Because it is 35$ ARM platform, possible working (who knows . Please advise me another cost-effective solution for command&control of FPGA boards.
I have several linux-based dev.boards on my desktop now (TI OMAP, Atmel's AT91RM9200, Atheros MIPS) but they all are over-sized and cost more than $300 each. 

Not a good idea in my opinion.

RS-485 is the same old-school as RS-232, it is slow (115200), unreliable, etc, but it may be kilometers long (at 2400 baud, this option definitely not for Icarus
CAN is definitely faster and reliable but is very complicated, I've worked with CAN node programming about a decade ago and it was a hell.

And both are not supported by Raspberry Pi.

All we need - fast synchronous addressed shift register (SPI). No overhead to FPGA design, easy to implement and debug, also we will reduce overall rig stale rate due to fast syncronous data exchange between master CPU and FPGAs.

And Raspberry PI has 3 SPI ports (I hope it is true).

For large rig with 12-16 boards packaged in the suitable case, it will be better to use two big fans instead of dozen smaller ones. Their rotation speed (and thus consumed power and noise level) may be adjusted by cgminer depending on highest temperature readout of all FPGAs (as cgminer does for GPU).
So please consider FPGA temperature readout in your future design if possible, regardless of sensor type you will use.

Will be very nice if you support SPI in the future bitstream.
I'm going to start developing such carrier board, but we should agree with daisy-chaining protocol details and pinout first.

The idea is to use SPI for connecting Icarus boards inside the rig (not just temp sensors). SPI will be used instead of USB, it is easy to implement on FPGA and fast serial interface. For individual use USB will remain (interface selection can be done by user with DIP switch).

Hi ngzhang,

I've heard you are developing next hardware revision of Icarus board.
Below is my 0.02$, comments are welcome.

One necessary thing is FPGA temperature sensors. For example TI digital sensors with serial output will be OK but I have no idea how to mechanically put them to the chips (why FPGA manufacturer didn't place one directly to the die?)

Second idea (which can be easily implemented with current revision boards btw) is simple but fast SPI interface using some DIMM connector pins. SPI address and SPI/USB interface selection can be done with existing DIP switches. The goal is to develop neat carrier board with 12-16 Icarus slots, ARM/MIPS/ATOM/etc CPU board slot and good 250-350W PSU behind. We can install several big 200mm fans for the whole rack and adjust their rotation speed according to temperature sensors output.

+3