> When you say, extremely early boot, it is device configuration?? 

Yes (well, at least the parts of device config which run before
interrupts are enabled and the process scheduler is started).

On some platforms (i386 comes to mind) there are dependancies which
make it difficult to correctly initialize thread state before certain
key drivers have been attached.

> And I assume that the kthread_run_deferred_queue() is the one which calls all
> these call-back functions. Am I right?

Yes.