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ucie_chiplet_soc

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Firmware-driven dual-die RV32 chiplet SoC with GCC/ISS co-verification, DMA/AES offload, UPF 4.0 low-power intent, async CDC, real UVM, formal, and open-source coverage.

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# RISC-V Chiplet SoC RTL and Verification

A dual-die RISC-V subsystem built to demonstrate firmware-driven SoC integration, queued DMA offload, low-power intent, and report-backed design verification with open-source tools. An RV32I/Zicsr core runs both lightweight assembly and GCC-built bare-metal C to program DMA through APB MMIO; payload traffic crosses a retry-capable behavioral UCIe-style link to an AES service die.

The flagship result is the chiplet extension under `chiplet_extension/`. The earlier `base_soc/` implementation is retained only as historical context and is excluded from current project metrics.

## Architecture

```mermaid
flowchart LR
    ROM["Assembly or GCC-built C firmware"] --> CPU["RV32I/Zicsr core + RVFI trace"]
    CPU --> APB["APB-to-CSR bridge"]
    TMR["64-bit mtime/mtimecmp"] --> CPU
    CPU --> MBX["Firmware result mailbox"]
    APB --> DMA["Queued DMA + banked scratchpads"]
    DMA --> PKT["Packetizer / retry link"]
    PKT --> CH["Behavioral UCIe-style channel"]
    CH --> AES["Die B AES service"]
    AES --> CH
    CH --> DMA
    PWR["UPF 4.0 intent + proxy controls"] -.-> DMA
    PWR -.-> PKT
    PWR -.-> AES
```

The payload path is the behavioral UCIe-style transport. APB is the RV32 firmware control path. AXI-Lite is optional CSR-integration collateral and is not part of the chiplet payload path.

### GCC/ISS Co-Verification Architecture

![Compiled firmware and ISS co-verification architecture](https://raw.githubusercontent.com/ed766/ucie_chiplet_soc/main/docs/images/gcc_iss_architecture.svg)

Freestanding C is compiled into split instruction/data images, executed by the RV32 RTL, and observed through RVFI-style retirement plus APB/device event traces. The repository-local ISS predicts architectural state independently; the existing DMA/AES golden models check device behavior and final memory rather than asking the DUT to check itself.

The extended lane also verifies an always-on machine timer, `WFI`, timer/external interrupt priority, and writable `mcycle/minstret` counters. A standard-RVFI adapter and pinned Spike, ACT4/Sail, and `riscv-formal` integration paths provide independent evidence. Standalone probes report missing tools as `SKIP`, never as passes; the release gate uses `--require` and fails on any missing or revision-mismatched dependency.

## Verification Snapshot

This table is generated from `chiplet_extension/reports/project_metrics.csv` by `make -C chiplet_extension readme-metrics`.

| Evidence | Current result |
| --- | ---: |
| Stable regression | `70 / 70` |
| Functional coverage | `60 / 60` |
| Low-power proxy targets | `26 / 26` |
| GCC C/ISS firmware scenarios | `85 / 85` |
| Compiled firmware coverage | `178 / 178` |
| Compiled firmware crosses | `94 / 94` |
| Compiled firmware focused line coverage | `96.49%` |
| Compiled firmware focused branch coverage | `90.00%` |
| True RV32 RTL mutations | `14 / 14` |
| Spike CPU differential | `12 PASS / 0 SKIP / 0 FAIL` |
| ACT4/Sail RTL tests | `45 PASS / 0 SKIP / 0 FAIL` |
| RV32 standard/custom formal | `3 PASS / 0 SKIP / 0 FAIL` |
| Supporting real-UVM lane | `4 / 4` |
| Integrated async CDC ratios | `4 / 4` |
| Raw design line coverage | `96.25%` |
| Raw design toggle coverage | `1933 / 2570 (75.21%)` |
| Reviewed design toggle coverage | `1910 / 2116 (90.26%); 454 excluded` |

The non-UVM Verilator regression remains the default functional closure gate. The pinned `4 / 4` UVM phase/TLM/RAL lane is supporting methodology evidence, not a replacement closure claim. Raw and reviewed code-coverage values, including exclusion rationale, remain separate from the `60 / 60` functional model.

