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9.5.6. Root File System in a Read-Only File in a FAT File System

This is a more complicated example that shows that - depending on project requirements - many other alternatives for chosing a root file system for your embedded system exist.

The szenario is as follows: on your embedded device you use a cheap and popular storage medium like CompactFlash, MMC or SD cards or USB memory sticks to store both the Linux kernel and your root file system. You want to distribute software updates over the internet: your customers can download the file from your web site, or you sent the images by email. Your customers may use any flash card or memory stick they happen to find, so you have no information about brand or size of the storage device.

Unfortunately most of your customers use Windows systems. And they don't want to be bothered with long instructions how to create special partitions on the storage device or how to write binary images or things like that. A simple "copy file" operation is nearly exhausting their capabilities.

What to do? Well, if copying a file is all your customers can do we should not ask for more. Storage devices like CompactFlash cards etc. typically come with a single partition on it, which holds a FAT or VFAT file system. This cannot be used as a Linux root file system directly, so we have to use some trickery.

Here is one possible solution: Your software distribution consistes of two files: The first file is the Linux kernel with a minimal ramdisk image attached (using the multi-file image format for U-Boot); U-Boot can load and boot such files from a FAT or VFAT file system. The second file is your root file system. For convenience and speed we use again an image of an ext2 file system. When Linux boots, it will initially use the attached ramdisk as root file system. The programs in this ramdisk will mount the FAT or VFAT file system - read-only. Then we can use a loop device (see losetup(8)) to associate the root file system image with a block device which can be used as a mount point. And finally we use pivot_root(8) to change the root file system to our image on the CF card.

This sounds not so complicated, and actually it is quite simple once you understand what needs to be done. Here is a more detailed description:

  1. The root file system image is easy: as mantioned before, we will use an ext2 file system image, and to avoid wearing the flash storage device we will use it in read-only mode - we did a read-only ext2 root file system image before, and here we can just re-use the existing image file.
  2. The initial ramdisk image that performs the pivot_root step must be created from scratch, but we already know how to create ramdisk images, so we just have to figure out what to put in it.

    The most important tool here is nash, a script interpreter that was specifically designed for such purposes (see nash(8)). We don't need any additional tools, and if we use static linking, that the nash binary plus a small script to control it is all we need for our initial ramdisk.

    To be precise, we need a couple of (empty) directories (bin, dev, etc, lib, loopfs, mnt, proc, and sysroot), the bin/nash binary, the linuxrc script and a symbolic link sbin pointing to bin:
    drwxr-xr-x    2 wd       users        4096 Apr 13 01:11 bin
    -rwxr-xr-x    1 wd       users      469512 Apr 11 22:47 bin/nash
    drwxr-xr-x    2 wd       users        4096 Apr 12 00:04 dev
    drwxr-xr-x    2 wd       users        4096 Apr 12 00:04 etc
    drwxr-xr-x    2 wd       users        4096 Apr 12 00:04 lib
    -rwxr-xr-x    1 wd       users         511 Apr 13 01:28 linuxrc
    drwxr-xr-x    2 wd       users        4096 Apr 12 00:04 loopfs
    drwxr-xr-x    2 wd       users        4096 Apr 12 00:09 mnt
    drwxr-xr-x    2 wd       users        4096 Apr 12 00:04 proc
    lrwxrwxrwx    1 wd       users           3 Jun 12 18:54 sbin -> bin
    drwxr-xr-x    2 wd       users        4096 Apr 12 00:04 sysroot
  3. We also need only a minimal device table for creating the initial ramdisk:
    #<name>    <type> <mode> <uid> <gid> <major> <minor> <start>  <inc>  <count>
    /dev            d  755  0       0       -       -       -       -       -
    /dev/console    c  640  0       0        5      1       -       -       -
    /dev/hda        b  640  0       0        3      0       -       -       -
    /dev/hda        b  640  0       0        3      1       1       1       8
    /dev/loop       b  640  0       0        7      0       0       1       4
    /dev/null       c  640  0       0        1      3       -       -       -
    /dev/ram        b  640  0       0        1      0       0       1       2
    /dev/ram        b  640  0       0        1      1       -       -       -
    /dev/tty        c  640  0       0        4      0       0       1       4
    /dev/tty        c  640  0       0        5      0       -       -       -
    /dev/ttyS       c  640  0       0        4      64      0       1       4
    /dev/zero       c  640  0       0        1      5       -       -       -
  4. To create the initial ramdisk we perform the usual steps:
    $ INITRD_DIR=initrd
    $ INITRD_SIZE=490
    $ INITRD_IMAGE=initrd.img
    $ genext2fs -U \
            -d ${INITRD_DIR} \
            -D ${INITRD_DEVICES} \
            -b ${INITRD_SIZE} \
            -r ${INITRD_FREE} \
            -i ${INITRD_INODES} \
    $ gzip -v9 ${INITRD_IMAGE}
    The result is a really small (233 kB) compressed ramdisk image.
  5. Assuming you already have your Linux kernel image, you can now use mkimage to build an U-Boot multi-file image that combines the Linux kernel and the initial ramdisk:
    $ LINUX_KERNEL=linuxppc_2_4_devel/arch/ppc/boot/images/vmlinux.gz
    $ mkimage -A ppc -O Linux -T multi -C gzip \
    > -n 'Linux with Pivot Root Helper' \
    > -d ${LINUX_KERNEL}:${INITRD_IMAGE}.gz linux.img
    Image Name:   Linux with Pivot Root Helper
    Created:      Mon Jun 13 01:48:11 2005
    Image Type:   PowerPC Linux Multi-File Image (gzip compressed)
    Data Size:    1020665 Bytes = 996.74 kB = 0.97 MB
    Load Address: 0x00000000
    Entry Point:  0x00000000
       Image 0:   782219 Bytes =  763 kB = 0 MB
       Image 1:   238433 Bytes =  232 kB = 0 MB
    The newly created file linux.img is the second image we have to copy to the CF card.

