regdrift

regdrift is a compatibility gate for CMSIS-SVD register maps. It compares two silicon descriptions, classifies every detected change as BREAKING, WARNING, or SAFE, and returns a CI-friendly verdict.

A moved register can leave a driver compiling cleanly while every write lands at the wrong address. regdrift catches that change in the pull request, before it becomes a logic-analyzer debugging session.

Status: 0.1.0a3 is a public alpha, published on PyPI. The implementation is exercised against 15 pinned vendor SVDs, but the explicit limitations in RULES.md still apply.

Highlights

• Resolves derivedFrom, register-property inheritance, nested clusters, dim arrays, enumerated values, interrupts, and read/write side effects.
• Uses deterministic structural matching with conservative rename detection.
• Applies a published 22-rule contract: 15 BREAKING, 4 WARNING, and 3 SAFE rules.
• Produces ranked text, schema-versioned JSON, or GitHub workflow annotations.
• Supports exact-path allowlisting, project-wide severity overrides, stdin for the baseline file, and configurable failure thresholds.
• Ships as both a Python CLI and a composite GitHub Action.
• Has unit, CLI, mutation, fuzz, golden, real-vendor corpus, and performance coverage on Python 3.11 and 3.12.

Installation

Install the current alpha from PyPI:

python -m pip install regdrift==0.1.0a3

or, as an isolated CLI tool:

pipx install regdrift==0.1.0a3

To install from source instead:

git clone https://github.com/Pranav-s79/regdrift.git
cd regdrift
python -m venv .venv
# POSIX: source .venv/bin/activate
# Windows PowerShell: .venv\Scripts\Activate.ps1
python -m pip install .

For development, install the editable package and validation tools instead:

python -m pip install -e ".[dev]"
python scripts/fetch_corpus.py

Quick start with the synthetic data

The committed demo/ folder contains two revisions of an imaginary accelerator. The candidate moves a register, renames a field, renumbers an interrupt, changes write semantics and a reset value, and adds a register.

regdrift check demo/chip_v1.svd demo/chip_v2.svd

BREAKING (4)
  RD015  interrupt ACCEL.ACCEL_DONE renumbered 17 -> 18
  RD005  field ACCEL.CTRL.ENABLE renamed (was EN; exact structural match)
  RD001  register ACCEL.STATUS address moved 0x8 -> 0xC
  RD017  field ACCEL.STATUS.DONE write semantics changed oneToClear -> oneToSet (what writing a bit does is inverted or altered)

WARNING (1)
  RD010  register ACCEL.STATUS reset value changed 0x1 -> 0x0

1 safe (1 added) - use --all to list

4 breaking, 1 warning, 1 safe, 0 allowed

The command exits 1 because the candidate contains unallowed breaking changes. Run it with --all to list the safe addition too.

CLI

Useful gate options:

# Show SAFE findings instead of rolling them up.
regdrift check old.svd new.svd --all

# Fail on warnings as well as breaking changes.
regdrift check old.svd new.svd --fail-on warning

# Produce stable machine-readable output.
regdrift check old.svd new.svd --format json

# Emit GitHub workflow commands.
regdrift check old.svd new.svd --format github

# Read the baseline from stdin.
git show origin/main:device.svd | regdrift check - device.svd

check uses these exit codes:

Rulebook and configuration

RULES.md is the normative compatibility contract. Every emitted change must map to exactly one documented rule or classification fails explicitly.

Create .regdrift.toml in the working directory to acknowledge intentional breaks or adjust policy for a specific toolchain:

allow = [
  "RD001:UART0.CTRL", # this rule at this exact path
  "RD030",            # this rule at every path
]

[severity]
RD013 = "WARNING"    # enum removals do not break this project's generator
RD010 = "BREAKING"   # reset-value drift must fail this project

Command-line entries add to the file allowlist:

regdrift check old.svd new.svd --allow RD001:UART0.CTRL --allow RD030

Configuration is strict: unknown keys, unpublished rule IDs, malformed allow entries, and invalid severities are tool errors rather than silent no-ops. Allowlisting wins over severity overrides and is reported separately as ALLOWED.

GitHub Actions

The bundled action extracts the baseline SVD from the pull request's base ref, runs the gate, writes a step summary, adds workflow annotations, and can update a sticky pull-request comment.

name: Register-map compatibility

on:
  pull_request:

permissions:
  contents: read
  pull-requests: write

jobs:
  regdrift:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0
      - uses: Pranav-s79/regdrift@v0.1.0a3
        with:
          svd-path: hardware/device.svd
          fail-on: breaking

The action installs the source at the selected action ref by default, so its runtime and action definition stay in sync. Set the optional version input to an exact published package version to install from PyPI instead. Disable the sticky comment with comment: "false" if the workflow only has read access.

For a fully manual workflow, install the package and run:

regdrift check /tmp/base.svd hardware/device.svd --format github

Output contracts

• Text output is optimized for review: breaking findings first, then warnings, allowed findings, and a safe-change rollup.
• JSON output includes schema_version: 1, device metadata, summary counts, the verdict, and every finding. See docs/json-schema.md.
• GitHub output escapes untrusted paths and SVD-derived text and caps each annotation severity at nine entries to keep workflow output usable.

Architecture

SVD/XML -> canonical parser -> structural diff -> rule classification -> report -> exit code

Parsing, factual diffing, policy classification, rendering, and CLI orchestration are separate modules. See docs/architecture.md for ownership boundaries and testing strategy.

Data and verification

• demo/ is committed, deterministic synthetic data. Regenerate it with python scripts/make_demo.py; CI verifies its generated content.
• tests/corpus/ is a gitignored, pinned 15-file vendor corpus downloaded by python scripts/fetch_corpus.py.
• tests/golden/ locks canonical parser output for selected corpus files.
• The mutation harness proves that every published rule fires against real vendor models.

Run the same checks enforced by CI:

ruff check .
mypy
pytest

Contributor setup and change requirements are documented in CONTRIBUTING.md.

Project documentation

• RULES.md — severities, rationales, calibration, and limitations
• docs/architecture.md — module boundaries and test strategy
• docs/json-schema.md — machine-readable output contract
• CHANGELOG.md — release history
• docs/release-checklist.md — maintainer release runbook

License

Apache-2.0. See LICENSE.