88E1111 Marvell PHY Chip Clock

This isn't the best board to start learning low level Ethernet with. The 88E1111 is a very flexible chip with a lot of settings that need to be correct before it will work. On top of that, the documentation is only available by contacting Marvell and signing an NDA. Since this is an old chip, and you aren't developing a product that will sell lots of their chips, I doubt they would let you have the documentation. But you can always try. You might also find some clues on the web.

The only demo for the DE0-115 that uses Ethernet is written for MicroC os-ii which is another commercial product, so it's not available unless you purchase it. So you won't even have working source to use as a place to start.

In answer to your questions. Yes either MII or RGMII will work. You won't receive anything unless the 88E1111 has been initialized. I think there are 8-10 settings needed. The Ethernet MAC in the FPGA takes care of monitoring CRS and receiving data each clock. I think there are tutorials available on how to set up the Altera Ethernet. It's fairly involved. Your software needs to have receive buffers available. The data will be coming in via DMA.

My advice would be to use a different board unless your goal is understanding Ethernet. One based on SoC would be nice. You can have Linux take care of Ethernet in the kernel.

If you do manage to get Ethernet going on the DE2-115 board without the Marvell docs and MicroC source, congratulations. This is not a project for someone new to embedded design.