reg_write64 MMIO register error in rocket chip

Question

I am trying to write a 64 bit word to the MMIO register using the reg_write64 construct defined in mmio.h . Everything works fine as long as the wire that I map this register to in chisel land is not decoupled. As soon as I make this wire decoupled the execution hangs and does not complete. For example, this is my Chisel code

val wireToConnect = Wire(Decoupled(Bits(64.W))) //This  works when I don't have decoupled
regmap(
0x00 -> Seq(RegField.w(64, wireToConnect))))

And this is my C code for writing into this register

#include "mmio.h"
#define WRITEVAL 0x2000
int main(void){
    uint64_t data = 123456;
    reg_write64(WRITEVAL, data);
}

However when I replace reg_write64 with reg_write32 this works. Is there any particular setting which we need to set to make sure that decoupled interfaces can work with 64 bit wide registers?

Answer 1

The problem is with the bit width. Since it is already 64 bits, a decoupled is going to add a ready and valid bit to this interface which exceeds the bitwidth allowed by registers. We get an error that regmap is overlapping if we have more than 64 bits.