Create DMAable memory for a device on 32 bit xilinx board

I have a Xilinx 32 bit board. So, total address id 0x00000000 to 0xffffffff. I need to partition the address for 512 MB DMA use and need to set some address for GPIO port. I have below doubts regarding same. Please grant some suggestion on the same. 1. How much memory we should reserve for kernel and user application (starting from 0x00000000 till what ?). Is there any constraint to reserve specific amount of memory for kernel and user application use. 2. What region should be preferable to make DMAable memory which could be assigned to some device and loaded by device tree. 3. I need to have two GPIO ports what memory should be assigned to GPIO ports.

Any help over this highly appreciated. Please share some links related to same if you guys have. As, I am new to the field of device driver any small stuffs will be helpful for me.