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Creating a root filesystem from scratch

To create a root file system image for storage in NAND flash, we use the mkfs.jffs2 program contained in the Debian package mtd-tools.

mkfs.jffs2 --pad=0x700000 -o rootfs.jffs2 -e 0x4000 -n -d/tmp/jffsroot/

is a typical command. It creates a FS image for our erase block size of 16k, padded to 0x700000 bytes in length from the files contained in the /tmp/jffsroot directory and stores the FS image in rootfs.jffs2.

Using pre-created root fs image

Prebuilt

See Repositories and update the list if you have more information.

OpenEmbedded

OpenEmbedded will itself create a jffs2 image at the end of the build process.

Once the build server is running, it will create root filesystem images for us.

demo JFFS2 root image

Check the Flashing openmoko page for pre-built root images. Please note that this image may take long to boot the first time, since this boot finishes the installation process.

Mounting the JFFS2 image on a loop back device (Block Memory MTD Emulation)

Configure the kernel for support

For a 2.6.25 kernel:

• Device Drivers
  • Block Devices
    • Loopback device support
  • Memory Technology Device (MTD) support
    • Caching block device access to MTD device (mtdblock module, required for block2mtd)
    • Self-contained MTD device drivers
      • MTD using block device (block2mtd)
• File systems
  • Miscellaneous filesystems
    • Journalling Flash File System v2 (JFFS2) support
      • Advanced compression for JFFS2
        • JFFS2 ZLIB compression support

Mount on loopback device

losetup is an application to manage loop devices, if you have the "losetup -f" option (for instance debian etch do not has such option) you can use the following commands:

export loop=$(losetup -f)
losetup $loop <rootfs.jffs2>
modprobe block2mtd block2mtd=$loop,131072
modprobe jffs2
modprobe mtdblock
mkdir <mountpoint>
mount -t jffs2 -o ro /dev/mtdblock0 <mountpoint>

"losetup -f" just returns the first unused loop device, if your system do not comes with "loosetup -f" just chose the first /dev/loop* device that is unused and change the first line with:

export loop=/dev/loop0

if loop0 is unused

Based on this script by fanoush.

The following is an alternate way of doing the above.

modprobe mtdcore
modprobe jffs2
modprobe mtdram total_size=59024  # default is 4 MByte - set to max. available size
modprobe mtdchar
modprobe mtdblock
dd if=<rootfs.jffs2> of=/dev/mtd0
mount -t jffs2 /dev/mtdblock0 <mountpoint>

Mounting the JFFS2 image on a loop back device (Kernel Memory MTD Emulation)

As mounting the JFFS2 image from /dev/loop0 device is not supported, use the mtdram device as described in http://www.handhelds.org/hypermail/familiar/62/6232.html

(On Fedora the first module to load is mtd, not mtdcore and you need to set e.g. vmalloc=256m as a kernel boot parameter for this to work.)

modprobe mtdcore
modprobe jffs2
modprobe mtdram total_size=59024  # default is 4 MByte - set to max. available size
modprobe mtdchar
modprobe mtdblock

Using total_size=59024 will set the filesystem size to the available space on /dev/mtd4 of the GTA1 device. So when you copy the image back it will be about 58 MB. You can set this to smaller values though.

Be careful to really use the mtd-ram-disk (maybe there is a real Flash-device somewhere in your development system)

So check for that looking into /proc/mtd:

cat /proc/mtd 
# dev:    size   erasesize  name
# mtd0: 039a4000 00020000 "mtdram test device"

Then write the image to that device and mount it:

dd if=<rootfs.jffs2> of=/dev/mtd0
mount -t jffs2 /dev/mtdblock0 <mountpoint>

The image is an Little Endian jffs2 image. To mount it on Big Endian architectures (like PowerPC) it'll have to be converted:

jffs2dump -l -c -e <rootfs.jffs2.be> <rootfs.jffs2> # only needed on non little endian archs

create the new image

To get the changed image back there are two ways. The obvious one is to unmount it and copy the data back from the MTD block device (the image will be 58 MB):

umount /dev/mtdblock0
dd if=/dev/mtdblock0 of=<rootfs.jffs2>

You can also create a new image using the mounted file system and the method described above (the image will have the size of its data):

mkfs.jffs2 --pad=0x700000 -o new_rootfs.jffs2 -e 0x4000 -n -d <mountpoint>

Flashing a root filesystem into NAND

See U-boot#Writing_rootfs_to_NAND

Converting a GTA02 image for GTA01

The techniques on this page may be used to convert a GTA02 (Freerunner) image for use on a GTA01, provided that it is small enough to fit into the GTA01's flash.

The steps I used are shown below (corrections welcome). Please read all of this page before executing these commands, in particular the warning to make sure that /dev/mtd0 does not map to a real flash device on your system.

modprobe jffs2
modprobe mtdchar
modprobe mtdblock
modprobe mtdram erase_size=128 total_size=262144
dd if=gta02_image.jffs2 of=/dev/mtd0
mount -t jffs2 -o ro /dev/mtdblock0 /mnt
mkfs.jffs2 --little-endian -e 16 -x lzo --pad -n -r /mnt -o for_gta01.jffs2

To clean up when you're done:

umount /mnt
rmmod mtdram

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