This is unfinished code (including modifications to the Vectrex
Interface library), for a "PiTrex Display" Linux kernel module.

The idea is to create a device file (/dev/pitrexvp0) that the
vector pipeline data is written to, which then allows for a
(hopefully) fast interrupt-driven drawing routine so that
game code can run and prepare the next frame while the current
frame is still being drawn on the Vectrex screen.

Initially the hrtimers system is being used to trigger interrupts
from the Pi/Linux's internal timer. Failing that the BCM2835's FIQ
"Fast Interrupt Request" system, usually used by the dwc_otg USB
driver (the dwc2 driver can be used instead), could be used to
trigger the interrupt from the IRQ signal generated by the 6522
VIA in the Vectrex. Failing that, a hardware solution could be
implemented where the PiTrex cartridge is set to write 
automatically after the required delay*.

It has been found that system interrupts will still need to be
disabled during parts of the data processing code for perfect
display without risk of lines/objects wobbling. Therefore the
system configuration steps (disabling HDMI, using the dwc2 USB
driver, CPU in performance mode) will still be needed. The
advantage therefore is purely in performance (and not needing
to run programs as root, but that's a very minor advantage in
the expected application, and requires more fuctionality from
the driver module than is currently shown).

While work on this code is suspended for now, advice or assistance,
especially from someone who actually knows what they're doing with
regards to the Linux kernel API, would be much welcomed!

Kevin Koster, Jan. 2021.

*A delay is required after reception of the IRQ signal from the
 6522 in order for correct positioning to be achieved, hence the
 original hardware solution of using IRQ_LATCH was unsuccessful
 (unless "optimisation" mode is disabled in the Vectrex Interface
 library and every line is positioned from the zero point).