when early_param("string", function_name) will be called?

Hi all, In kernel code at lot of places i came across "early_param("string", function_name)" Eg: early_param("vmalloc", early_vmalloc); in arch/arm/mm/mmu.c when this function will be called? and what is the significance of the string "vmalloc" there? Thank you.. -- With regards, Sandeep Kumar Anantapalli,