    We are done.

But wait - one essential part was not mentioned yet: the linuxrc script in our initial ramdisk image which contains all the magic. This script is quite simple:


echo Mounting /proc filesystem
mount -t proc /proc /proc

echo Creating block devices
mkdevices /dev

echo Creating root device
mkrootdev /dev/root
echo 0x0100 > /proc/sys/kernel/real-root-dev

echo Mounting flash card
mount -o noatime -t vfat /dev/hda1 /mnt

echo losetup for filesystem image
losetup /dev/loop0 /mnt/rootfs.img

echo Mounting root filesystem image
mount -o defaults --ro -t ext2 /dev/loop0 /sysroot

echo Running pivot_root
pivot_root /sysroot /sysroot/initrd
umount /initrd/proc
Let's go though it step by step: There is one tiny flaw in this method: since we mount the CF card on a directory in the ramdisk to be able to access to root file system image. This means that we cannot unmount the CF card, which in turn prevents us from freeing the space for the inital ramdisk. The consequence is that you permanently lose approx. 450 kB of RAM for the ramdisk. [We could of course re-use this ramdisk space for temporary data, but such optimization is beyond the scope of this document.]

And how does this work on our target?

  1. First we copy the two images to the CF card; we do this on the target under Linux:
    bash-2.05b# fdisk -l /dev/hda
    Disk /dev/hda: 256 MB, 256376832 bytes
    16 heads, 32 sectors/track, 978 cylinders
    Units = cylinders of 512 * 512 = 262144 bytes
       Device Boot    Start       End    Blocks   Id  System
    /dev/hda1   *         1       978    250352    6  FAT16
    bash-2.05b# mkfs.vfat /dev/hda1
    mkfs.vfat 2.8 (28 Feb 2001)
    bash-2.05b# mount -t vfat /dev/hda1 /mnt
    bash-2.05b# cp -v linux.img rootfs.img /mnt/
    `linux.img' -> `/mnt/linux.img'
    `rootfs.img' -> `/mnt/rootfs.img'
    bash-2.05b# ls -l /mnt
    total 4700
    -rwxr--r--    1 root     root      1020729 Jun 14 05:36 linux.img
    -rwxr--r--    1 root     root      3788800 Jun 14 05:36 rootfs.img
    bash-2.05b# umount /mnt
  2. We now prepare U-Boot to load the "uMulti" file (combined Linux kernel and initial ramdisk) from the CF card and boot it:
    => setenv fat_args setenv bootargs rw
    => setenv fat_boot 'run fat_args addip;fatload ide 0:1 200000 linux.img;bootm'
    => setenv bootcmd run fat_boot
  3. And finally we try it out:
    U-Boot 1.1.3 (Jun 13 2005 - 02:24:00)
    CPU:   XPC86xxxZPnnD4 at 50 MHz: 4 kB I-Cache 4 kB D-Cache FEC present
    Board: TQM860LDB0A3-T50.202
    DRAM:  16 MB
    FLASH:  8 MB
    In:    serial
    Out:   serial
    Err:   serial
    PCMCIA: 3.3V card found: Transcend    256M
                Fixed Disk Card
                IDE interface 
                [silicon] [unique] [single] [sleep] [standby] [idle] [low power]
    Bus 0: OK 
