The flush_cache_all() is empty, but it is still called by some function? And what's the difference between flush_cache_all() and __flush_cache_all()?

["Lennox Wu" <lennox.wu@xxxxxxxxx>]

Hi,
    I traced the cache operations of R3000 and R4000  (the Kernel version is 2.6.27).
I found the flush_cache_all() is empty, but it is still called by some function, e.g. remap_area_pages() .
I don't understand why the function is empty and what's the difference between flush_cache_all() and 
__flush_cache_all(). Does calling a empty flush_cache_all() make some problem ?
    
Thanks, 
Lennox