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DENX . DULG . RootFileSystemInAReadOnlyFile
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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 scenario 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:
- 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.
- 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
- 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 - - -
- To create the initial ramdisk we perform the usual steps:
$ INITRD_DIR=initrd
$ INITRD_SIZE=490
$ INITRD_FREE=0
$ INITRD_INODES=54
$ INITRD_DEVICES=initrd_devices.tab
$ INITRD_IMAGE=initrd.img
$ genext2fs -U \
-d ${INITRD_DIR} \
-D ${INITRD_DEVICES} \
-b ${INITRD_SIZE} \
-r ${INITRD_FREE} \
-i ${INITRD_INODES} \
${INITRD_IMAGE}
$ gzip -v9 ${INITRD_IMAGE}
The result is a really small (233 kB) compressed ramdisk image.
- 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
Contents:
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:
#!/bin/nash
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:
- The first line says that it's a script file for the
/bin/nash
interpreter.
Note: even if this file looks like a shell script
it is NOT interpreted by a shell,
but by the nash interpreter.
For a complete list of available nash commands and their syntax
please refer to the manual page, nash(8).
- The first action is to mount the
/proc pseudo file system
which is needed to find out some required information.
- Then we create block device entries for all partitions
listed in
/proc/partitions (mkdevices command).
- In the next step a block device for our new root file system
is created (
mkrootdev command).
- Then we mount the CF card.
We assume that there is only a single partition on it (
/dev/hda1)
which is of type VFAT
(which also will work with FAT file systems).
These assumptions work fine with basicly all memory devices
used under Windows.
- We further assume that the file name of the
root file system image on the
CF card is
"rootfs.img" - this file now gets mounted
using a loop device (losetup and mount commands).
- Our file system image,
is now mounted on the
/sysroot directory.
In the last step we use pivot_root to make
this the new root file system.
- As a final cleanup we unmount the
/proc file system which
is not needed any more.
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?
- 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
- 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
- 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
Net: SCC ETHERNET, FEC ETHERNET [PRIME]
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
Contents:
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=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.
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
Contents:
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.
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