtdblock3 rw rootfstype=ext2 console=ttyS0,9600n8 mem=64M ip= : : : ::et h0:off ether=12:34:56:78:90:ab)
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17. Change File System Image Size
The size of root filesystem is determined by Linux kernel (in arch/arm/mach-pxa/mach-creator-pxa270.c)
Then the corresponding option is required to set
% ./mkfs.jffs2 -v -e 0x pad=0xf r \
<root filesystem folder’s path> -o rootfs.jffs2
% ./genext2fs -b d <root filesystem folder’s path> \
-e 0 rootfs.ext2
static struct mtd_partition creator_pxa270_partitions[] = { …
name: "Filesystem",
size: x00f00000, /* 15M for rootfs */ }
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18. Exercise
Refer to previous page, increase the size of the root filesystem to 20Mbytes
You can use “df -h” command in Linux shell to check
The tftp client utility is not built by default, please add tftp client utility to BusyBox
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2019/4/9