The rootfs is a squashfs image and my bootloader is loading it into some address in SDRAM. What parameters do I need to pass to the kernel so It can mount the rootfs from there? Squashfs support is built-in and it already works with
root=/dev/mtdblock2 rootfstype=squashfs
for booting from the flash.
EDIT: This is a MIPS based embedded device, using a custom bootloader. Normally, the bootloader extracts the compressed kernel from the flash into the SDRAM, and then kernel mounts /dev/mtdblock2 as the rootfs.
I am trying to improve the bootloader so it can download an image to its RAM and boot without writing to the flash.
I cannot figure out how to make Linux mount a filesystem image in the RAM as the rootfs.
Best Answer
I would use an initramfs. (http://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt)
Many Linux distributions use an initramfs (not to be confused with an initrd, they are different) during the boot process, mostly to be able to start userspace programs very early in the boot process. However, you can use it for whatever you want.
The benefit of an initramfs over an initrd is that an initramfs uses a tmpfs filesystem while an initrd uses a ram block device. The key difference here is that for an initrd, you must preallocate all the space for the filesystem, even if you're not going to use all that space. So if you don't use the filesystem space, you waste ram, which on an embedded device, is often a scarce resource. Tmpfs is a filesystem which runs out of ram, but only uses as much ram as is currently in use on the filesystem. So if you delete a file from a tmpfs, that ram is immediately freed up.
Now normally an initramfs is temporary, only used to run some programs extremely early in the boot process. After those programs run, control is turned over to the real filesystem running on a physical disk. However, you do not have to do that. There is nothing stopping you from running out of the initramfs indefinitely.