gary/vram/MemoryArbiter.bsv

package MemoryArbiter;

import Vector::*;

export MemoryArbiterWriter(..);
export MemoryArbiterReader(..);
export MemoryArbiter(..);
export mkMemoryArbiter;

// A MemoryArbiterWriter can request use of a memory port to write to
// an address. When a request is feasible, grant() returns True in the
// same cycle.
(* always_ready *)
interface MemoryArbiterWriter#(type addr);
   method Action request(Bool write, addr address);
   method Bool grant();
endinterface

// MemoryArbiterReader can request use of a memory port to read from
// an address. When a request is feasible, grant() returns True in the
// same cycle.
(* always_ready *)
interface MemoryArbiterReader#(type addr);
   method Action request(addr address);
   method Bool grant();
endinterface

// A MemoryArbiter manages concurrent access to memory ports. It
// mediates access between 2 writers and 4 readers, which are
// statically allocated to one of 2 internal memory ports.
interface MemoryArbiter#(type addr);
   // Assigned to port A.
   interface MemoryArbiterWriter#(addr) cpu;
   interface MemoryArbiterWriter#(addr) debugger;
   interface MemoryArbiterReader#(addr) palette;

   // Assigned to port B.
   interface MemoryArbiterReader#(addr) tile1;
   interface MemoryArbiterReader#(addr) tile2;
   interface MemoryArbiterReader#(addr) sprite;
endinterface

// mkMemoryArbiter builds a GARY memory arbiter.
//
// Port A arbitrates with strict priority: CPU requests always proceed
// first, followed by debugger requests, then the palette DAC.
//
// Port B does round-robin arbitration, giving each client a fair
// chance of having its requests processed.
module mkMemoryArbiter(MemoryArbiter#(addr) ifc)
   provisos(Bits#(addr, _),
            Eq#(addr),
            Alias#(write_req, Tuple2#(Bool, addr)));

   //////
   // Port A users

   RWire#(write_req) cpu_req <- mkRWire();
   RWire#(write_req) debugger_req <- mkRWire();
   PulseWire palette_req <- mkPulseWire();

   PulseWire cpu_ok <- mkPulseWire();
   PulseWire debugger_ok <- mkPulseWire();
   PulseWire palette_ok <- mkPulseWire();

   // Address written to by port A, if any. Used to block port B
   // clients that are trying to read the same address.
   RWire#(addr) written_addr <- mkRWire();

   // We could be fancy with rule conditions to express the priorities
   // between clients, but Bluespec has the preempts annotation to
   // express the ranking directly.
   (* preempts = "grant_cpu, (grant_debugger, grant_palette)" *)
   (* preempts = "grant_debugger, grant_palette" *)

   (* fire_when_enabled *)
   rule grant_cpu (cpu_req.wget matches tagged Valid {.write, .addr});
      cpu_ok.send();
      if (write)
         written_addr.wset(addr);
   endrule

   rule grant_debugger (debugger_req.wget matches tagged Valid {.write, .addr});
      debugger_ok.send();
      if (write)
         written_addr.wset(addr);
   endrule

   rule grant_palette (palette_req);
      palette_ok.send();
   endrule

   //////
   // Port B users

   Vector#(3, RWire#(addr)) portB_req <- replicateM(mkRWire);
   Wire#(Vector#(3, Bool)) portB_grant <- mkBypassWire();

   Vector#(3, Bool) init = replicate(False); init[0] = True;
   Reg#(Vector#(3, Bool)) priority_vec <- mkReg(init);

   rule grant_portB;
      Vector#(3, Bool) grants = replicate(False);
      Bool port_available = False;

      // This algorithm is a little mystifying at first glance, but it
      // works. priority_vec has one bool per client, only one of
      // which is True. That True bit identifies the client with the
      // highest priority on the next request.
      //
      // This loop goes through each client twice, using
      // port_available to track whether a client can grab the port or
      // not. When we start iterating, the port is marked unavailable
      // until we reach the top priority client, at which point we
      // mark the port available and keep scanning. That effectively
      // makes the search for a requesting client start at the top
      // priority one.
      //
      // As we loop back around a second time, the availability bool
      // gets reset again, but if you take the example of the True bit
      // being in the middle of the vector, and consider cases where
      // the first requestor is before/after that starting point,
      // you'll see that it all works out, and at the end of the loop
      // we have a new bit vector where only one client is True - the
      // one whose request is granted.
      for (Integer i = 0; i < 6; i=i+1) begin
         Integer idx = i % 3;
         if (priority_vec[idx])
            port_available = True;
         let req = portB_req[idx].wget();
         if (port_available && isValid(req) && req != written_addr.wget()) begin
            port_available = False;
            grants[idx] = True;
         end
      end
      portB_grant <= grants;
      // If we granted a request, the grantee becomes the lowest
      // priority client for the next round of requests. If nobody
      // requested anything, keep the same priority as before.
      if (any(id, grants))
         priority_vec <= rotateR(grants);
   endrule

   //////
   // External interface

   interface MemoryArbiterWriter cpu;
      method Action request(write, addr);
         cpu_req.wset(tuple2(write, addr));
      endmethod
      method grant = cpu_ok;
   endinterface

   interface MemoryArbiterWriter debugger;
      method Action request(write, addr);
         debugger_req.wset(tuple2(write, addr));
      endmethod
      method grant = debugger_ok;
   endinterface

   interface MemoryArbiterReader palette;
      method Action request(addr);
         palette_req.send();
      endmethod
      method grant = palette_ok;
   endinterface

   interface MemoryArbiterReader tile1;
      method Action request(addr);
         portB_req[0].wset(addr);
      endmethod
      method grant = portB_grant[0];
   endinterface

   interface MemoryArbiterReader tile2;
      method Action request(addr);
         portB_req[1].wset(addr);
      endmethod
      method grant = portB_grant[1];
   endinterface

   interface MemoryArbiterReader sprite;
      method Action request(addr);
         portB_req[2].wset(addr);
      endmethod
      method grant = portB_grant[2];
   endinterface
endmodule

endpackage