Burst support

[mbertuletti]

This PR adds burst support to the variable latency interconnect.

In systems where multiple interconnects are instantiated hierarchically, the arbitration across multiple initiators at the boundary between hierarchies causes a bottleneck in the request-per-cycle injected into the interconnect. The burst functionality solves the problem. We provide:

• A burst_req_grouper module, that groups a parallel valid request in a burst request,
• A burst_cutter module, that issues two bursts when the parallel request crosses the boundary between hierarchies and targets different arbitrators,
• A burst_manager module, that receives a burst request and translates it into a parallel request,
• A burst_rsp_grouper module, that increases the bandwidth of the response channel to send multiple words from a parallel response with the same resp_valid, thus reducing congestion to the arbiters also on the response path.

More details on the burst protocol applied to a system with hierarchical crossbars and wide memory requests initiators are in the following publication:

TCDM Burst Access: Breaking the Bandwidth Barrier in Shared-L1 RVV Clusters Beyond 1000 FPUs

@misc{shen2025tcdmburstaccessbreaking,
      title={TCDM Burst Access: Breaking the Bandwidth Barrier in Shared-L1 RVV Clusters Beyond 1000 FPUs}, 
      author={Diyou Shen and Yichao Zhang and Marco Bertuletti and Luca Benini},
      year={2025},
      eprint={2501.14370},
      archivePrefix={arXiv},
      primaryClass={cs.AR},
      url={https://arxiv.org/abs/2501.14370}, 
}

This paper is available on arXiv:2501.14370 [cs.AR].

[DiyouS]

Looks good to me!

[Paolo309]

MemPool relies on this line to override burst_resp_t with its own definition, which is slightly different from the default burst_pkg::burst_gresp_t. Removing it therefore seems to break the current MemPool interface.

Was the intention to update MemPool’s side of the interface as part of this change too?

MemPool's definition of the struct: https://github.com/pulp-platform/mempool/blob/b4bd2b1ff61b0fdce1a0321306a99dbf3fcf44d4/hardware/src/mempool_tile.sv#L506-L518

MemPool's burst_resp_t override: https://github.com/pulp-platform/mempool/blob/b4bd2b1ff61b0fdce1a0321306a99dbf3fcf44d4/hardware/src/mempool_tile.sv#L541

[mbertuletti]

Correct, this was updated in the current WIP of MemPool (https://github.com/pulp-platform/mempool/blob/86049e831a5869a6faf6018a90bd726d89893718/hardware/src/mempool_tile.sv#L522-L563).