14.4.1. How to Add Files to a SELF Ramdisk

It is not always necessary to rebuild a SELF based ramdisk image if you want to modify or to extend it. Especially during development it is often eaiser to unpack it, modify it, and re-pack it again. To do so, you have to understand the internal structure of the uRamdisk (resp. pRamdisk) images files as used with the U-Boot (old: PPCBoot) boot loader:

The uRamdisk image contains two parts:

• a 64 byte U-Boot header
• a (usually gzip compressed) ramdisk image

To modify the contents you have to extract, uncompress and mount the ramdisk image. This can be done as follows:

1. Extract compressed ramdisk image (ramdisk.gz)

   bash$ dd if=uRamdisk bs=64 skip=1 of=ramdisk.gz
   21876+1 records in
   21876+1 records out
   

2. Uncompress ramdisk image (if it was a compressed one)

   bash$ gunzip -v ramdisk.gz
   ramdisk.gz:      66.6% -- replaced with ramdisk
   

3. Mount ramdisk image

   bash# mount -o loop ramdisk /mnt/tmp
   

Now you can add, remove, or modify files in the /mnt/tmp directory. If you are done, you can re-pack the ramdisk into a U-Boot image:

1. Unmount ramdisk image:

   bash# umount /mnt/tmp
   

2. Compress ramdisk image

   bash$ gzip -v9 ramdisk
   ramdisk:         66.6% -- replaced with ramdisk.gz
   

3. Create new U-Boot image (new-uRamdisk)

   bash$ mkimage -T ramdisk -C gzip -n 'Simple Embedded Linux Framework' \
   > -d ramdisk.gz new-uRamdisk
   Image Name:   Simple Embedded Linux Framework
   Created:      Sun May  4 13:23:48 2003
   Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
   Data Size:    1400121 Bytes = 1367.31 kB = 1.34 MB
   Load Address: 0x00000000
   Entry Point:  0x00000000
   

Instead of re-packing into a U-boot ramdisk image you can of course also just extract the contents of the SELF image and re-use it as base of a (known to be working) root filesystem.

• For example, you can create a JFFS2 filesystem using the mkfs.jffs2 command that comes with the MTD Tools^?:

  bash# mkfs.jffs2 -r /mnt/tmp -e 0x10000 -o image.jffs2
  

• Or you can create a CramFS filesystem with mkcramfs:

  bash# mkcramfs -r /mnt/tmp image.cramfs
  Swapping filesystem endian-ness
  ...
  Everything: 1656 kilobytes
  Super block: 76 bytes
  CRC: 7f34cae4