Enhance AXI4-Lite CPU Interface to support high performance pipelined transactions

docs/architecture.rst

@@ -51,4 +51,5 @@ optimally reduced.
 
 A second optional read response retiming register can be enabled in-line with the
 path back to the CPU interface layer. This can be useful if the CPU interface protocol
-used has a fully combinational response path, and needs to be retimed further.
+used has a fully combinational response path, and the design's complexity requires
+this path to be retimed further.

docs/cpuif/axi4lite.rst

@@ -0,0 +1,11 @@
+AMBA AXI4-Lite
+==============
+
+TODO: Describe the following
+
+* List of interface signals
+
+    * interface name & modports (link to advanced topics in case user wants to override)
+    * flattened equivalents
+
+* Download link to SV interface definition

docs/index.rst

@@ -47,6 +47,7 @@ Links
 
    cpuif/addressing
    cpuif/apb3
+   cpuif/axi4lite
    cpuif/advanced
    cpuif/internal_protocol
 

peakrdl/regblock/addr_decode.py

@@ -87,7 +87,7 @@ class DecodeLogicGenerator(RDLForLoopGenerator):
 
 
     def enter_Reg(self, node: RegNode) -> None:
-        s = f"{self.addr_decode.get_access_strobe(node)} = cpuif_req & (cpuif_addr == {self._get_address_str(node)});"
+        s = f"{self.addr_decode.get_access_strobe(node)} = cpuif_req_masked & (cpuif_addr == {self._get_address_str(node)});"
         self.add_content(s)
 
 

peakrdl/regblock/cpuif/axi4lite/__init__.py

@@ -14,6 +14,28 @@ class AXI4Lite_Cpuif(CpuifBase):
     def data_width_bytes(self) -> int:
         return self.data_width // 8
 
+    @property
+    def regblock_latency(self) -> int:
+        return max(self.exp.min_read_latency, self.exp.min_write_latency)
+
+    @property
+    def max_outstanding(self) -> int:
+        """
+        Best pipelined performance is when the max outstanding transactions
+        is the design's latency + 2.
+        Anything beyond that does not have any effect, aside from adding unnecessary
+        logic and additional buffer-bloat latency.
+        """
+        return self.regblock_latency + 2
+
+    @property
+    def resp_buffer_size(self) -> int:
+        """
+        Response buffer size must be greater or equal to max outstanding
+        transactions to prevent response overrun.
+        """
+        return self.max_outstanding
+
 
 class AXI4Lite_Cpuif_flattened(AXI4Lite_Cpuif):
     @property

peakrdl/regblock/cpuif/axi4lite/axi4lite_tmpl.sv

@@ -1,102 +1,216 @@
-enum logic [1:0] {
+// LATENCY = {{cpuif.regblock_latency}}
-    CPUIF_IDLE,
+// MAX OUTSTANDING = {{cpuif.max_outstanding}}
-    CPUIF_BRESP,
+logic [{{clog2(cpuif.max_outstanding+1)-1}}:0] axil_n_in_flight;
-    CPUIF_RRESP
+logic axil_prev_was_rd;
-} cpuif_state;
+logic axil_arvalid;
-
+logic [{{cpuif.addr_width-1}}:0] axil_araddr;
-logic cpuif_prev_was_rd;
+logic axil_ar_accept;
+logic axil_awvalid;
+logic [{{cpuif.addr_width-1}}:0] axil_awaddr;
+logic axil_wvalid;
+logic [{{cpuif.data_width-1}}:0] axil_wdata;
+logic axil_aw_accept;
+logic axil_resp_acked;
 always_ff {{get_always_ff_event(cpuif.reset)}} begin
     if({{get_resetsignal(cpuif.reset)}}) begin
-        cpuif_state <= CPUIF_IDLE;
+        axil_prev_was_rd <= '0;
-        cpuif_prev_was_rd <= '0;
+        axil_arvalid <= '0;
+        axil_araddr <= '0;
+        axil_awvalid <= '0;
+        axil_awaddr <= '0;
+        axil_wvalid <= '0;
+        axil_wdata <= '0;
+        axil_n_in_flight <= '0;
+    end else begin
+        // AR* acceptance register
+        if(axil_ar_accept) begin
+            axil_prev_was_rd <= '1;
+            axil_arvalid <= '0;
+        end
+        if({{cpuif.signal("arvalid")}} && {{cpuif.signal("arready")}}) begin
+            axil_arvalid <= '1;
+            axil_araddr <= {{cpuif.signal("araddr")}};
+        end
 