## Five-Minute Reviewer Path

1. [Project metrics](docs/project_metrics.md)
2. [Verification traceability](docs/verification_traceability_matrix.md)
3. [Firmware-driven verification](docs/firmware_soc_verification.md)
4. [Compiled-C and ISS evidence](docs/reference/compiled_firmware_verification.md)
5. [Power and UPF verification](docs/power_verification_plan.md)
6. [Privileged architecture validation](docs/reference/privileged_architecture_validation.md)
7. [Coverage closure](docs/coverage_closure_case_study.md)
8. [Formal evidence](docs/formal_appendix.md)
9. [Bug diary](docs/bug_diary.md)
10. [Performance characterization](docs/performance_characterization.md)
11. [UVM status](docs/uvm_status.md)
12. [Documentation index](docs/README.md)

Core evidence refresh:

```bash
make -C chiplet_extension project-check
make -C chiplet_extension upf-check
```

Supporting lanes:

```bash
make -C chiplet_extension firmware-c-release-check
make -C chiplet_extension firmware-c-coverage
make -C chiplet_extension formal-prove
make -C chiplet_extension async-cdc-check
make -C chiplet_extension uvm-ci
```

## Firmware-Driven DMA Flow

Two independently reported firmware lanes execute the same software-visible DMA flow. The compatibility lane uses twelve hand-assembled programs. The compiled lane uses freestanding GCC C built for `rv32i_zicsr`, precise traps, machine external interrupts, and an independent architectural retirement checker.

1. Stage source, destination, length, and tag fields through APB-visible CSRs.
2. Ring the DMA doorbell.
3. Wait through APB ready-state stalls without retiring the load or store early.
4. Poll IRQ and completion state.
5. Read and pop the completion record.
6. Verify destination memory and software-visible error status.
7. Publish the program result through an explicit MMIO mailbox; compiled firmware does not use the legacy EBREAK halt shortcut.

The twelve programs cover nominal and back-to-back DMA, queue-full rejection, completion-FIFO pressure, timeout, parity and invalid-memory errors, IRQ pending-then-enable, sleep/resume, CRYPTO_ONLY rejection, APB wait/error handling, and reset during a pending transfer. Testbench control may inject power, reset, or link conditions, but it does not program DMA CSRs in this lane.

The compiled lane adds 35 named C/assembly-linked scenarios, 25 deterministic generated CPU streams, and 25 seeded firmware/DMA workloads. It closes `178 / 178` RV32/firmware coverage points and `94 / 94` event-correlated crosses using split code/data images, RVFI-style retirement traces, APB/device event traces, interrupt/trap handlers, exact access-mask checks, and all six Zicsr forms on `mscratch`. A separate privileged lane closes `8 / 8` scenarios, `32 / 32` points, and `16 / 16` same-window crosses for `misa`, `mtval`, direct/vectored `mtvec`, interrupt source selection, and `MRET` state restoration. Mutation evidence is intentionally split into `14 / 14` true RTL mutations and `10 / 10` trace-checker self-tests. GCC/binutils are checksum-pinned through `firmware_c/toolchain.lock.json`. The repository-local ISS independently validates the compiled instruction image and models GPRs, initialized SRAM, machine CSRs including `mscratch`, PC flow, reset epochs, traps, and interrupt state while the existing Python DMA/AES model remains authoritative for device behavior; it is not an official RISC-V compliance framework.

