From 2060a5c2f8426beff138892f44d3ddaf268d4205 Mon Sep 17 00:00:00 2001
From: David Harris <david_harris@hmc.edu>
Date: Wed, 23 Jun 2021 02:31:50 -0400
Subject: [PATCH] Reduced complexity of pmpadrdec

---
 wally-pipelined/src/mmu/pmpadrdec.sv | 61 ++++++++++++++++------------
 1 file changed, 34 insertions(+), 27 deletions(-)

diff --git a/wally-pipelined/src/mmu/pmpadrdec.sv b/wally-pipelined/src/mmu/pmpadrdec.sv
index f3e874a3..11c239b2 100644
--- a/wally-pipelined/src/mmu/pmpadrdec.sv
+++ b/wally-pipelined/src/mmu/pmpadrdec.sv
@@ -30,11 +30,8 @@
 `include "wally-config.vh"
 
 module pmpadrdec (
-  input  logic [31:0]      HADDR,
-
+  input  logic [31:0]      HADDR, // *** replace with PAdr
   input  logic [1:0]       AdrMode,
-
-  // input  logic [`XLEN-1:0] PreviousPMPAdr,
   input  logic [`XLEN-1:0] CurrentPMPAdr,
   input  logic             AdrAtLeastPreviousPMP,
   output logic             AdrAtLeastCurrentPMP,
@@ -45,37 +42,48 @@ module pmpadrdec (
   localparam NA4   = 2'b10;
   localparam NAPOT = 2'b11;
 
-  logic TORMatch, NA4Match, NAPOTMatch;
-
+  logic TORMatch, NAMatch;
   logic AdrBelowCurrentPMP;
+  logic [`PA_BITS-1:0] CurrentAdrFull;
+  logic [`PA_BITS-1:0] FakePhysAdr; 
 
-  logic [31:0] CurrentAdrFull;
-  // logic [31:0] PreviousAdrFull;
-
-  logic [33:0] Range;
-  
-  //assign PreviousAdrFull = {PreviousPMPAdr[29:0], 2'b00};
-  assign CurrentAdrFull  = {CurrentPMPAdr[29:0],  2'b00};
-
+  // ***replace this when the true physical address from MMU is available
+  assign FakePhysAdr = {{(`PA_BITS-32){1'b0}}, HADDR};
+ 
   // Top-of-range (TOR)
-  // *** Check if this synthesizes
-  // if not, literally do comparison (HADDR - PreviousAdrFull == 0)
-  assign AdrBelowCurrentPMP = HADDR < CurrentAdrFull;
+  // Append two implicit trailing 0's to PMPAdr value
+  assign CurrentAdrFull  = {CurrentPMPAdr[`PA_BITS-3:0],  2'b00};
+  assign AdrBelowCurrentPMP = /*HADDR */FakePhysAdr < CurrentAdrFull; // *** make sure unsigned comparison works correctly
   assign AdrAtLeastCurrentPMP = ~AdrBelowCurrentPMP;
   assign TORMatch = AdrAtLeastPreviousPMP && AdrBelowCurrentPMP;
 
-  // Naturally aligned four-byte region
-  // *** need to switch to Physical Address and extend to proper number of bits
-  assign NA4Match = &(HADDR[31:2] ~^ CurrentAdrFull[31:2]); // check if address matches all but bottom 2 bits;
-  //adrdec na4dec(HADDR, CurrentAdrFull, (2**2)-1, NA4Match);
+  // Naturally aligned regions
+  // *** should be able to optimize away bottom 2 bits
 
+  // verilator lint_off UNOPTFLAT
+  logic [`PA_BITS-1:0] Mask;
+  genvar i;
+  
+  // create a mask of which bits to ignore
   generate
-    if (`XLEN == 32 || `XLEN == 64) begin
+    assign Mask[1:0] = 2'b11;
+    assign Mask[2] = ~CurrentPMPAdr[0] & (AdrMode == NAPOT); // mask has 0s in upper bis for NA4 region
+    for (i=3; i < `PA_BITS; i=i+1) 
+      assign Mask[i] = Mask[i-1] & CurrentPMPAdr[i-3]; // NAPOT mask: 1's indicate bits to ignore
+   endgenerate
+  // verilator lint_on UNOPTFLAT
+
+  assign NAMatch = &((FakePhysAdr ~^ CurrentAdrFull) | Mask);
+
+ /* generate
+    if (`XLEN == 32 || `XLEN == 64) begin // ***redo for various sizes
       // priority encoder to translate address to range
       // *** We'd like to replace this with a better priority encoder
       // *** We should not be truncating 64 bit physical addresses to 32 bits...
+      // *** there is an easy combinatinoal way to do this with a cascade of AND gates O(32) rather than O(32^2) dh
       always_comb
-        casez (CurrentPMPAdr[31:0])
+        if (AdrMode == NA4) Range = (2**2) - 1;
+        else casez (CurrentPMPAdr[31:0]) // NAPOT regions
           32'b???????????????????????????????0: Range = (2**3)  - 1;
           32'b??????????????????????????????01: Range = (2**4)  - 1;
           32'b?????????????????????????????011: Range = (2**5)  - 1;
@@ -114,16 +122,15 @@ module pmpadrdec (
     end else begin
       assign Range = '0;
     end
-  endgenerate
+  endgenerate 
 
   // *** Range should not be truncated... but our physical address space is
   // currently only 32 bits wide.
   // with a bit of combining of range selection,  this could be shared with NA4Match ***
-   assign NAPOTMatch = &((HADDR ~^ CurrentAdrFull) | Range[31:0]);
+   assign NAMatch = &((HADDR ~^ CurrentAdrFull) | Range[31:0]);*/
 
   assign Match = (AdrMode == TOR) ? TORMatch : 
-                 (AdrMode == NA4) ? NA4Match :
-                 (AdrMode == NAPOT) ? NAPOTMatch :
+                 (AdrMode == NA4 || AdrMode == NAPOT) ? NAMatch :
                  0;
 
 endmodule