-        cpuif_req <= '0;
+        // AW* & W* acceptance registers
-        cpuif_req_is_wr <= '0;
+        if(axil_aw_accept) begin
-        cpuif_addr <= '0;
+            axil_prev_was_rd <= '0;
-        cpuif_wr_data <= '0;
+            axil_awvalid <= '0;
+            axil_wvalid <= '0;
+        end
+        if({{cpuif.signal("awvalid")}} && {{cpuif.signal("awready")}}) begin
+            axil_awvalid <= '1;
+            axil_awaddr <= {{cpuif.signal("awaddr")}};
+        end
+        if({{cpuif.signal("wvalid")}} && {{cpuif.signal("wready")}}) begin
+            axil_wvalid <= '1;
+            axil_wdata <= {{cpuif.signal("wdata")}};
+        end
 
-        {{cpuif.signal("arready")}} <= '0;
+        // Keep track of in-flight transactions
-        {{cpuif.signal("awready")}} <= '0;
+        if((axil_ar_accept || axil_aw_accept) && !axil_resp_acked) begin
-        {{cpuif.signal("wready")}} <= '0;
+            axil_n_in_flight <= axil_n_in_flight + 1'b1;
+        end else if(!(axil_ar_accept || axil_aw_accept) && axil_resp_acked) begin
+            axil_n_in_flight <= axil_n_in_flight - 1'b1;
+        end
+    end
+end
+
+always_comb begin
+    {{cpuif.signal("arready")}} = (!axil_arvalid || axil_ar_accept);
+    {{cpuif.signal("awready")}} = (!axil_awvalid || axil_aw_accept);
+    {{cpuif.signal("wready")}} = (!axil_wvalid || axil_aw_accept);
+end
+
+// Request dispatch
+always_comb begin
+    cpuif_wr_data = axil_wdata;
+    cpuif_req = '0;
+    cpuif_req_is_wr = '0;
+    cpuif_addr = '0;
+    axil_ar_accept = '0;
+    axil_aw_accept = '0;
+
+    if(axil_n_in_flight < 'd{{cpuif.max_outstanding}}) begin
+        // Can safely issue more transactions without overwhelming response buffer
+        if(axil_arvalid && !axil_prev_was_rd) begin
+            cpuif_req = '1;
+            cpuif_req_is_wr = '0;
+            cpuif_addr = axil_araddr;
+            if(!cpuif_req_stall_rd) axil_ar_accept = '1;
+        end else if(axil_awvalid && axil_wvalid) begin
+            cpuif_req = '1;
+            cpuif_req_is_wr = '1;
+            cpuif_addr = axil_awaddr;
+            if(!cpuif_req_stall_wr) axil_aw_accept = '1;
+        end else if(axil_arvalid) begin
+            cpuif_req = '1;
+            cpuif_req_is_wr = '0;
+            cpuif_addr = axil_araddr;
+            if(!cpuif_req_stall_rd) axil_ar_accept = '1;
+        end
+    end
+end
+
+
+// AXI4-Lite Response Logic
+{%- if cpuif.resp_buffer_size == 1 %}
+always_ff {{get_always_ff_event(cpuif.reset)}} begin
+    if({{get_resetsignal(cpuif.reset)}}) begin
+        {{cpuif.signal("rvalid")}} <= '0;
+        {{cpuif.signal("rresp")}} <= '0;
+        {{cpuif.signal("rdata")}} <= '0;
         {{cpuif.signal("bvalid")}} <= '0;
         {{cpuif.signal("bresp")}} <= '0;
-        {{cpuif.signal("rvalid")}} <= '0;
-        {{cpuif.signal("rdata")}} <= '0;
-        {{cpuif.signal("rresp")}} <= '0;
     end else begin
-        // Load response transfers as they arrive
+        if({{cpuif.signal("rvalid")}} && {{cpuif.signal("rready")}}) begin
+            {{cpuif.signal("rvalid")}} <= '0;
+        end
+
+        if({{cpuif.signal("bvalid")}} && {{cpuif.signal("bready")}}) begin
+            {{cpuif.signal("bvalid")}} <= '0;
+        end
+
         if(cpuif_rd_ack) begin
             {{cpuif.signal("rvalid")}} <= '1;
             {{cpuif.signal("rdata")}} <= cpuif_rd_data;
             if(cpuif_rd_err) {{cpuif.signal("rresp")}} <= 2'b10; // SLVERR
             else {{cpuif.signal("rresp")}} <= 2'b00; // OKAY
         end
+
         if(cpuif_wr_ack) begin
             {{cpuif.signal("bvalid")}} <= '1;
             if(cpuif_wr_err) {{cpuif.signal("bresp")}} <= 2'b10; // SLVERR
             else {{cpuif.signal("bresp")}} <= 2'b00; // OKAY
         end
-
-        // Transaction state machine
-        case(cpuif_state)
-            CPUIF_IDLE: begin
-                // round-robin arbitrate between read/write requests
-                // Allow read if previous transfer was not a read, or no write is active
-                if({{cpuif.signal("arvalid")}} && (!cpuif_prev_was_rd || !{{cpuif.signal("awvalid")}} || !{{cpuif.signal("wvalid")}})) begin
-                    cpuif_req <= '1;
-                    cpuif_req_is_wr <= '0;
-                    {%- if cpuif.data_width == 8 %}
-                    cpuif_addr <= {{cpuif.signal("araddr")}}[{{cpuif.addr_width-1}}:0];
-                    {%- else %}
-                    cpuif_addr <= { {{-cpuif.signal("araddr")}}[{{cpuif.addr_width-1}}:{{clog2(cpuif.data_width_bytes)}}], {{clog2(cpuif.data_width_bytes)}}'b0};
-                    {%- endif %}
-                    {{cpuif.signal("arready")}} <= '1;
-                    cpuif_state <= CPUIF_RRESP;
-                end else if({{cpuif.signal("awvalid")}} && {{cpuif.signal("wvalid")}}) begin
-                    {{cpuif.signal("awready")}} <= '1;
-                    {{cpuif.signal("wready")}} <= '1;
-                    if({{cpuif.signal("wstrb")}} != {{"%d'b" % cpuif.data_width_bytes}}{{"1" * cpuif.data_width_bytes}}) begin
-                        // Unaligned writes or use of byte strobes is not supported yet
-                        {{cpuif.signal("bvalid")}} <= '1;
-                        {{cpuif.signal("bresp")}} <= 2'b10; // SLVERR
-                    end else begin
-                        cpuif_req <= '1;
-                        cpuif_req_is_wr <= '1;
-                        {%- if cpuif.data_width == 8 %}
-                        cpuif_addr <= {{cpuif.signal("awaddr")}}[{{cpuif.addr_width-1}}:0];
