But I am afraid, I have to dig into linux quite deep
That scares me too
So I was very happy to find something that is not Linux.
You have not said what you are using the FPGA for, do you need Linux?
If so, then that kernel driver will give you clues.
If you make your FPGA look like a NAND flash it might be one way.
Gert's IDE/SMI interface is similar to NAND flash/compact flash...
The Motorola 6800 interface or Intel 8080 interface are both old, well known to grumpy old guys like me.
These were common to some things like LCDs but at slower rates.
I really need to dig into that 125MHz clock option for pio, that sounds way too fast.
There must be wait and divide by clock cycles or something.
From the manual, the GPCLKs can go to 125MHz but at 1.2V, but I don't think GPIO can go that fast.
Two or three wait cycles?
62.5 and 41.67MHz do seem close to the bitbang results.
125MHz at 1.2V is consistent with 62.5MHz at 3.3V swing.
As far as I can tell the GPCLK pins are just normal GPIO with ALT functions.
New silicon chip in the Pi3B+, it is a flip chip with a mirrored die, what this means it there are no internal bond wires.
Shorter wires in RF terms means higher freqs.
It would be interesting to compare GPCLK0 output on new 3B+ to old 3B on a cro to see what the signal levels and waveshape looks like.
At 40um and flipped die the 3B+ should have the fastest I/O until 28um or less versions come online in 20xx?