Feature request: frame counter accessible from HPS

[tonytoon]

How feasible would it to be to update a region of memory accessible from the HPS every time a core generates vblank?

It could be implemented as a simple counter that is incremented with every vblank and rolls over if it overflows. This would provide the HPS side with a timer that self adjusts to the core's speed.

As an example: autofire could be rewritten to simply trigger every x frames. The input loop would check the assigned memory address and if the value changes, it knows it's a new frame and acts accordingly. The need to check the system clock would be removed and the resulting code would be more maintainable.

I am willing to rewrite the existing autofire code in input.cpp to take advantage of this if it was implemented. I've mocked up a version that tries to calculate frame counts based on video timings from video.cpp but I know that's probably not a good way to do it.

I can't personally think of other uses but any HPS code that would benefit from synchronizing with the core's video output would have a much easier pathway to do it.

I've not touched vhdl/verilog since college or I'd submit a sample implementation myself. If this is a reasonable feature but out prioritized by other issues I may take a stab at it myself as a learning exercise.

[sorgelig]

Writing to memory from FPGA is very costly operation. It's impossible to write like in normal computer. You have to reserve a time on memory bus and then write it my some module.
So, I don't see a reason to waste so much resources for autofire which is already working fine.

[birdybro]

Since the hdmi_config stuff was moved to Main_MiSTer isn't the vblank already captured on the ARM/HPS side, or is that just an approximation based on the pixel clock and other factors?

[sorgelig]

vblank can be captured in HPS, but again resources (in this context time resource) not worth the feature.