Add multicast support to XBAR #398

colluca · 2025-09-22T12:15:36Z

This PR adds multicast support to the AXI XBAR, such that write requests from a master can be multicast to multiple slaves.

To support multicast with low area overhead, we adopt the mask-based encoding proposed in https://arxiv.org/pdf/2502.19215.

In detail, this PR implements the following changes:

The axi_mcast_demux_simple implements a superset of the functionality of the axi_demux_simple, including all multicast capabilities, which require a few more signals on the interface, namely mst_is_mcast_o and mst_aw_commit_o. So axi_demux_simple is rewritten as a wrapper around axi_mcast_demux_simple, tying off unused ports and parameters.
The same is done for the axi_mcast_mux and axi_mux modules, the axi_mcast_xbar_unmuxed and axi_xbar_unmuxed modules, and the axi_mcast_xbar and axi_xbar modules.
The axi_demux_id_counters module is moved to its own file, which is anyways good practice, and it would otherwise not be clear whether it should now belong to axi_demux_simple.sv or axi_mcast_demux_simple.sv.
Placing the spill registers downstream of the address decoders requires registering the ax_select signals from the address decoders. To eliminate this overhead (in the case of multicast it would even increase due to additional output signals from the decoders), we move the address decoders downstream of the spill registers in theaxi_mcast_demux_mapped module. The _mapped prefix is to identify a demux whose slaves are memory-mapped, i.e. that uses address decoders to generate the ax_select signals. Since several modules use the demux with a non-address-based ax_select signal generation, the axi_demux module preserves the same interface and functionality as before, and does not wrap the axi_mcast_demux_mapped module, unlike the other unicast and multicast IP pairs.

To be improved: - 50% multicast AW throughput - Potential for combinational loops in mux

Multiple outstanding multicast transactions are allowed only if the slaves selected by these transactions are the same.

--------- Co-authored-by: Luca Colagrande <[email protected]> Co-authored-by: Raphael <[email protected]>

micprog

This PR is quite cumbersome to review, as most of the 5k line changes are code movement changes, and the code changes are not directly visible in the interface here. For some files, I added the corresponding git commands I used to check the changes. It might make sense to move some of these changes not related to multicast to dedicated PRs, simplifying the review and traceability of changes.

A further complication in reviewing this PR is the non-standard feature, as multicast is not supported in any available AXI specification. Please add documentation in the corresponding README files for what multicast is supported, how it works, and how it is intended to be used.

Also, given that this is not part of the spec, it might make sense to put this in a separate project, keeping this axi repository to what is available in the spec, but we can have that discussion offline.

Primarily, I was under the impression that this PR does not change the existing functionality when multicast is disabled or the existing modules are used, but this does not seem to be the case!

Along with comments placed to help understanding, there are a few other items that need to be addressed. Please see below.

micprog · 2026-01-15T14:46:08Z

scripts/run_vsim.sh

-        axi_xbar)
-            for NumMst in 1 6; do
-                for NumSlv in 1 8; do
-                    for Atop in 0 1; do
-                        for Exclusive in 0 1; do
-                            for UniqueIds in 0 1; do
-                                call_vsim tb_axi_xbar -gTbNumMasters=$NumMst -gTbNumSlaves=$NumSlv \
-                                        -gTbEnAtop=$Atop -gTbEnExcl=$Exclusive \
-                                        -gTbUniqueIds=$UniqueIds
-                            done
-                        done
-                    done
-                done
-            done
-            ;;


Is the AXI xbar test no longer valid?

micprog · 2026-01-15T14:54:36Z

Bender.yml


 dependencies:
-  common_cells: { git: "https://github.com/pulp-platform/common_cells.git", version: 1.37.0 }
+  common_cells:        { git: "https://github.com/pulp-platform/common_cells.git", rev: "multicast-xbar" }


Please update this to use a proper version, not a branch name.

micprog · 2026-01-15T14:55:28Z

Bender.yml

  - src/axi_cdc_src.sv
  - src/axi_cut.sv
  - src/axi_delayer.sv
  - src/axi_demux_simple.sv


axi_demux_simple is no longer considered Level 2, as it now depends on the mcast module. Please update this ordering (also for other updated files).

micprog · 2026-01-15T14:57:24Z

src/axi_demux_id_counters.sv

+  // outstanding transactions
+  output logic                 any_outstanding_trx_o


Just so I understand correctly, the only adjustment you made to this module is adding this port, otherwise it is copied from axi_demux_simple.sv.

git diff master:src/axi_demux_simple.sv multicast:src/axi_demux_id_counters.sv

micprog · 2026-01-15T15:03:14Z

src/axi_demux_simple.sv

+  logic [NoMstPorts-1:0] aw_select_mask;
+
+  assign aw_select_mask = 1'b1 << slv_aw_select_i;


The main changes here seem to be the deletion of all code to reference the new demux module, and the translation from select to onehot masking.

git diff master:src/axi_demux_simple.sv multicast:src/axi_demux_simple.sv

micprog · 2026-01-15T17:50:58Z

src/axi_mcast_demux_mapped.sv

+);
+
+  // Account for additional error slave
+  localparam int unsigned NoMstPortsExt = NoMstPorts + 1;


Is an error slave needed when a default port is enabled?

micprog · 2026-01-15T17:56:10Z

src/axi_mcast_demux_mapped.sv

+  spill_register #(
+    .T       ( aw_chan_t  ),
+    .Bypass  ( ~SpillAw   )
+  ) i_aw_spill_reg (
+    .clk_i,
+    .rst_ni,
+    .valid_i ( slv_req_i.aw_valid    ),
+    .ready_o ( slv_resp_o.aw_ready   ),
+    .data_i  ( slv_req_i.aw          ),
+    .valid_o ( slv_req_cut.aw_valid  ),
+    .ready_i ( slv_resp_cut.aw_ready ),
+    .data_o  ( slv_req_cut.aw        )
+  );


