metronome/docs/rust-port.md

# Rust port — staged plan

This is the plan for the native-Rust direction first discussed alongside the A/B-bootloader
idea. **What changed since then:** the track format is now formally specced
(`docs/track-format.md`) with a golden-vector conformance suite (`tests/run.mjs`). That suite is
the thing that makes a port *safe* — any Rust engine has a precise, executable definition of
"correct" to validate against, the same one `engine.js` and `app.py` already pass.

## The core idea

Port from the **inside out**, lowest-risk first. The pure logic (track codec, then the
scheduler) is host-testable with zero hardware and is gated by the existing golden vectors. Only
once that's proven do we touch drivers, A/B, and the actual firmware. We do **not** rip out
CircuitPython until the Rust engine passes the vectors *and* the drivers are proven on hardware.

## Architecture: one firmware core, modular drivers per form factor

Trying many form factors (Kit, Explorer, **Grid**/Scroll Pack, …) is how we *discover the line
between core and driver*. In Rust that line is enforced by the type system instead of copied by
hand — today each CircuitPython form factor is its own ~1,500-line `app.py` clone; the Rust build
is one core crate plus a thin per-board binary.

**`pm-core` — the core (`no_std`, zero hardware):**
- the track-format codec (`rust/track-format`, Stage 1) and the scheduler/clock (Stage 2, already
  `no_std` and building for RP2350),
- playback-flow (rep/end/continue, segment seams), app state, set-list model,
- the **USB-MIDI / live-sync / firmware-update protocol** logic (the SysEx opcode handling, which
  is form-factor-independent).
It is host-testable and gated by the golden vectors — the same suite `engine.js` and `app.py`
pass. **This is "core."**

**Driver traits — what the core is generic over (the swappable part):** define small project
traits — `Display` (or render straight to an `embedded-graphics` `DrawTarget`), `Inputs` (yields
button / touch events), `Clicker` (audio out), `Indicator` (RGB) — and write each concrete driver
against **`embedded-hal`** bus traits (`I2c`, `SpiBus`, `OutputPin`, `DelayNs`). The core's UI code
then doesn't care whether the target is a 17×7 mono matrix or a 320×480 colour TFT.

**Per-board binary crates — `pm-kit`, `pm-explorer`, `pm-grid`:** a thin `main.rs` BSP that
instantiates the right concrete drivers and hands them to the generic core:
- **Grid** (Scroll Pack): IS31FL3731 over I²C (a `DrawTarget` for a 17×7 mono frame) + 4 GPIO buttons.
- **Kit:** ST7796 320×480 over **SPI** — driven by a **custom `St7796` struct** (direct port of
  `pico/main.py`), **not** mipidsi, which fought the panel's geometry/CS (see [[rust-st7796-cs-gotcha]]);
  UI still renders through an `embedded-graphics` framebuffer. GT911 touch over I²C; WS2812 via
  `ws2812-pio`; I²S to the PCM5102A via PIO.
- **Explorer:** ST7789V 320×240 over an **8-bit parallel (8080) bus** via `mipidsi`'s
  `ParallelInterface` — a different driver story from the Kit's SPI (see the matrix below).

### Display driver matrix (researched 2026-06-03)

Displays are not the gate — every controller has a real Rust path; the buses differ:

| Form factor | Controller | Bus | Rust driver | Status |
|---|---|---|---|---|
| Kit (`pm-kit`) | ST7796 320×480 | SPI | **custom `St7796`** (port of `pico/main.py`) + `embedded-graphics` framebuffer; **mipidsi dropped** | ✅ on hardware — **but tearing** (see below) |
| Explorer (`pm-explorer`) | ST7789V 320×240 | 8-bit parallel 8080 | `mipidsi` `ParallelInterface` *(start here; be ready to port directly if geometry fights, as on the Kit)* | path proven upstream; not yet built |
| Grid (`pm-grid`) | IS31FL3731 17×7 mono | I²C | **vendored bulk-framebuffer driver** (port of `pico-scroll/app.py`'s `Matrix`); `is31fl3731` crate *not* used | ✅ **built + compiles** (LED-first milestone) — see below |
| Kit touch | GT911 | I²C | `gt911` or `gt9x` crate | ✅ mature (blocking + async, 5-point) |