-                        {%- else %}
-                        cpuif_addr <= { {{-cpuif.signal("awaddr")}}[{{cpuif.addr_width-1}}:{{clog2(cpuif.data_width_bytes)}}], {{clog2(cpuif.data_width_bytes)}}'b0};
-                        {%- endif %}
-                        cpuif_wr_data <= {{cpuif.signal("wdata")}};
-                    end
-                    cpuif_state <= CPUIF_BRESP;
-                end
-            end
-
-            CPUIF_BRESP: begin
-                cpuif_req <= '0;
-                {{cpuif.signal("awready")}} <= '0;
-                {{cpuif.signal("wready")}} <= '0;
-                cpuif_prev_was_rd <= '0;
-                if({{cpuif.signal("bvalid")}} && {{cpuif.signal("bready")}}) begin
-                    {{cpuif.signal("bvalid")}} <= '0;
-                    cpuif_state <= CPUIF_IDLE;
-                end
-            end
-
-            CPUIF_RRESP: begin
-                cpuif_req <= '0;
-                {{cpuif.signal("arready")}} <= '0;
-                cpuif_prev_was_rd <= '1;
-                if({{cpuif.signal("rvalid")}} && {{cpuif.signal("rready")}}) begin
-                    {{cpuif.signal("rvalid")}} <= '0;
-                    cpuif_state <= CPUIF_IDLE;
-                end
-            end
-
-            default: begin
-                cpuif_state <= CPUIF_IDLE;
-            end
-        endcase
     end
 end
+
+always_comb begin
+    axil_resp_acked = '0;
+    if({{cpuif.signal("rvalid")}} && {{cpuif.signal("rready")}}) axil_resp_acked = '1;
+    if({{cpuif.signal("bvalid")}} && {{cpuif.signal("bready")}}) axil_resp_acked = '1;
+end
+
+{%- else %}
+struct {
+    logic is_wr;
+    logic err;
+    logic [{{cpuif.data_width-1}}:0] rdata;
+} axil_resp_buffer[{{cpuif.resp_buffer_size}}];
+logic [{{clog2(cpuif.resp_buffer_size)}}:0] axil_resp_wptr;
+logic [{{clog2(cpuif.resp_buffer_size)}}:0] axil_resp_rptr;
+
+always_ff {{get_always_ff_event(cpuif.reset)}} begin
+    if({{get_resetsignal(cpuif.reset)}}) begin
+        for(int i=0; i<{{cpuif.resp_buffer_size}}; i++) begin
+            axil_resp_buffer[i].is_wr = '0;
+            axil_resp_buffer[i].err = '0;
+            axil_resp_buffer[i].rdata = '0;
+        end
+        axil_resp_wptr <= '0;
+        axil_resp_rptr <= '0;
+    end else begin
+        // Store responses in buffer until AXI response channel accepts them
+        if(cpuif_rd_ack || cpuif_wr_ack) begin
+            if(cpuif_rd_ack) begin
+                axil_resp_buffer[axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].is_wr = '0;
+                axil_resp_buffer[axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].err = cpuif_rd_err;
+                axil_resp_buffer[axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].rdata = cpuif_rd_data;
+
+            end else if(cpuif_wr_ack) begin
+                axil_resp_buffer[axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].is_wr = '1;
+                axil_resp_buffer[axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].err = cpuif_wr_err;
+            end
+            {%- if is_pow2(cpuif.resp_buffer_size) %}
+            axil_resp_wptr <= axil_resp_wptr + 1'b1;
+            {%- else %}
+            if(axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] == {{cpuif.resp_buffer_size-1}}) begin
+                axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] <= '0;
+                axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)}}] <= ~axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)}}];
+            end else begin
+                axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] <= axil_resp_wptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] + 1'b1;
+            end
+            {%- endif %}
+        end
+
+        // Advance read pointer when acknowledged
+        if(axil_resp_acked) begin
+            {%- if is_pow2(cpuif.resp_buffer_size) %}
+            axil_resp_rptr <= axil_resp_rptr + 1'b1;
+            {%- else %}
+            if(axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] == {{cpuif.resp_buffer_size-1}}) begin
+                axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] <= '0;
+                axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)}}] <= ~axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)}}];
+            end else begin
+                axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] <= axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)-1}}:0] + 1'b1;
+            end
+            {%- endif %}
+        end
+    end
+end
+
+always_comb begin
+    axil_resp_acked = '0;
+    {{cpuif.signal("bvalid")}} = '0;
+    {{cpuif.signal("rvalid")}} = '0;
+    if(axil_resp_rptr != axil_resp_wptr) begin
+        if(axil_resp_buffer[axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].is_wr) begin
+            {{cpuif.signal("bvalid")}} = '1;
+            if({{cpuif.signal("bready")}}) axil_resp_acked = '1;
+        end else begin
+            {{cpuif.signal("rvalid")}} = '1;
+            if({{cpuif.signal("rready")}}) axil_resp_acked = '1;
+        end
+    end
+
+    {{cpuif.signal("rdata")}} = axil_resp_buffer[axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].rdata;
+    if(axil_resp_buffer[axil_resp_rptr[{{clog2(cpuif.resp_buffer_size)-1}}:0]].err) begin
+        {{cpuif.signal("bresp")}} = 2'b10;
+        {{cpuif.signal("rresp")}} = 2'b10;
+    end else begin
+        {{cpuif.signal("bresp")}} = 2'b00;
+        {{cpuif.signal("rresp")}} = 2'b00;
+    end
+end
+{%- endif %}

