Design Issues and Solutions for Embedded Linux Systems

1
Design Issues and Solutions for Embedded Linux
Systems

• Shanker Balan
• Exocore Consulting
• Bangalore
• http//shankerbalan.com/

2
Client Hardware

• Intel PIII/PIV Processor
• Mini-ITX Motherboard (VIA/Intel Chipset)
• 256MB SDRAM
• 128MB / 256MB DiskOnChip Storage Module
• AC97 Compatible Sound Chipset
• Conexant Soft Modem
• CDROM / DVD Drive
• USB Pen Drive

3
Feature Request

• Fully integrated and customisable desktop
  environment
• Java Runtime Environment
• Browser, Messenger and Word Processor
• Media Player
• File Manager with SMB Browsing Support
• XDMCP and RDP Support

4
The Big Plan

• Choose a distribution
• Find a suitable Compression method (cramfs,
  cloop)
• rootfs on ramdisk
• Build custom kernel and write bootup scripts
• Create a core distribution and build around it

5
Debian The Distribution Of Choice

• Mix of testing And unstable
• Testing for base packages (libs and tools)
• Unstable for user applications (GNOME)
• Advantages
• Powerful package management tools (dpkg, apt-get)
• Low cruft and minimalistic approach to package
  creation
• Customisable - dpkg reconfigure,
  /etc/alternatives etc
• Exhaustive package collection (8000)

6
Building the core

• Do 2 Debian installs on seperate partitions on
  the target hardware
• Regular install for a build environment with
  compiler
• Base only install for core
• Under build
• mount /mnt/core chroot /mnt/core /bin/sh
• apt-get update apt-get install foo
• Configure GRUB to boot build and core
• Boot into core for testing

7
Building Custom Kernel

• Target platform is x86, not much effort needed
• From build, cd /usr/src/linux/
• make menuconfig. Note ext2 and iso9660
  should not be modules
• Use make-kpkg to build kernel package and extra
  modules (if any)
• Build cloop as module
• Install new kernel in core and build
• dpkg -i custom_kernel.deb
• Boot into new kernel and test

8
Image Compression

• Compress core to lt size of flash device
• Cramfs has file size limitation. Use cloop
• apt-get install cloop-utils
• cd /mnt/core
• mkisofs -R -l -V "Debian -hide-rr-moved -v
  -exclude-list mkisofs.exclude .
  create_compressed_fs - 65536 gt ../CORE
• Test compressed image
• insmod cloop file/mnt/CORE
• mount -r /dev/cloop /mnt/cloop

9
Space Saved is Space Earned

• mkisofs.exclude
• /var/, /home, man pages, doc, unused wallpapers,
  themes etc
• Use localepurge to remove superfluous locale
  data
• Keep only misc and TrueType font packages

10
Make Flash Device Bootable

• Appears as a regular IDE drive
• mke2fs -m0 /dev/hdc1
• mount /dev/hdc1 /mnt/flash
• mkdir /mnt/flash/boot
• cp /vmlinuz /mnt/flash/boot/
• Install boot record using GRUB/LILO
• Configure splash screen for eye candy

11
The Ramdisk

• Ramdisk provides
• Provides a / like environment
• R/W /home area, /tmp and /var
• /linuxrc init script for setting up a suitable
  environment
• Mounts cloop image
• Create symlinks to /cloop for all binaries (/etc,
  /bin, /sbin/, /usr etc)
• Run init
• Transfer control to compressed Debian

12
Creating The Ramdisk

• dd if/dev/zero oframdisk bs1k count2880
• mke2fs -m0 -Fq -b 1024 -N 8192 ramdisk
• mkdir /mnt/ramdisk
• mount -o loop,rw ramdisk /mnt/ramdisk
• Copy static sh and linuxrc script, cloop module
  etc
• umount /mnt/ramdisk
• gzip -9 ramdisk

13
Things to put into the Ramdisk

• /dev entries, /static/sh,init (BusyBox),
  /modules/cloop.o
• /linuxrc
• Set PATH /static
• Mount proc and devpts
• Mount -t ext2 -r /dev/hdc1 /mnt/flash
• Insmod cloop and mount compressed CORE
  /compressed
• Copy the library cache ld.so.cache and set
  symlinks to /compressed

14
/linuxrc

• Set PATH to "/sbin/bin/usr/sbin/usr/bin/usr/X1
  1R6/bin." hash -r
• Clean Up! rm -rf /modules /static
• Create a nMB tmpfs RamDisk
• mount -t tmpfs -o "size10000k" ramdisk /ramdisk
• Point /home, /var etc to /ramdisk
• exit 0 to start init
• Init now follows /etc/inittab

15
RamDisk Gotchas

• Use small block size and high inode count when
  formatting
• Certain services wont work with symlinked /etc/.
  Use cp for those.
• Bigger ramdisk less free RAM

16
Automate! Automate! Automate!

• Create Makefile targets
• make kernel
• make ramdisk
• make boot
• make compressed ...

17
The Desktop Experience

• Desktop Environment
• GNOME2/Gtk2 or KDE3/Qt3?
• We chose GNOME2
• Clean desktop interface
• Usual eye candy
• Most of the apps (Gaim, Abiword) uses Gtk
• I am biased towards Gtk applications
• Gconf2 based framework makes setting user
  defaults very easy

18
USB HotPlug Support

• apt-get install hotplug
• Add Disk Mounter applet to GNOME2 Panel
• Clicking on the applet would mount the drive as
  vfat and open the file manager
• User uses the USB drive for permenant storage

19
The Final Product

• Original Size 500MB
• Compressed Size 90MB
• Kernel 2.4.22 Xfree86 4.3.0
• GNOME 2.4.1 Gaim 0.71
• Mozilla 1.5 Abiword 2.0.0
• Rdesktop
• core is now feature frozen

20
What Next?

• Value Add
• core Java
• core OpenOffice
• core Insert Application Here
• cp -r /mnt/core /mnt/corejava
• chroot /mnt/corejava /bin/sh
• apt-get install jre exit
• make compressed
• TaDa!

21
Resources

• http//www.debian.org
• Thank you!