"What are you trying to do that you want SPI slave support for?"
"If you are going to bit bang SPI, why not use UART instead? Simpler to implement and Linux already has built in support for it. Are you thinking of a particular device that can only talk SPI?"
Ah ... we are getting into the long story . I'm hoping to use the Pi to reverse engineer an SPI interface.
My 'hobby' for the last while has been trying to get the code off a Tamagotchi so I can figure out how everything works. Modern Tamagotchis have contacts on the top that allow an SPI ROM to be attached (so you can play extra games on your Tamagotchi). I've dumped one of these ROMs, and I suspect it has functionality that might let me dump the code ROM, but I haven't been able to acquire a R/W chip that's compatible with the proprietary (GeneralPlus) ROM that the Tamagotchi uses. So I've been trying to use a MCU to 'simulate' this chip. I tried to us an Arduino, but it was just a hair too slow to do this. So I was thinking of using the Raspberry Pi for this, and hoping it might be useful for other reversing projects as well.
More info is here: http://www.kwartzlab.ca/author/natalies/
, but the short story is yes, it must be SPI slave, because the Tamagotchi is (only) a master, and I'm hoping for a lot of fine-grained control so I can match the master as best as possible.