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Instead, we create a separate tarball which contains only the device entries so we can use them when
necessary (with cramfs):
bash# tar -zcf /tmp/devices.tar.gz dev/
bash# cd /tmp
4.
Unmount ramdisk image:
bash# umount /mnt/tmp
5.
We will use the /tmp/rootfs.tar.gz tarball as master file in all following experiments.
9.5.1. Root File System on a Ramdisk
Ram disks are used very often to hold the root file system of embedded systems. They have several
advantages:
well-known•
well-supported by the Linux kernel•
simple to build•
simple to use - you can even combine the ramdisk with the Linux kernel into a single image file•
RAM based, thus pretty fast•
writable file system•
original state of file system after each reboot = easy recovery from accidental or malicious data
corruption etc.
•
On the other hand, there are several disadvantages, too:
big memory footprint: you always have to load the complete filesystem into RAM, even if only small
parts of are actually used
•
slow boot time: you have to load (and uncompress) the whole image before the first application
process can start
•
only the whole image can be replaced (not individual files)•
additional storage needed for writable persistent data•
Actually there are only very few situations where a ramdisk image is the optimal solution. But because they
are so easy to build and use we will discuss them here anyway.
In almost all cases you will use an ext2 file system in your ramdisk image. The following steps are needed to
create it:
Create a directory tree with the content of the target root filesystem. We do this by unpacking our
master tarball:
$ mkdir rootfs
$ cd rootfs
$ tar zxf /tmp/rootfs.tar.gz
1.
We use the genext2fs tool to create the ramdisk image as this allows to use a simple text file to
describe which devices shall be created in the generated file system image. That means that no root
permissions are required at all. We use the following device table rootfs_devices.tab:
#<name> <type> <mode> <uid> <gid> <major> <minor> <start> <inc> <count>
/dev d 755 0 0 - - - - -
/dev/console c 640 0 0 5 1 - - -
/dev/fb0 c 640 0 0 29 0 - - -
/dev/full c 640 0 0 1 7 - - -
/dev/hda b 640 0 0 3 0 - - -
2.
9.5.1. Root File System on a Ramdisk 112
- 1. Abstract 6
- 2. Introduction 7
- 3.1. ELDK Availability 10
- 3.2. ELDK Getting Help 10
- 3.3. Supported Host Systems 10
- 3.5. Installation 11
- 3.5.1. Product Packaging 11
- 3.5.2. Downloading the ELDK 11
- 3.5.3. Initial Installation 12
- Packages 13
- 3.6. Working with ELDK 14
- $ arm-linux-gcc -c myfile.c 15
- 3.9. ELDK Packages 18
- 3.10.3. build.sh Usage 28
- 4. System Setup 30
- 4.8. Configuring a NFS Server 34
- 5. Das U-Boot 36
- 5.1. Current Versions 36
- 5.3. Configuration 37
- 5.4. Installation 38
- 5.4.1. Before You Begin 38
- 5.5. Tool Installation 39
- 5.6. Initialization 39
- 5.7. Initial Steps 40
- 5.8. The First Power-On 41
- 5.9.1. Information Commands 42
- 5.9.2. Memory Commands 44
- 5.9.2.3. cmp - memory compare 45
- 5.9.2.4. cp - memory copy 46
- 5.9.2.5. md - memory display 46
- 5.9.3. Flash Memory Commands 50
- 5.9.3.6. UBI Usage in U-Boot 54
- Useful definitions: 58
- 5.9.5. Download Commands 61
- 5.9.9. Storage devices 72
- 5.9.9.1. MMC devices 72
- 5.9.8. Special Commands 72
- 5.9.9.1. MMC devices 74 74
- 5.9.9.1. MMC devices 75 75
- 5.9.9.2. NAND devices 76
- 5.13. U-Boot Image Formats 84
- 5.14.1. Boot Count Limit 84
- 6.3. Installation 86
- 7. Booting Embedded Linux 87
- 7.1. Introduction 87
- 7.3. Passing Kernel Arguments 88
- 7.4. Boot Arguments Unleashed 88
- 8. Building and Using Modules 96
- 9. Advanced Topics 97
- 9.1. Flash Filesystems 97
- 9.1.3. Second Version of JFFS 99
- 9.1.5.1. Create Device Files 101
- 9.2.1. Mount Parameters 107
- $ mkfs.jffs2 -U \ 114
- Using FEC ETHERNET device 115
- Load address: 0x100000 115
- => printenv addip 115
- /proc on /proc type proc (rw) 116
- FAT File System 121
- 9.7. Overlay File Systems 126
- What it is good for? 127
- Examples 127
- # insmod mini_fo.o 128
- Performance overhead 128
- Further information 129
- 9.8.1. Mount Parameters 129
- 9.8.2. Example 129
- 10. Debugging 130
- 10.1. Debugging of U-Boot 130
- 10.2. Linux Kernel Debugging 131
- 10.4. Tips and Tricks 135
- 10.5. Application Debugging 135
- 10.5.2. Remote Debugging 136
- 12.1. Application Notes 137
- 12.2. Further Reading 137
- 12.2.2. Linux kernel 138
- Articles 138
- Standards: 139
- 12.2.4. Network Programming 139
- 12.2.10. Embedded Topics 141
- 12.3. Mailing Lists 142
- 12.4. Links 142
- 12.5. Tools 143
- 13. Appendix 143
- 13.1. Flat Device Tree 144 144
- 13.1. Flat Device Tree 145 145
- 13.1. Flat Device Tree 146 146
- ELDK Installation Hangs 151
- 14.1.11. Enable SSH Access 158
- 14.2. U-Boot 159
- -mrelocatable 160
- Compiler 164
- 14.2.12. Erasing Flash Fails 165
- $ make tools/gen_eth_addr 166
- $ tools/gen_eth_addr 166
- 14.2.17.2. Hush shell 169
- 14.2.17.3. Hush shell scripts 170
- 14.2.17.4. General rules 171
- => setenv clocks_in_mhz 1 173
- => saveenv 173
- => setenv clocks_in_mhz 173
- Rebooting in 1 seconds 178
- # cat /etc/blank_off 181
- # cat /etc/init_tty 181
- Multi-File Image 183
- 14.3.26.1. XIP Kernel 191
- 14.3.26.2. Cramfs Filesystem 192
- 14.3.26.3. Hints and Notes 193
- 14.3.30. Swapping over NFS 196
- 14.4. Self 198
- 14.5. RTAI 200
- 14.6. BDI2000 201
- 14.7. Motorola LITE5200 Board 203
- 15. Glossary 204
- YellowDog 210
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