Root File System on a JFFS2 File System

JFFS2 (Journalling Flash File System version 2) was specifically designed for use on flash memory devices in embedded systems. It is a log-structured file system which means that it is robust against loss of power, crashes or other unorderly shutdowns of the system ("robust" means that data that is just being written when the system goes down may be lost, but the file system itself does not get corrupted and the system can be rebootet without need for any kind of file system check).

Some of the advantages of using JFFS2 as root file system in embedded systems are:

• file system uses compression, thus making efficient use of flash memory
• log-structured file system, thus robust against unorderly shutdown
• flash sector wear-leveling
• writable flash file system

Disadvantages are:

• long mount times (especially older versions)
• slow when reading: files to be read get uncompressed on the fly which eats cycles and takes time
• slow when writing: files to be written get compressed, which eats cycles and takes time, but it may even take much longer until data gets actually stored in flash if the file system becomes full and blocks must be erased first or - even worse - if garbage collection becomes necessary
• The garbage collector thread may run at any time, consuming cycles and blocking accesses to the file system.

Despite the aforementioned disadvantages, systems using a JFFS2 based root file system are easy to build, make efficient use of the available resources and can run pretty reliably.

To create a JFFS2 based root file system please proceed as follows:

1. 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
   

2. We can now create a JFFS2 file system image using the mkfs.jffs2 tool:

   $ ROOTFS_DIR=rootfs                 # directory with root file system content
   $ ROOTFS_EBSIZE=0x20000             # erase block size of flash memory
   $ ROOTFS_ENDIAN=b                   # target system is big endian
   $ ROOTFS_DEVICES=rootfs_devices.tab # device description file
   $ ROOTFS_IMAGE=jffs2.img            # generated file system image
   
   $ mkfs.jffs2 -U \
           -d ${ROOTFS_DIR} \
           -D ${ROOTFS_DEVICES} \
           -${ROOTFS_ENDIAN} \
           -e ${ROOTFS_EBSIZE} \
           -o ${ROOTFS_IMAGE}
   mkfs.jffs2: skipping device_table entry '/dev': no parent directory!
   

Note: When you intend to write the JFFS2 file system image to a NAND flash device, you should also add the "-n" (or "--no-cleanmarkers") option, as cleanmarkers are not needed then.

When booting the Linux kernel prints the following messages showing the default partition map which is used for the flash memory on the TQM8xxL boards:

 TQM flash bank 0: Using static image partition definition
Creating 7 MTD partitions on "TQM8xxL0":
0x00000000-0x00040000 : "u-boot"
0x00040000-0x00100000 : "kernel"
0x00100000-0x00200000 : "user"
0x00200000-0x00400000 : "initrd"
0x00400000-0x00600000 : "cramfs"
0x00600000-0x00800000 : "jffs"
0x00400000-0x00800000 : "big_fs"

We use U-Boot to load and store the JFFS2 image into the last partition and set up the Linux boot arguments to use this as root device:

1. Erase flash:

   => era 40400000 407FFFFF
   
   ................. done
   Erased 35 sectors
   

2. Download JFFS2 image:

   => tftp 100000 /tftpboot/TQM860L/jffs2.img
   Using FEC ETHERNET device
   TFTP from server 192.168.3.1; our IP address is 192.168.3.80
   Filename '/tftpboot/TQM860L/jffs2.img'.
   Load address: 0x100000
   Loading: #################################################################
            #################################################################
            #################################################################
            #################################################################
            #################################################################
            #################################################################
            ########
   done
   Bytes transferred = 2033888 (1f08e0 hex)
   

3. Copy image to flash:

   => cp.b 100000 40400000 ${filesize}
   Copy to Flash... done
   

4. set up boot arguments to use flash partition 6 as root device:

   => setenv mtd_args setenv bootargs root=/dev/mtdblock6 rw rootfstype=jffs2
   => printenv addip
   addip=setenv bootargs ${bootargs} ip=${ipaddr}:${serverip}:${gatewayip}:${netmask}:${hostname}:${netdev}:off panic=1
   => setenv flash_mtd 'run mtd_args addip;bootm ${kernel_addr}'
   => run flash_mtd
   Using FEC ETHERNET device
   TFTP from server 192.168.3.1; our IP address is 192.168.3.80
   Filename '/tftpboot/TQM860L/uImage'.
   Load address: 0x200000
   Loading: #################################################################
            #################################################################
            ###########
   done
   Bytes transferred = 719233 (af981 hex)
   ## Booting image at 40040000 ...
      Image Name:   Linux-2.4.25
      Created:      2005-06-12  16:32:24 UTC
      Image Type:   PowerPC Linux Kernel Image (gzip compressed)
      Data Size:    782219 Bytes = 763.9 kB
      Load Address: 00000000
      Entry Point:  00000000
      Verifying Checksum ... OK
      Uncompressing Kernel Image ... OK
   Linux version 2.4.25 (wd@xpert) (gcc version 3.3.3 (DENX ELDK 3.1.1 3.3.3-9)) #1 Sun Jun 12 18:32:18 MEST 2005
   On node 0 totalpages: 4096
   zone(0): 4096 pages.
   zone(1): 0 pages.
   zone(2): 0 pages.
   Kernel command line: root=/dev/mtdblock6 rw rootfstype=jffs2 ip=192.168.3.80:192.168.3.1::255.255.255.0:tqm860l:eth1:off panic=1
   Decrementer Frequency = 187500000/60
   Calibrating delay loop... 49.86 BogoMIPS
   ...
   NET4: Unix domain sockets 1.0/SMP for Linux NET4.0.
   VFS: Mounted root (jffs2 filesystem).
   Freeing unused kernel memory: 56k init
   
   
   BusyBox v0.60.5 (2005.03.07-06:54+0000) Built-in shell (msh)
   Enter 'help' for a list of built-in commands.
   
   # ### Application running ...
   # mount
   rootfs on / type rootfs (rw)
   /dev/mtdblock6 on / type jffs2 (rw)
   /proc on /proc type proc (rw)
   # df /
   Filesystem           1k-blocks      Used Available Use% Mounted on
   rootfs                    4096      2372      1724  58% /