**ST7796 (Kit) — only *partially* working: tearing.** Pixels are correct and the panel boots, but the
image tears badly. The cause is structural, not a bug: `mipidsi` has **no TE-pin / vsync / partial-update
/ double-buffer support** (confirmed against the upstream repo `github.com/almindor/mipidsi` — it offers
only optional draw *batching*), and this ST7796 module doesn't break out the TE (tearing-effect) line to
sync writes against scan-out. So writes race the panel's refresh. Mitigations available to us, none from
the crate: (a) redraw only changed full-width row-bands to shrink the tear window — already done; (b) DMA
each band as one tight burst; (c) sync to a TE GPIO *only if* a module that exposes that pin is sourced.
Treat tearing as an **open hardware/firmware item**, not "display done." See [[rust-st7796-cs-gotcha]].

**Explorer parallel bus — correctness and performance are decoupled.** `mipidsi`'s `ParallelInterface`
drives the data pins through an `OutputBus` *trait*. The shipped `Generic8BitBus` is plain GPIO bit-bang
(`embedded-hal` `OutputPin`s) — works immediately, just CPU-bound. For speed, implement `OutputBus` over
**RP2350 PIO** (the PIO supports 8080/6800 bus timing; the C/TFT_eSPI world hits ~4 ms for a full 320×480
clear this way) — a drop-in swap that leaves mipidsi + `embedded-graphics` + `pm-ui` untouched. Worst case
is "slow but functional," never "impossible," so the bit-bang fallback de-risks the whole Explorer bring-up.

**The honest caveat (what the Grid prototype is teaching us):** a 17×7 mono grid and a 320×480
touch TFT are too different for *one* pixel-identical UI. So the clean split is **core engine +
protocol + state = fully shared; the *view* = per-display-class.** The Grid is the most extreme,
minimal display in the lineup, which makes it the best forcing-function for finding exactly where
that boundary falls before we commit drivers to Rust. The CircuitPython `pico-scroll/` build exists
to nail that UI down on real hardware first.

## Stages

### Stage 0 — toolchain in a container
Add a Rust toolchain image (mirroring `hardware/eda/`): a `Containerfile` with `rustup`, the
`thumbv8m.main-none-eabihf` target (RP2350 is Cortex-M33), `flip-link`, `probe-rs`, `elf2uf2`.
Driven by a `run.sh` like the EDA one. **Never on the host.**

### Stage 1 — `track-format` crate  ✅ DONE (`rust/track-format/`)
Implemented and **passing**: `./rust/run.sh` builds the container and runs `cargo test`, which
validates the crate against `tests/fixtures/track-format.json` (conformance + idempotency). The
Rust codec agrees with `engine.js` and `app.py` on every vector — and carries `vol`/`cd`, so it's
the most spec-complete of the three. Original scope below.

#### (original) Stage 1 — `track-format` crate  ← the concrete first PR
A pure, `no_std`-compatible crate: `parse(&str) -> Track` and `serialize(&Track) -> String`,
plus a `normalize()` that emits the neutral structure from `docs/track-format.md` §5. Then a
`cargo test` that reads `tests/fixtures/track-format.json` and asserts each case's `norm` and
round-trip — i.e. a **third adapter alongside `js_adapter.mjs` / `py_adapter.py`**. When this is
green, the Rust engine provably agrees with web + device on every groove, euclid, swing, ghost,
polymeter, and the playback-flow tokens. No hardware, fully testable in the container.

This is the highest-value slice: small, gated by work already done, and it proves the toolchain.

### Stage 2 — scheduler/engine  ✅ DONE (`rust/track-format/src/schedule.rs`)
Ported the look-ahead step scheduler (the `durs` math from `app.py` `tick`/`_prepare_next`).
`render(track, bars)` produces the deterministic click timeline; `tests/schedule.rs` asserts the
timings — quarter-note spacing, subdivisions, swing 2/3:1/3, polymeter 5:4, accents/ghosts, mute,
multi-bar looping. All green on the host, no hardware. The real-time firmware loop will just play
this timeline against the wall clock.

