command line. Here is an example of a kernel command line for a popular ARM-based reference board containing the TI OMAP 5912 processor.
console=ttyS0,115200 root=/dev/nfs
nfsroot=192.168.1.9:/home/chris/sandbox/omap-target
initrd=0x10800000,0x14af47
The previous kernel command line has been separated into several lines to fit in the space provided. In actual practice, it is a single line, with the individual elements separated by spaces. This kernel command line defines the following kernel behavior:
• Specify a single console on device ttyS0 at 115 kilobaud
• Mount a root file system via NFS, the network file system
• Find the NFS root file system on host 192.168.1.9 (from directory /home/chris/sandbox/omap-target)
• Load and mount an initial ramdisk from physical memory location 0x10800000, which has a size of 0x14AF47 (1,355,591 bytes)
One additional note regarding this example: Almost universally, the initrd image is compressed. The size specified on the kernel command line is the size of the compressed image.
6.4.3. Bootloader Support for initrd
Let's look at a simple example based on the popular U-Boot bootloader running on an ARM processor. This bootloader has been designed with Linux kernel support. Using U-Boot, it is easy to include an initrd image with the kernel image. Listing 6-10 examines a typical boot sequence containing an initial ramdisk image.
Listing 6-10. Booting Kernel with Ramdisk Support
# tftpboot 0x10000000 kernel-uImage
...
Load address: 0x10000000
Loading: ############################ done
Bytes transferred = 1069092 (105024 hex)
# tftpboot 0x10800000 initrd-uboot
...
Load address: 0x10800000
Loading: ########################################### done
Bytes transferred = 282575 (44fcf hex)
# bootm 0x10000000 0x10800040
Uncompressing kernel.................done.
...
RAMDISK driver initialized: 16 RAM disks of 16384K size 1024 blocksize
...
RAMDISK: Compressed image found at block 0
VFS: Mounted root (ext2 filesystem).
Greetings: this is linuxrc from Initial RAMDisk
Mounting /proc filesystem
BusyBox v1.00 (2005.03.14-16:37+0000) Built-in shell (ash)
Enter 'help' for a list of built-in commands.
# (<<<< Busybox command prompt)
Here in Listing 6-10, we get a glimpse of the U-Boot bootloader, which we examine in more detail in the next chapter. The tftpboot command causes U-Boot to download the kernel image from a tftp server. The kernel image is downloaded and placed into the base of this target system's memory at the 256MB address (0x10000000 hex[53]). Then a second image, the initial ramdisk image, is downloaded from a tftp server into memory at a higher memory address (256MB + 8MB, in this example). Finally, we issue the U-Boot bootm command, which is the 'boot from memory' command. The bootm command takes two arguments: the address of the Linux kernel image, optionally followed by an address representing the location of the initial ramdisk image.
Take special note of one feature of the U-Boot bootloader. It fully supports loading kernel and ramdisk images over an Ethernet connection. This is a very useful development configuration. You can get a kernel and ramdisk image onto your board in other ways as well. You can flash them into your Flash memory using a hardware-based flash programming tool, or you can use a serial port and download the kernel and file system images via RS-232. However, because these images are typically large (a kernel can be about a megabyte, and a ramdisk can be tens of megabytes), you will save a significant amount of engineering time if you invest in this Ethernet-based tftp download method. Whatever bootloader you choose, make sure it supports network download of development images.
6.4.4. initrd Magic: linuxrc
When the kernel boots, it detects the presence of the initrd image, and copies the compressed binary file from the specified physical location in RAM into a proper kernel ramdisk and mounts it as the root file system. The magic of the initrd comes from the contents of a special file within the initrd image. When the kernel mounts the initial ramdisk, it looks for a specific file called linuxrc. It treats this file as a script file and proceeds to execute the commands contained therein. This mechanism enables the system designer to specify the behavior of initrd. Listing 6-11 contains a sample linuxrc file.
Listing 6-11. Example linuxrc File
#!/bin/sh
echo 'Greetings: this is 'linuxrc' from Initial Ramdisk'
echo 'Mounting /proc filesystem'
mount -t proc /proc /proc
busybox sh
In practice, this file would contain directives required before we mount the real root file system. One example might be to load CompactFlash drivers to obtain a real root file system from a CompactFlash device. For purposes of this example, we simply spawn a busybox shell and halt the boot process for examination. You can see the # command prompt from Listing 6-10 resulting from this busybox shell. If one were to type the exit command here, the kernel would continue its boot process until complete.
After the kernel copies the ramdisk from physical memory into a kernel ramdisk, it returns this physical memory back to the available memory pool. You can think of this as transferring the initrd image from physical memory at the hard-coded address into the kernel's own virtual memory (in the form of a kernel ramdisk device).
One last comment about Listing 6-11: The mount command in which the /proc file system is mounted seems redundant in its use of the word proc. This command would also work:
mount -t proc none /proc
