implemented a simple bootloader for initializing the free_queue

devlog/2025-07-02-Bootloader.md

@@ -0,0 +1,49 @@
+# The Bootloader
+Date: 2025-07-02
+
+## Goals and expectations
+Writing the bootloader logic for the fabric.  This is for easy scaling
+of the buffer size.
+
+And also add indicators (output logic) to signal the user and
+interfaces that the fabric is ready.
+
+## Results
+Done.  A simple bootloader, much simpler than I expected, just a
+simple FSM with some initializing logic.
+
+Just some logic inside the `free_queue` module to initialize it when
+the FPGA boots.
+
+And in the meanwhile, I spawned a new server (one specifically for
+running long term simulations), and also changed my shell to `fish`
+from `zsh` for some more modern feel.
+
+## Reflections
+1. Once again, I am bemused by the simplicity of combinational logic.
+   FSMs translate much better to SystemVerilog than to some
+   sequentially executed logic like C.
+2. Keep it simple.  A simple bootloader is much easier for future
+   implementations compared to initializing through other means, and
+   once complete, it's very scalable.
+3. The plan isn't everything.  This wasn't part of the plan, the plan
+   was to burn the initial values inside a hex file or some other
+   methods that would allow instant initialization.  However, due to
+   the elements of the `free_queue` being not byte-aligned, this would
+   become much more trouble in the end.  The plan was modified
+   accordingly.
+4. Make the best out of the existing tools on hand.  This is a
+   follow-up to the previous point.  Don't go into coding with only
+   the plan and goals in mind, reassess when you've set foot on its
+   grounds, weigh the options, and something else might just come up.
+   
+## Final thoughts
+Find the right tools to do the right things.  Trying to simulate
+clocked behaviors in C++ is as awkward as trying to write sequential
+logic in SystemVerilog.
+
+The first choice isn't always the best one, but the final choice
+should the better one after weighing the options.
+
+## Next steps
+THORN!

src/fabric/hub.sv

@@ -15,7 +15,8 @@ module hub(input logic                         rst,
            input logic [QUEUE_ADDR_LEN - 1:0]  tx_queue_addr [INTERFACE_CNT],
            output logic [QUEUE_ADDR_LEN - 1:0] tx_new_queue[INTERFACE_CNT],
            output logic                        tx_new_queue_valid [INTERFACE_CNT],
-           input logic                         tx_new_queue_ready [INTERFACE_CNT]);
+           input logic                         tx_new_queue_ready [INTERFACE_CNT],
+           output logic                        hub_ready);
     timeunit 1ns;
     timeprecision 1ps;
 
@@ -36,7 +37,8 @@ module hub(input logic                         rst,
                       .empty_slot_addr(empty_slot_addr),
                       .empty_slot_enqueue(empty_slot_enqueue),
                       .new_slot_addr(new_slot_addr),
-                      .queue_empty(free_queue_empty));
+                      .queue_empty(free_queue_empty),
+                      .queue_ready(hub_ready));
     
     logic [QUEUE_ADDR_LEN - 1:0]       rx_queue_addr [INTERFACE_CNT];
     logic [MEMORY_POOL_ADDR_LEN - 1:0] mem_read_addr;
@@ -148,7 +150,8 @@ module free_queue(input logic                         sys_clk,
                   input logic [QUEUE_ADDR_LEN - 1:0]  empty_slot_addr,
                   input logic                         empty_slot_enqueue,
                   output logic [QUEUE_ADDR_LEN - 1:0] new_slot_addr,
-                  output logic                        queue_empty);
+                  output logic                        queue_empty,
+                  output logic                        queue_ready);
     timeunit 1ns;
     timeprecision 1ps;
 
@@ -157,32 +160,39 @@ module free_queue(input logic                         sys_clk,
     logic [QUEUE_ADDR_LEN - 1:0] tail;
     shortint                     queue_size;
 
