-The "USI" controller, using J405, can be used for a second SPI bus. That
-would let you talk to the AVR over SPI, running firmware that makes it act
-as an SPI slave, while letting either Linux or the AVR use the DataFlash.
-There are plenty of spare parport pins to wire this one up, such as:
+Or you could flash firmware making the AVR into an SPI slave (keeping the
+DataFlash in reset) and tweak the spi_butterfly driver to make it bind to
+the driver for your custom SPI-based protocol.
+
+The "USI" controller, using J405, can also be used for a second SPI bus.
+That would let you talk to the AVR using custom SPI-with-USI firmware,
+while letting either Linux or the AVR use the DataFlash. There are plenty
+of spare parport pins to wire this one up, such as: