hw: Add Reduction Feature by Lore0599 · Pull Request #163 · pulp-platform/FlooNoC

Lore0599 · 2026-03-03T08:40:00Z

Reduction

This PR introduces reduction support in the FlooNoC architecture. The reduction functionality is integrated with the previously supported multicast feature.

🔎 Feature Overview:

Reduction support can be divided into two different cases:

Parallel: Parallel reduction is used for synchronization mechanisms and leverages the previously supported CollectB feature. In particular, synchronization is enabled by the LsbAnd operation. This feature is typically supported in the narrow router.
Sequential: To support scalar or floating-point reductions, a sequential reduction mechanism has been added. This is supported in both wide and narrow routers.
To perform arithmetic operations, the router exposes wide and narrow offload ports towards a functional unit. Floating-point operations can leverage the Direct Compute Access (DCA) logic to reuse existing functional units in the target SoC.

⚙️ Configuration

To simplify the top-level configuration, all previously separate collective-related parameters have been merged into theCollectiveCfg structure.
At the top level, the user only needs to select which macro operations should be enabled (collect_op_fe_cfg_t). Internally, these operations are mapped into micro operations, which determine which features must be enabled at a finer granularity.
For example:

Multicast support requires both multicast functionality in the router and a primitive parallel reduction mechanism to collect incoming responses (CollectB).
Synchronization requires parallel reduction support for the requests and multicast support for the responses.

This fine-grained configuration is hidden from the user through the use of frontend operations (collect_op_fe_cfg_t), which are internally remapped to backend operations (collect_op_be_cfg_t).

For an example of how to integrate and enable the different operations, see the picobello SoC.

➕ Additions

floo_pkg.sv:
- Adds CollectiveCfgto the route configuration structure.
- OpCfg selects which macro operations are supported.
- RedCfg specifies how many pipeline stages the reduction unit should implement.
- Includes helper functions to abstract configuration complexity.
floo_reduction_unit.sv: Implements the main logic responsible for handling and offloading sequential reductions.
floo_alu.sv: An ALU used for narrow sequential reductions. Ideally, the IPU from the snitch cluster could be reused, but this would require adding Snitch as a dependency of FlooNoC.
floo_reduction_arbiter.sv/floo_output_arbiter.sv: Refactors the router and arbiter logic.
reduction/typedef.svh: Adds typedef definitions for the reduction offload interface.

➖ Removed

Removed parameters used to enable multicast, parallel reduction, and sequential reduction.
These features are now automatically enabled based on the selected operations.

⚠️ IMPORTANT

With the introduction of reduction support, all collective operations (multicast, synchronization, and reduction) can now be executed entirely within the NoC, provided that the loopback feature is enabled. This ensures that the full collective operation is handled exclusively within the NoC system.
This was not the case in the multicast implementation supported in v0.7.0, which relied on the multicast capabilities of the endpoint interconnect.

TODO

Rebase on top of main
Check FlooNoC CI
Remove or adapt the reduction TB
Split the synthesis wrapper contribution

When an endpoint initiates a wide multicast DMA transfer from another endpoint to itself (and possibly other endpoints), the following deadlock occurs. The DMA issues an AR, causing a read burst to come back from the router to the initiator endpoint. When the first read beats arrive, the DMA issues a write burst. This write burst loops back to the initiator endpoint, and may take control of the physical link. As writes lock the link for the entire burst, but the burst cannot complete as it needs the stalling read beats to feed the write burst (a DMA requirement), there is a deadlock. Note, this can happen also without multicast, so long as the system uses loopback. hw: Add spill registers for VCs in chimney hw: Fix reduction support with VC for read and write (working) hw: Merging collective configuration parameters hw: Initial setup for PnR experiments hw: Add internal offload cuts hw: Fix parametrization multicast hw: Fix parametrization adding micro collective ops pnr: Adapt synth wrapper to script fixes to script synthesis runs hw: Create two physical channels on wide_in interface of eject port hw: Support outstanding barriers with overlapping inputs hw: Fix `floo_reduction_sync` hw: Add comment to add assertion in `floo_route_xymask` synt: Fix synth wrapper and include chimney hw: Clean reduction_sync hw: Re-implement reduction simple controller (reduction_unit) synth: Adapt supprot for chimney with VC

fischeti

I had a first look and have a first round of feedback. In general it looks quite clean. Good job at cleaning this up and making it ready!

So far, I mainly looked at the top-level modules (routers, chimney), and I don't see any bigger issues there. I would just make sure that the default configuration is not affected. This seems to be true mostly with proper parametrization, but sometimes it is a bit hard to see in the Github diff view.

fischeti · 2026-03-13T12:54:19Z

hw/synth/snitch_cluster_pkg.sv

Is this file intentionally tracked?

fischeti · 2026-03-13T12:55:26Z

Bender.lock

    version: 0.39.9
    source:
-      Git: https://github.com/pulp-platform/axi.git
+      Git: https://github.com/Lura518/axi.git


fischeti · 2026-03-13T12:56:56Z

Bender.yml

  common_verification: 0.2.3
  idma: {version: 0.6.2, upstream_name: iDMA}
  floo_noc_pd: {path: ./pd, target: "floo_synth"}
+  FPnew: { git: "https://github.com/pulp-platform/cvfpu.git",        rev:      pulp-v0.1.3 }


Use the newest version pulp-v0.2.3, which is also used in snitch_cluster and cheshire afaik.

fischeti · 2026-03-13T12:58:31Z

Makefile


 PD_REMOTE ?= git@iis-git.ee.ethz.ch:axi-noc/floo_noc_pd.git
-PD_BRANCH ?= master
+PD_BRANCH ?= feature/reduction


Is there a Gitlab MR already open?

fischeti · 2026-03-13T13:04:28Z

hw/floo_pkg.sv

+    EnNarrowMulticast : 1'b0,
+    EnWideMulticast   : 1'b0,
+    EnLSBAnd          : 1'b0,
+    EnF_Add           : 1'b0,
+    EnF_Mul           : 1'b0,
+    EnF_Min           : 1'b0,
+    EnF_Max           : 1'b0,
+    EnA_Add           : 1'b0,
+    EnA_Mul           : 1'b0,
+    EnA_Min_S         : 1'b0,
+    EnA_Min_U         : 1'b0,
+    EnA_Max_S         : 1'b0,
+    EnA_Max_U         : 1'b0


'{default: '0} would be a bit shorter here.

fischeti · 2026-03-13T13:47:41Z

hw/floo_nw_router.sv

+  /// Parameter to define which type of collective operation support
+  parameter floo_pkg::collect_op_fe_cfg_t CollectiveOpCfg = floo_pkg::CollectiveOpDefaultCfg,
+  /// Parameter for the wide reduction configuration
+  parameter floo_pkg::reduction_cfg_t RdWideCfg     = floo_pkg::ReductionDefaultCfg,
+  /// Parameter for the narrow reduction configuration
+  parameter floo_pkg::reduction_cfg_t RdNarrowCfg   = floo_pkg::ReductionDefaultCfg


nit: Move up as well here

fischeti · 2026-03-13T13:48:48Z

hw/floo_nw_router.sv

@@ -11,46 +12,58 @@
 /// Wrapper of a multi-link router for narrow and wide links
 module floo_nw_router #(


You can also import floo_pkg globally in this module since it is used a lot.

fischeti · 2026-03-13T13:58:40Z

hw/floo_nw_chimney.sv

+      user_aw = axi_narrow_out_req_o.aw.user;
+      user_aw.collective_mask = '0;
+      user_aw.collective_op = Unicast;
+      axi_narrow_out_req_o.aw.user = user_aw;


Suggested change

user_aw = axi_narrow_out_req_o.aw.user;

user_aw.collective_mask = '0;

user_aw.collective_op = Unicast;

axi_narrow_out_req_o.aw.user = user_aw;

axi_narrow_out_req_o.aw.user.collective_mask = '0;

axi_narrow_out_req_o.aw.user.collective_op = Unicast;

Wouldn't that be equivalent? or are there some type casts that I am missing?

fischeti · 2026-03-13T14:00:49Z

hw/floo_nw_chimney.sv

+    assign collective_mask = '0;
  end

+  // Because the W doesn't have a user field it is required to store the AW user field


but W does have a use field, no?

fischeti · 2026-03-13T14:02:11Z

hw/floo_nw_chimney.sv

+    if (is_en_narrow_reduction(CollectOpCfg)) begin
+      if (is_reduction_op(red_coll_operation[NarrowAw])) begin


you can combine the two if statements

Lore0599 changed the title ~~Add Reduction Feature~~ hw: Add Reduction Feature Mar 3, 2026

Lore0599 requested review from Lura518, colluca, fischeti and imchenwu March 6, 2026 09:33

Lore0599 and others added 8 commits March 6, 2026 10:40

hw: Improve VC parametrization

5ca5e4e

hw: Align reduction to VC support

42d0eb6

hw: Remove complex reduction modules

921c2c3

Initial RTL clenaing

09e9dd0

Remove unused cfg parameters

4267a2e

Remove RdLoopBack: now coll only with loopback in the NoC

79a7be6

hw: Add support for reduction interface

e8d8e3e

Lore0599 force-pushed the reduction-vc-rebase branch from d19db12 to e8d8e3e Compare March 6, 2026 09:43

Lore0599 marked this pull request as ready for review March 6, 2026 11:00

Lore0599 force-pushed the reduction-vc-rebase branch from d123d89 to d3fb3f4 Compare March 6, 2026 12:27

Lore0599 mentioned this pull request Mar 6, 2026

hw: Align chimney and cut modules to updated floo_pkg #162

Open

Fix bender yml

8d19d92

Lore0599 force-pushed the reduction-vc-rebase branch from d3fb3f4 to 8d19d92 Compare March 6, 2026 14:01

fischeti reviewed Mar 13, 2026

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hw: Add Reduction Feature#163

hw: Add Reduction Feature#163
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reduction-vc-rebase

Lore0599 commented Mar 3, 2026 •

edited

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		@@ -11,46 +12,58 @@
		/// Wrapper of a multi-link router for narrow and wide links
		module floo_nw_router #(

		if (is_en_narrow_reduction(CollectOpCfg)) begin
		if (is_reduction_op(red_coll_operation[NarrowAw])) begin

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Lore0599 commented Mar 3, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Reduction

🔎 Feature Overview:

⚙️ Configuration

➕ Additions

➖ Removed

⚠️ IMPORTANT

TODO

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Lore0599 commented Mar 3, 2026 •

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