Linux Kernel Booting Approach Pre built rootfs - linux-kernel

Question

I am learning the linux kernel booting process and trying to install linux on my beagleboard xM. I came across two approach both using the SD card.
1. Have the MLO, initrd, uboot.bin and uImage in one partition.
2. Have the MLO, uboot.bin and uImage in one partition and a pre-built rootfs (Angstrom) in 2nd partition.
In the first approach how is the initrd transformed into a complete file system on the 2nd partition. And what happens internally when uboot extracts the kernel from uImage and paste it on the 2nd partition. which directories would get modified of the init rootfs. How the init process of the kernel is called.

Answer 1

In general, the booting sequence in the Beagleboard-XM is something like this :
Firmware -> MLO -> uboot.bin -> uImage -> initrd(optional) -> rootfs
The initrd itself is a simple RAM based file system. It is generally used to mount the actual file system. Some of the modules which are required to mount the rootfs are bundled up into the initrd. Once the rootfs is mounted, the initramfs is cleaned up. If the features that are required to mount the rootfs is built into the kernel (uImage) itself, then there is no need of initrd. As I said earlier, initrd is a simple RAM based file system. That means if you use this as your filesystem, anything you write onto it will not be preserved. A few embedded applications actually use this as their rootfs.
Now answering to your question,
How is the initrd transformed into a complete file system on the 2nd partition?
Initially u-boot loads the kernel(uImage), initrd onto RAM and pass the appropriate the command line arguments to the kernel. Once the u-boot transfers the control over to the kernel, it first initializes the drivers and other modules and finally looks for initrd. The initrd is generally put as a cpio image. The kernel then uncompresses it and looks for 'init'. The init processes few things and then mounts the rootfs. There is no such thing as 2nd partition when it comes to initrd. It resides on the RAM.
What happens internally when uboot extracts the kernel from uImage and paste it on the 2nd partition?
Again, there is no such things as 2nd partition when it comes to uImage. uImage is a format which is understood by u-boot. The u-boot first loads the uImage onto the RAM and before actually passing the control over to the kernel, it extracts the contents from uImage, moves the binary to a predefined location in the RAM and then passes the control over to the kernel. Similar to initrd, the kernel also resides on the RAM itself.
Which directories would get modified of the init rootfs?
No directories gets modifies. But yes, a few of them are mounted, like procfs, sysfs, etc.
How the init process of the kernel is called?
Please refer to the kernel_init() function under init/main.c to see how the kernel executes init (refer http://lxr.free-electrons.com/source/init/main.c#L871).
I am attaching a simple code snippet from it :
if (!try_to_run_init_process("/sbin/init") ||
!try_to_run_init_process("/etc/init") ||
!try_to_run_init_process("/bin/init") ||
!try_to_run_init_process("/bin/sh"))
return 0;
panic("No working init found. Try passing init= option to kernel. "
"See Linux Documentation/init.txt for guidance.");
Therefore, the kernel searches in these default paths for the init binary. If it is not found, the kernel panics!
Hope this helps.