+    enum logic [1:0] {VOID, INIT, READY} state = VOID;
+
     assign queue_empty = queue_size == 0;
 
-    initial begin
-        // TODO: pre-load the free queue with every slot possible
-    end
-
-    // IMPORTANT: rst must be held high for at least 2 sys_clk cycles
-    always_ff @ (posedge sys_clk or rst) begin
-        if (rst) begin
+    always_ff @ (posedge sys_clk) begin
+        new_slot_addr <= fqueue[head];
+        
+        if (state == VOID) begin
+            queue_ready <= '0;
             head <= '0;
-            tail <= {QUEUE_ADDR_LEN{1'd1}};
-            queue_size <= QUEUE_SIZE;
-            new_slot_addr <= '0;
+            tail <= '0;
+            queue_size <= 0;
+            state <= INIT;
+        end else if (state == INIT) begin
+            if (tail != {QUEUE_ADDR_LEN{1'b1}}) begin
+                fqueue[tail] <= queue_size[QUEUE_ADDR_LEN - 1:0];
+                tail <= tail + 1;
+                queue_size <= queue_size + 1;
+            end else begin
+                state <= READY;
+                queue_ready <= 1;
+            end
         end else begin
             if (request_new_slot) begin
                 head <= head + 1;
                 queue_size <= queue_size - 1;
             end
-            new_slot_addr <= fqueue[head];
-
             if (empty_slot_enqueue) begin
                 fqueue[tail] <= empty_slot_addr;
                 tail <= tail + 1;
                 queue_size <= queue_size + 1;
             end
-        end
+        end // else: !if(fqueue_state_t == INIT)
     end
     
 endmodule // free_queue

src/fabric/main.sv

@@ -9,16 +9,20 @@ module fabric(input logic                        rst_raw,
 	      input logic [INTERFACE_CNT - 1:0]  cs_n,
 	      input logic [INTERFACE_CNT - 1:0]  mosi,
 	      output logic [INTERFACE_CNT - 1:0] miso,
-              output logic [INTERFACE_CNT - 1:0] tx_active);
+              output logic [INTERFACE_CNT - 1:0] tx_active,
+              output logic [INTERFACE_CNT - 1:0] sig_fabric_ready,
+              output logic                       fabric_ready,
+              output logic                       hub_ready);
 
     timeunit 1ns;
     timeprecision 1ps;
     
     logic sys_clk;
-    logic rst;
-
     // TODO: add PPL module for a boosted system clock
     
+    logic rst;
+    assign fabric_ready = hub_ready & (~rst);
+    
     reset_sync rst_sync(.sys_clk(sys_clk),
                         .rst_raw(rst_raw),
                         .rst(rst));
@@ -39,6 +43,8 @@ module fabric(input logic                        rst_raw,
     genvar i;
     generate
         for (i = 0; i < INTERFACE_CNT; i = i + 1) begin : gen_interfaces
+            assign sig_fabric_ready[i] = fabric_ready;
+            
             spi_interface spi(.rst(rst),
                               .sys_clk(sys_clk),
                               .sclk(sclk[i]),
@@ -74,7 +80,8 @@ module fabric(input logic                        rst_raw,
                 .tx_queue_addr(tx_queue_addr),
                 .tx_new_queue(tx_new_queue),
                 .tx_new_queue_valid(tx_new_queue_valid),
-                .tx_new_queue_ready(tx_new_queue_ready));
+                .tx_new_queue_ready(tx_new_queue_ready),
+                .hub_ready(hub_ready));
     
 endmodule // fabric
 
@@ -86,7 +93,8 @@ module reset_sync (input logic  sys_clk,
     timeprecision 1ps;
 
     
-    logic rst_sync_0, rst_sync_1;
+    logic rst_sync_0 = 0;
+    logic rst_sync_1 = 0;
 
     always_ff @(posedge sys_clk or posedge rst_raw) begin
 	if (rst_raw) begin