in realtek 8139cp driver code, where does the start_xmit function triggers the chip?

[Chan Kim]

Hello all,
Hope somebody could give me an explanation to this. I was looking at
http://lxr.free-electrons.com/source/drivers/net/ethernet/realtek/8139cp.c#L731
and found in cp_start_xmit(..) function, there are all descriptor parameter setting for the driver private data structure. Where exactly the code triggers the chip?
regards,
Chan Kim