### Reproducing the GCC/ISS Evidence

| Gate | Measured result | Canonical evidence |
| --- | ---: | --- |
| Directed compiled firmware | `35 / 35` | [`firmware_c_directed_summary.csv`](chiplet_extension/reports/firmware_c_directed_summary.csv) |
| Full directed + seeded closure | `85 / 85` | [`firmware_c_summary.csv`](chiplet_extension/reports/firmware_c_summary.csv) |
| Trace-derived functional points | `178 / 178` | [`firmware_c_coverage_summary.csv`](chiplet_extension/reports/firmware_c_coverage_summary.csv) |
| Same-window interaction crosses | `94 / 94` | [`firmware_c_cross_coverage_summary.csv`](chiplet_extension/reports/firmware_c_cross_coverage_summary.csv) |
| Trace-checker mutation self-tests | `10 / 10` | [`firmware_c_trace_mutation_summary.csv`](chiplet_extension/reports/firmware_c_trace_mutation_summary.csv) |
| True RTL mutation sensitivity | `14 / 14` | [`firmware_c_rtl_mutation_summary.csv`](chiplet_extension/reports/firmware_c_rtl_mutation_summary.csv) |
| Generated-C differential programs | `25 / 25` | [`firmware_c_generated_c_summary.csv`](chiplet_extension/reports/firmware_c_generated_c_summary.csv) |
| Optimizer/ABI executions and points | `34 / 34`, `16 / 16` | [`firmware_c_compiler_matrix_summary.csv`](chiplet_extension/reports/firmware_c_compiler_matrix_summary.csv) |
| Timer/WFI/counter scenarios and coverage | `10 / 10`, `16 / 16`, `22 / 22` | [`timer_wfi_summary.csv`](chiplet_extension/reports/timer_wfi_summary.csv) |
| Two-contributor robustness | `118 / 118` points, `58 / 58` crosses | [`firmware_c_coverage_robustness.csv`](chiplet_extension/reports/firmware_c_coverage_robustness.csv) |
| Focused RTL line / branch coverage | `96.49% / 90.00%` | [`firmware_c_code_coverage_summary.md`](chiplet_extension/reports/firmware_c_code_coverage_summary.md) |
| Spike CPU differential | `12 / 12` | [`rv32_external_iss_summary.csv`](chiplet_extension/reports/rv32_external_iss_summary.csv) |
| ACT4/Sail RTL architectural tests | `45 / 45` | [`rv32_act_summary.csv`](chiplet_extension/reports/rv32_act_summary.csv) |
| Standard/custom RVFI formal groups | `3 / 3` | [`rv32_formal_summary.csv`](chiplet_extension/reports/rv32_formal_summary.csv) |
| External-oracle RTL mutation sensitivity | `5 / 5` | [`rv32_external_mutation_matrix.csv`](chiplet_extension/reports/rv32_external_mutation_matrix.csv) |
| Privileged architecture scenarios / points / crosses | `8 / 8`, `32 / 32`, `16 / 16` | [`privileged_arch_summary.csv`](chiplet_extension/reports/privileged_arch_summary.csv) |

```bash
make -C chiplet_extension firmware-c-closure
make -C chiplet_extension firmware-c-coverage
make -C chiplet_extension firmware-c-mutation-check
make -C chiplet_extension firmware-c-release-check
```

The closure runner records the first architectural mismatch, ELF symbol/source line, surrounding disassembly, waveform cycle, seed, instruction count, APB activity, traps/interrupts, DMA outcomes, and memory-scoreboard status. Coverage receives no scenario-name-only credit; provenance is checked in [`firmware_c_evidence_audit.csv`](chiplet_extension/reports/firmware_c_evidence_audit.csv). [External validation status](docs/reference/rv32_external_validation.md), [privileged architecture validation](docs/reference/privileged_architecture_validation.md), and [timer/WFI verification](docs/reference/timer_wfi_verification.md) describe the independent and architectural evidence.

## DMA and Memory

- Four-entry ordered submit queue and four-entry completion FIFO.
- Staged CSR descriptor submission with source, destination, length, and tag fields.
- Success, submission-reject, timeout, parity, and invalid-memory completion status.
- Interrupt masking, pending state, completion polling, and software acknowledgement.
- Two-bank source and destination scratchpads with parity, maintenance arbitration, wait/conflict accounting, and invalid-bank tracking.
- Independent Python transaction-level golden model for descriptor flow, AES output, return ordering, and final memory image.

## Link and Service Path

The link models credits, backpressure, latency, jitter, CRC faults, retry, lane faults, and reset/recovery behavior. It is intentionally UCIe-style rather than standards-certified UCIe. Die B provides the AES service used by the DMA return path; scoreboards compare packet ordering, ciphertext, completion records, and final destination contents.