      Device 0: Model: Transcend    256M Firm: 1.1 Ser#: SSSC256M04Z27A25906T
                Type: Removable Hard Disk
                Capacity: 244.5 MB = 0.2 GB (500736 x 512)
    Type "run flash_nfs" to mount root filesystem over NFS
    Hit any key to stop autoboot:  0 
    reading linux.img
    1025657 bytes read
    ## Booting image at 00200000 ...
       Image Name:   Linux with Pivot Root Helper
       Created:      2005-06-13   0:32:41 UTC
       Image Type:   PowerPC Linux Multi-File Image (gzip compressed)
       Data Size:    1025593 Bytes = 1001.6 kB
       Load Address: 00000000
       Entry Point:  00000000
       Image 0:   787146 Bytes = 768.7 kB
       Image 1:   238433 Bytes = 232.8 kB
       Verifying Checksum ... OK
       Uncompressing Multi-File Image ... OK
       Loading Ramdisk to 00f3d000, end 00f77361 ... OK
    Linux version 2.4.25 (wd@xpert) (gcc version 3.3.3 (DENX ELDK 3.1.1 3.3.3-9)) #1 Mon Jun 13 02:32:10 MEST 2005
    On node 0 totalpages: 4096
    zone(0): 4096 pages.
    zone(1): 0 pages.
    zone(2): 0 pages.
    Kernel command line: rw ip= panic=1
    Decrementer Frequency = 187500000/60
    Calibrating delay loop... 49.86 BogoMIPS
    NET4: Unix domain sockets 1.0/SMP for Linux NET4.0.
    RAMDISK: Compressed image found at block 0
    Freeing initrd memory: 232k freed
    VFS: Mounted root (ext2 filesystem).
    Red Hat nash version 4.1.18 starting
    Mounting /proc filesystem
    Creating block devices
    Creating root device
    Mounting flash card
     hda: hda1
     hda: hda1
    losetup for filesystem image
    Mounting root filesystem image
    Running pivot_root
    Freeing unused kernel memory: 60k 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 ...

Kernel with a Flattened Device Tree Blob

When booting an arch/powerpc kernel that requires a flattened device tree blob, the above procedure must be slightly modified. Namely, the multi-image file has to include the blob as the thrid image. Here's an example of the mkimage command to create it:

mkimage -A ppc -O Linux -T multi -C gzip -n 'Kernel + Pivot Root Helper initrd + FDT blob' -d vmlinux.bin.gz:ramdisk_image.gz:canyonlands.dtb kernel+initrd+blob.img
Image Name:   Kernel + Pivot Root Helper initrd + FDT blob
Created:      Fri Sep 14 18:24:29 2007
Image Type:   PowerPC Linux Multi-File Image (gzip compressed)
Data Size:    2894576 Bytes = 2826.73 kB = 2.76 MB
Load Address: 0x00000000
Entry Point:  0x00000000
   Image 0:  1351205 Bytes = 1319 kB = 1 MB
   Image 1:  1531063 Bytes = 1495 kB = 1 MB
   Image 2:    12288 Bytes =   12 kB = 0 MB

The newly created file kernel+initrd+blob.img needs to be copied to the CF card.

9.5.5. Root File System on a Flash Card 1. Abstract 9.6. Root File System Selection
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