gary/lib/DelayLine.bsv

package DelayLine;

import List::*;

// A DelayLine mirrors writes back as reads after a number of delay
// cycles. Delay lines are pipelined and multiple values can be "in
// flight" at once.
//
// Reads block until a value is available, but the DelayLine doesn't
// stall if there are no readers - the value is available for reading
// after the set delay, and is lost if not read by anything.
//
// For callers that need to check for a value without tripping over an
// implicit condition, ready() can poll the delay line's output
// without blocking.
interface DelayLine#(type value);
   (* always_ready *)
   method Action _write (value v);
   method value _read();
   (* always_ready *)
   method Bool ready();
endinterface

// mkDelayLine constructs a DelayLine with delay_cycles of latency
// between writes and reads. delay_cycles may be zero, with the result
// being Wire/RWire like scheduling semantics.
module mkDelayLine(Integer delay_cycles, DelayLine#(a) ifc)
   provisos (Bits#(a, a_sz));

   // In most cases a DelayLine is a pipeline of registers through
   // which values flow, and so it inherits the scheduling properties
   // of registers: all reads from the delay line happen before any
   // writes take effect.
   //
   // The exception is when the caller asks for zero cycles of delay:
   // in that case, a write must be visible to reads within the same
   // cycle, and thus the write to the delay line must execute before
   // any reads.
   //
   // So, handle a delay of 0 cycles specially and implement it as an
   // RWire, which has write-before-read behavior. Otherwise, build a
   // pipeline of registers with read-before-write behavior.
   //
   // You might wonder why even bother allowing a DelayLine with zero
   // delay. It's because doing so is handy in parameterized
   // modules. Say for example you're wrapping a blackbox module that
   // has optional input and output registers. Depending on the
   // parameters, the in->out delay could be 0 cycles (pure
   // combinatorial circuit), 1 cycle (input or output register) or 2
   // cycles (input and output register). It's very handy to be able
   // to plop down a DelayLine regardless of the requested
   // configuration, and have it gracefully go all the way to zero
   // latency.
   if (delay_cycles == 0) begin
      RWire#(a) w <- mkRWire();

      method Action _write(a value);
         w.wset(value);
      endmethod
      method a _read() if (w.wget matches tagged Valid .val);
         return val;
      endmethod
      method Bool ready();
         return isValid(w.wget);
      endmethod
   end
   else begin
      RWire#(a) inputVal <- mkRWire;

      // Note that in rules and modules, for loops get unrolled
      // statically at compile time. We're not specifying a circuit
      // that runs a loop in hardware here, this is shorthand for
      // "splat out N registers and wire them together in a line".
      List#(Reg#(Maybe#(a))) delay = Nil;
      for (Integer i = 0; i < delay_cycles; i = i+1) begin
         let r <- mkReg(Invalid);
         delay = cons(r, delay);
      end

      // pump_line ingests a new value into the first register in the
      // line, and shifts older values one register down. The
      // registers always shift forward even when nobody's writing, in
      // which case that slot in the pipeline is occupied by an empty
      // Maybe.
      (* no_implicit_conditions, fire_when_enabled *)
      rule pump_line;
         delay[0] <= inputVal.wget();
         for (Integer i = 0; i < delay_cycles-1; i = i+1)
            delay[i+1] <= delay[i];
      endrule

      method Action _write(a value);
         inputVal.wset(value);
      endmethod
      method a _read() if (delay[delay_cycles-1] matches tagged Valid .val);
         return val;
      endmethod
      method Bool ready();
         return isValid(delay[delay_cycles-1]);
      endmethod
   end
endmodule

endpackage