Nios II - Data cache problem

sync() would be a unix system call to write all the disk buffer cache to disk.

If you are running in user mode, how are you mapping the required physical addresses into (process) virtual ones? That is usually only allowed in a device driver. That mapping operation should have an option to enable/disable the cache - put you may not have the desired control over it (even in the kernel).

To flush the data cache you need to execute the flushd or flushda instruction, you might need initda (initd is supervisor/kernel only). Not sure what is in which .h file for these...