dev->hard_header ?

In Layer 3 Protocols(eg. IP, IPv6, ...),
they calls dev_queue_xmit() to send their data.

Before calling dev_queue_xmit(),
does the layer 3 protocols always call dev->hard_header ?

Actually, I'm not sure when dev->hard_header is called.

I think dev->hard_start_xmit can include the part of dev->hard_header.
Why did the developer seperate the header building and sending frame
fucntion ?

thanks in advance.