Книга: Embedded Linux Primer: A Practical, Real-World Approach

6.4.2. Booting with initrd

6.4.2. Booting with initrd

To use the initrd functionality, the bootloader gets involved on most architectures to pass the initrd image to the kernel. A common scenario is that the bootloader loads a compressed kernel image into memory and then loads an initrd image into another section of available memory. In doing so, it becomes the bootloader's responsibility to pass the load address of the initrd image to the kernel before passing control to it. The exact mechanism differs depending on the architecture, bootloader, and platform implementation. However, the kernel must know where the initrd image is located so it can load it.

Some architectures and platforms construct a single composite binary image. This scheme is used when the bootloader does not have specific Linux support for loading initrd images. In this case, the kernel and initrd image are simply concatenated together. You will find reference to this type of composite image in the kernel makefiles as bootpImage. Presently, this is used only for arm architecture.

So how does the kernel know where to find the initrd image? Unless there is some special magic in the bootloader, it is usually sufficient simply to pass the initrd image start address and size to the kernel via the kernel 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.

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