Design Issues and Solutions for Embedded Linux Systems

1. Shanker Balan Exocore Consulting Bangalore http://shankerbalan.com/ Design Issues and Solutions for Embedded Linux Systems

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 < 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 > ../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 of=ramdisk bs=1k count=2880 • “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/X11R6/bin:." && “hash -r” • Clean Up! “rm -rf /modules /static” • Create a nMB “tmpfs” RamDisk • mount -t tmpfs -o "size=10000k" 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/core+java” • “chroot /mnt/core+java /bin/sh” • “apt-get install jre && exit” • “make compressed” • TaDa!

21. Resources • http://www.debian.org Thank you!