On my ARM system (Tegra based), I'm running the mainline linux kernel. It uses the device tree system.
I have enabled a hardware driver for the General-Memory-Bus (part of the SoC) in the .dts file by setting its status="okay". Recompiled the dtb and booted the kernel. But no device (/dev/xx) appears.
The driver is compiled into the kernel and can be seen by
cat /lib/modules/$(uname -r)/modules.builtin
The command
cat /sys/firmware/devicetree/base/<path to device>/status
returns "okay".
Do I need to make some kind of "mknod"? What else is nessesary?
The traditional UNIX "stream of bytes" device model is a pretty high-level abstraction of most modern hardware, and as such there are plenty of drivers which do not create /dev entries for the devices they control largely because they don't fit that model. Bus drivers in particular are very much a case of that - they exist, but only for the sake of discovering and allowing access to the devices behind them; there is no /dev/sata that lets you interact with the actual host controller, sending out raw commands on any old port regardless of what's connected or not; there is no /dev/usb that lets you attempt arbitrary transfers to arbitrary endpoints which may or may not exist.
Furthermore, your typical 'external interface' controller as in this case is orders of magnitude less complex than an interface like SATA or USB - the 'device' itself is often little more than a register block controlling some clocks and a chip-select multiplexer. Even if the driver did create something you could interact with directly, there's not exactly much you could do with it.
The correct way to proceed in this situation is to describe your FPGA device in the DT as a child of the GMI bus, accurately reflecting the hardware, no less, then develop your own driver for that. The bus driver itself just sits transparently in the middle. And if you do want a quick and dirty way to get started by just reading and writing bus addresses directly, well, it's behind a memory-mapped I/O region; that's exactly what /dev/mem exists for.