peakrdl/regblock/cpuif/base.py

@@ -1,6 +1,6 @@
 from typing import TYPE_CHECKING, Optional
 
-from ..utils import get_always_ff_event, clog2
+from ..utils import get_always_ff_event, clog2, is_pow2
 
 if TYPE_CHECKING:
     from ..exporter import RegblockExporter
@@ -25,6 +25,7 @@ class CpuifBase:
             "get_always_ff_event": lambda resetsignal : get_always_ff_event(self.exp.dereferencer, resetsignal),
             "get_resetsignal": self.exp.dereferencer.get_resetsignal,
             "clog2": clog2,
+            "is_pow2": is_pow2,
         }
 
         template = self.exp.jj_env.get_template(self.template_path)

peakrdl/regblock/exporter.py

@@ -31,6 +31,8 @@ class RegblockExporter:
         self.field_logic = FieldLogic(self)
         self.readback = None # type: Readback
         self.dereferencer = Dereferencer(self)
+        self.min_read_latency = 0
+        self.min_write_latency = 0
 
         if user_template_dir:
             loader = jj.ChoiceLoader([
@@ -76,12 +78,12 @@ class RegblockExporter:
         if kwargs:
             raise TypeError("got an unexpected keyword argument '%s'" % list(kwargs.keys())[0])
 
