SiLabs had distributed an appnote AN139, "USB Firmware Programmer's Guide".
This document explains well on the behavior of SiLabs SIE.
(Though I don't like the firmware code attached :-)
Unfortunately, SiLabs dropped this appnote from their web site.
But copies are still floating over the net.
Search "Silicon Laboratories" AN139
As of the SETUP transaction handling, the appnote describes it as follows.
Setup Transactions AN139.pdf p5
Setup packets are similar to OUT packets: hard- ware sets the OPRDY bit to '1' (to indicate a data packet ready in the FIFO), and generates an inter- rupt. There is no flag in hardware to indicate that received data in the FIFO is from a Setup packet (versus an OUT packet); firmware must keep state information that indicates what type of packet should be received next. I'm guessing it will be the same with there new uC the C8051f38x.
Maybe. SiLabs don't open its full datasheet yet.
Tsuneo