The minidriver program requires a space to buffer the received hardware data that is later passed to the full driver at process time. You will need to determine the amount of data you require and allocate the memory.
You have the choice of specifying where the memory is located or having startup choose a safe location. In order to allocate the memory, you make the following function call:
paddr_t alloc_ram(phys_addr, size, alignment);
The address returned is the physical address of your memory area. This is used when you register your minidriver with the startup program. Since you are reserving an area of RAM, make sure you call this function after RAM has been setup, i.e. after calling init_raminfo().
This area of memory isn't internally managed by the system, it's your drivers responsibility to make sure it doesn't overwrite system memory and cause the startup to crash.