-        min_read_latency = 0
+        self.min_read_latency = 0
-        min_write_latency = 0
+        self.min_write_latency = 0
         if retime_read_fanin:
-            min_read_latency += 1
+            self.min_read_latency += 1
         if retime_read_response:
-            min_read_latency += 1
+            self.min_read_latency += 1
 
         # Scan the design for any unsupported features
         # Also collect pre-export information
@@ -120,8 +122,8 @@ class RegblockExporter:
             "readback": self.readback,
             "get_always_ff_event": lambda resetsignal : get_always_ff_event(self.dereferencer, resetsignal),
             "retime_read_response": retime_read_response,
-            "min_read_latency": min_read_latency,
+            "min_read_latency": self.min_read_latency,
-            "min_write_latency": min_write_latency,
+            "min_write_latency": self.min_write_latency,
         }
 
         # Write out design

peakrdl/regblock/module_tmpl.sv

@@ -36,10 +36,12 @@ module {{module_name}} (
 
     {{cpuif.get_implementation()|indent}}
 
+    logic cpuif_req_masked;
 {% if min_read_latency == min_write_latency %}
     // Read & write latencies are balanced. Stalls not required
     assign cpuif_req_stall_rd = '0;
     assign cpuif_req_stall_wr = '0;
+    assign cpuif_req_masked = cpuif_req;
 {%- elif min_read_latency > min_write_latency %}
     // Read latency > write latency. May need to delay next write that follows a read
     logic [{{min_read_latency - min_write_latency - 1}}:0] cpuif_req_stall_sr;
@@ -54,6 +56,7 @@ module {{module_name}} (
     end
     assign cpuif_req_stall_rd = '0;
     assign cpuif_req_stall_wr = cpuif_req_stall_sr[0];
+    assign cpuif_req_masked = cpuif_req & !(cpuif_req_is_wr & cpuif_req_stall_wr);
 {%- else %}
     // Write latency > read latency. May need to delay next read that follows a write
     logic [{{min_write_latency - min_read_latency - 1}}:0] cpuif_req_stall_sr;
@@ -68,6 +71,7 @@ module {{module_name}} (
     end
     assign cpuif_req_stall_rd = cpuif_req_stall_sr[0];
     assign cpuif_req_stall_wr = '0;
+    assign cpuif_req_masked = cpuif_req & !(!cpuif_req_is_wr & cpuif_req_stall_rd);
 {%- endif %}
 
     //--------------------------------------------------------------------------
@@ -84,7 +88,7 @@ module {{module_name}} (
     end
 
     // Pass down signals to next stage
-    assign decoded_req = cpuif_req;
+    assign decoded_req = cpuif_req_masked;
     assign decoded_req_is_wr = cpuif_req_is_wr;
     assign decoded_wr_data = cpuif_wr_data;
 

peakrdl/regblock/utils.py

@@ -32,4 +32,7 @@ def get_always_ff_event(dereferencer: 'Dereferencer', resetsignal: 'Optional[Sig
     return "@(posedge clk)"
 
 def clog2(n: int) -> int:
-    return n.bit_length() - 1
+    return (n-1).bit_length()
+
+def is_pow2(x: int) -> bool:
+    return (x > 0) and ((x & (x - 1)) == 0)

test/lib/cpuifs/axi4lite/axi4lite_intf_driver.sv

@@ -136,7 +136,7 @@ interface axi4lite_intf_driver #(
             begin
                 txn_b_mutex.get();
                 @cb;
-                while(cb.BREADY !== 1'b1 && cb.BVALID !== 1'b1) @(cb);
+                while(!(cb.BREADY === 1'b1 && cb.BVALID === 1'b1)) @(cb);
                 assert(!$isunknown(cb.BRESP)) else $error("Read from 0x%0x returned X's on BRESP", addr);
                 txn_b_mutex.put();
             end