This driver is a simplified version of altpciechdma.c (which lived for a while in the staging area of the Linux kernel, but was not maintained so it was removed), with all the complexity of chaining-DMA removed in favour of a simple direct-I/O (i.e no kernel involvement on the hot-path) scheme which exposes a bunch of FPGA registers and a pair of single-producer/single-consumer queues, in order to orchestrate data transfer between an FPGA and its host computer over PCI-Express.
This repo is not much use on its own; the intent is for it to be a git submodule of your project, along with some other submodules which implement the FPGA side of the system. See the altera-pcie repo for an example of how to do this.
The driver implements a couple of ioctl()s to initialize and control things, but the bulk of the real work happens in userspace, so the driver just implements mmap(). Note that it receives definitions of the queue sizes, etc from a defs.h file which is generated from a defs.vh file located in the application directory (e.g $PROJ_HOME/apps/demo/defs.vh). So before you can build the driver, you need to go there and (at the very least) run make defs.