## Low-Power Intent

The tool-neutral IEEE 1801 / UPF 4.0 package declares always-on control plus switchable Die A traffic/DMA/link, Die B crypto/link, and channel domains. It includes power switches, clamp-to-zero output isolation, DMA sleep-context retention, retention-capable DMA memories, and a four-state PST.

| State | Powered behavior | Low-power behavior |
| --- | --- | --- |
| `RUN` | All switchable domains available | Isolation released; normal DMA/link/crypto activity |
| `CRYPTO_ONLY` | DMA, link, channel, and Die B crypto remain available | Legacy Die A traffic off; new DMA submissions blocked by policy |
| `SLEEP` | Switchable traffic, link, channel, crypto, and DMA are off | DMA queue/control context retained; memory validity follows retention masks |
| `DEEP_SLEEP` | All switchable domains off | DMA control context cleared; non-retained banks become invalid/poisoned |

Verilator power tests verify proxy behavior and monitor-driven functional coverage. `make upf-check` validates static power-intent structure. Verilator does not directly simulate UPF semantics, and no commercial UPF-aware signoff is claimed.

## Verification Architecture

- SystemVerilog drivers, monitors, scoreboards, protocol assertions, and event-derived functional/cross coverage.
- Negative tests and five compile-time bug modes validating checker sensitivity.
- Forty optional, executed seeded-random stress rows with reproducible manifests.
- Named assertions covering DMA accounting, retry ordering, AXI-Lite handshakes, parity containment, isolation, and retention sequencing.
- Twenty-four compiled-firmware architectural assertions cover `x0`, aligned retirement, exact load/store masks, APB wait/error atomicity, interrupt boundaries, precise trap state, retirement order, reset suppression, and `MRET` return state.
- Bounded Verilator properties plus separate solver-backed proofs, reachability covers, and expected mutation counterexamples.
- Supporting real-UVM agents, TLM analysis paths, coverage subscribers, and AXI-Lite RAL frontdoor smoke.
- Integrated two-clock CDC matrix plus standalone synchronizer/RDC collateral.

## Characterization and Implementation Evidence

Behavioral characterization reports link latency, retry overhead, backpressure, DMA queue depth, bank conflicts, parity cost proxies, retention, and invalid-memory recovery. Front-end evidence includes Verilator lint and code coverage, Yosys/LibreLane implementation proxies where supported, and generated documentation. These results are useful architecture and DV evidence, not silicon timing, power, CDC, or implementation signoff.

## Repository Layout

```text
chiplet_extension/rtl/       chiplet RTL, DMA, memory, link, CDC, and integration
chiplet_extension/sim/       procedural and UVM verification collateral
chiplet_extension/formal/    solver and bounded-property harnesses
chiplet_extension/upf/       tool-neutral UPF 4.0 power intent
chiplet_extension/scripts/   regressions, models, reports, and checks
chiplet_extension/firmware_c/ GCC C, startup code, and linker script
chiplet_extension/reports/   canonical normalized evidence
docs/                        primary reviewer documentation
docs/reference/              generated and specialist supporting collateral
base_soc/                    historical predecessor, excluded from flagship metrics
```

## Limitations

- The RV32 core implements RV32I plus the machine-mode CSR/interrupt subset needed by the firmware tests; it is not a production privileged RISC-V implementation or compliance-certified core.
- The die-to-die link is behavioral UCIe-style transport, not UCIe compliance.
- AXI-Lite is optional CSR wrapper collateral; firmware uses APB MMIO.
- UVM is a supporting lane; the stable non-UVM Verilator suite remains the closure gate.
- UPF, CDC/RDC, formal, synthesis, timing, power, and physical results are open-source verification proxies rather than commercial signoff.
- Cache coherence, DRAM, PCIe, production firmware, and additional power domains are intentionally out of scope.

## Project History

The project began as a small RV32 SoC and was extended in stages into the current firmware-controlled chiplet subsystem. Historical material remains available for provenance, but current claims and release metrics are generated only from the chiplet extension’s canonical reports.

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