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

6.4.4. initrd Magic: linuxrc

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

Notice that the device field of the mount command has been changed to none. The mount command ignores the device field because no physical device is associated with the proc file system. The -t proc is enough to instruct mount to mount the /proc file system on the /proc mount point. I use the former invocation as a mental reminder that we are actually mounting the kernel pseudo device (the /proc file system) on /proc. The mount command ignores this argument. Use the method that you prefer.

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