This module has a different timing behavior than the existing implementation, as outlined in the PR description. While this simplifies some things and reduces the number of registers, the address decoding can add quite a bit of timing overhead and can now no longer be pipelined out of the demux path when using any xbar configuration. Do you have an analysis of this timing impact before we integrate this change?

micprog · 2026-01-15T18:00:52Z

src/axi_mcast_demux_mapped.sv

+      dec_aw_addr = '0;
+      dec_aw_mask = '0;
+
+      if (slv_req_cut.aw.user.collective_mask == '0) begin


I'm not sure this will work for all tools if the custom user field is not present in the passed struct...

micprog · 2026-01-15T18:07:47Z

src/axi_mcast_demux_simple.sv

@@ -0,0 +1,739 @@
+// Copyright (c) 2019 ETH Zurich and University of Bologna.


git diff master:src/axi_demux_simple.sv multicast:src/axi_mcast_demux_simple.sv

micprog · 2026-01-15T18:13:38Z

src/axi_mcast_demux_simple.sv

+      else $fatal(1, "slv_ar_select_i illegal while ar_valid.");
+    w_underflow: assert property( @(posedge clk_i)
+        ((w_open == '0) && (w_cnt_up ^ w_cnt_down) |-> !w_cnt_down)) else
+        $fatal(1, "W counter underflowed!");


Are the following assertions that you removed fully no longer applicable, or is there a version we could keep?

- aw_select: assume property( @(posedge clk_i) (slv_req_i.aw_valid |-> - (slv_aw_select_i < NoMstPorts))) else - $fatal(1, "slv_aw_select_i is %d: AW has selected a slave that is not defined.\ - NoMstPorts: %d", slv_aw_select_i, NoMstPorts); - internal_aw_select: assert property( @(posedge clk_i) - (aw_valid |-> slv_aw_select_i < NoMstPorts)) - else $fatal(1, "slv_aw_select_i illegal while aw_valid.");

colluca and others added 16 commits September 17, 2025 15:07

axi_xbar: Correct signal names in waveform file

9cd8157

docs: Correct typos

bab24f7

axi_mcast_xbar: Create copies of original IPs as starting point

d416ee9

axi_mcast_xbar: Add basic multicast logic

28071b7

axi_mcast_xbar: Correct deadlock condition

6972683

axi_mcast_xbar: Cut valid->ready->commit combinational path

5813d9f

To be improved: - 50% multicast AW throughput - Potential for combinational loops in mux

axi_mcast_demux: Retrieve AW select index form from the mask

9d6df68

axi_mcast_xbar: Move AW address decoders after spill registers in demux

7bce784

axi_mcast_demux: Merge B responses appropriately

29f1302

axi_mcast_xbar: Allow both regular and mask-based address rules

3992e5b

axi_mcast_xbar: Allow multiple outstanding multicast transactions

1a401d0

Multiple outstanding multicast transactions are allowed only if the slaves selected by these transactions are the same.

axi_mcast_xbar: Filter multicast requests to unconnected slaves

a64189a

axi_mcast_xbar: Add to CI

a7c1423

Improved route decoding for collective operations (#1)

1e34421

--------- Co-authored-by: Luca Colagrande <[email protected]> Co-authored-by: Raphael <[email protected]>

Rebase onto latest master

bd1abff

Continue rebase, make tests pass again

3fb5376

colluca force-pushed the multicast branch from 272e442 to 5feb39b Compare September 23, 2025 11:54

Implement unicast XBAR IPs from multicast IPs

eb5e61e

colluca force-pushed the multicast branch from 5feb39b to eb5e61e Compare September 23, 2025 17:03

colluca added 2 commits September 23, 2025 19:17

Restrict UniqueIds to 0 when multicast is enabled

674d26e

Rename axi_mcast_demux to axi_mcast_demux_mapped

3de08ff

colluca force-pushed the multicast branch from 33d764c to 5020b85 Compare September 23, 2025 18:12

colluca added 3 commits September 24, 2025 13:58

Minor cleanup

5f90fb2

Add assertions on multicast rule conversion

05af798

Address final TODOs

fbc7c91

colluca force-pushed the multicast branch from 5020b85 to fbc7c91 Compare September 24, 2025 11:58

colluca added 3 commits September 25, 2025 15:25

axi_mcast_mux: Remove dependency of valid on ready

cda42b9

axi_mcast_demux_mapped: Improve readability of assertions

6544830

axi_mcast_demux_mapped: Allow multicast XBAR with NoMulticastRules == 0

8e04779

colluca force-pushed the multicast branch from d7b7105 to 8e04779 Compare September 26, 2025 17:48

colluca marked this pull request as ready for review October 6, 2025 06:58

fischeti mentioned this pull request Dec 9, 2025

hw: Support custom axi_xbar config structs pulp-platform/cheshire#254

Merged

axi_mcast_demux_mapped: Fix VCS compilation when NoMulticastPorts==0

06410c3

micprog requested changes Jan 15, 2026

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Add multicast support to XBAR #398

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colluca commented Sep 22, 2025 •

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		// outstanding transactions
		output logic any_outstanding_trx_o

		logic [NoMstPorts-1:0] aw_select_mask;

		assign aw_select_mask = 1'b1 << slv_aw_select_i;

		@@ -0,0 +1,739 @@
		// Copyright (c) 2019 ETH Zurich and University of Bologna.

Add multicast support to XBAR #398

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colluca commented Sep 22, 2025 •

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