What is CONFIG_INITRAMFS_SOURCE ? How can I use it to update my Debian GNU/Linux system kernel to 2.6.30 ?
You cannot "use it" to update your kernel. It is merely one kernel configuration option among many.
The CONFIG_INITRAMFS_SOURCE option is for providing a text file (during the kernel build procedure) which lists files and directories to be included _inside_ the kernel binary image. This makes it possible to provide files to the kernel before ordinary disk filesystems have been mounted.
Ordinarily, that is the purpose of an initial ramdisk ("initrd") -- an archive file loaded by the boot manager (grub, lilo, etc.) during the boot process allowing access to files (usually kernel modules) needed to complete the boot process. Packing an initramfs archive directly into the kernel binary makes it possible to avoid using an initrd -- for example, if you only have a small number of files that must be available during boot and you didn't want the initrd baggage. I use an initramfs archive for exactly this purpose: I compile my own kernels and normally would not need an initrd, but I need a small userspace binary (v86d) that a 64-bit kernel can use as a helper to set video modes through a 32-bit BIOS. I just let the kernel know about the list of files I want packed with the kernel (CONFIG_INITRAMFS_SOURCE) and make sure that those files are actually present and, bingo, no need for an initrd!
You can read about this in the documentation provided with the kernel sources. Take a look at this file:
<path-to-kernel-source>/Documentation/filesystems/ramfs-rootfs-initramfs.txt HTH, Dave W.