**Also done:** the crate is now `#![no_std]` + `alloc` and **builds for the RP2350 target**
(`cargo build --lib --target thumbv8m.main-none-eabihf`) — the codec + scheduler are firmware-ready.

### Stage 3 — drivers (hardware)  🔧 IN PROGRESS (`rust/pm-kit/`)
**✅ Milestone 1 (boot) — confirmed on Pico 2:** GP25 blink. Toolchain + RP2350 boot block + flash work.
**🟡 Milestone 2 (display) — draws correctly on Pico 2, but TEARS:** ST7796 320×480 over SPI0 via
`rp235x-hal`, drawing the shared `pm-ui` through an `embedded-graphics` framebuffer. Driver is a
**custom `St7796` struct** ported from `pico/main.py` — mipidsi was tried first and **dropped**: its
split-transaction CS and orientation/offset math mangled the geometry; the port uses correct
per-command CS framing (CS low → cmd → params → CS high) and full-width row bands (see
[[rust-st7796-cs-gotcha]]). **Not done:** the image tears badly — no TE/vsync sync is possible because
this module exposes no TE pin, so writes race scan-out (no crate fixes this; see the tearing note in
the display driver matrix above). Open item before the display can be called finished. Diagnosed off-bench with host tools in `rust/uisim`: `uisim` renders
pm-ui to PNG; `--bin panelsim` decodes mipidsi's real command/pixel stream into a PNG (proved the
protocol correct → bug was physical); `--bin initdump` dumps the init + CASET/RASET sequence.

**🟡 Milestone 3 (live metronome) — built, pending on-device check:** the firmware is now an actual
metronome. `embedded-alloc` heap → parses tracks with `track-format` on-device; 4 built-in grooves;
Timer-driven clock; **audio clicks** on the master lane's hits (GP13 PWM, short edge-triggered
pulses, accent louder); **controls** — A = play/stop, B = grid/notation view, joystick (rotated 90°
CCW) up/down = tempo, left/right = groove. Renders `pm-ui::draw_metronome` / `draw_notation`, with a
cheap `draw_progress` strip animating the bar position every frame (full redraw only on change → no
flicker). All loop input reads use `unwrap_or` (no panics) — addresses the self-test crash.
Compile + simulator verified; **needs a flash to confirm** audio timing, joystick directions, no crash.

**pm-ui views (sim-verified, PNGs):** metronome grid (accents/ghosts/polymeter), and **drum notation**
(5-line staff, time sig, hands stem-up / feet stem-down, shared stems, beamed eighths, ledger lines).

**Still to do:** GT911 touch (GP8/9), WS2812 RGB (GP12), USB-MIDI, set-lists from programs.json,
per-cell live playhead, the rest of the practice features. Then split `pm-core` out as its own crate
and add `pm-explorer`/`pm-grid` binaries. HAL stays `rp235x-hal` (embassy later if async earns it).

On `embassy` / `rp-hal`:
- ST7789 240×320 display → `mipidsi` + `embedded-graphics` (mature; the parts are well-supported).
- I²S to the PCM5102A → RP2350 PIO.
- WS2812 → `ws2812-pio`.  USB-MIDI → `usbd-midi` / `embassy-usb`.
- GT911 touch (Kit) over I²C.

#### `pm-grid` — Scroll Pack firmware  🟢 BUILT (LED-first milestone), pending on-device check
The Rust sibling of `pico-scroll/app.py` (`rust/pm-grid/`). Target is a **plain RP2040** (Cortex-M0+,
`thumbv6m-none-eabi`) — NOT the Pico 2 — so it has its own HAL (`rp2040-hal` 0.10 + `rp2040-boot2`),
`.cargo/config.toml`, `memory.x` (BOOT2 + flash + 264 KB RAM) and `build.sh`+`uf2.py` (RP2040 family
id `0xe48bff56`). `thumbv6m-none-eabi` added to `rust/Containerfile`. Compiles clean → **48 KB
`pm-grid.uf2`** (BOOTSEL drag-flash; no probe needed). Kept out of the host workspace like `pm-kit`.

What's implemented (faithful port of `pico-scroll`): the **IS31FL3731 driver** (vendored bulk
144-byte framebuffer, one I²C block write per frame — the right architecture, mirrors the
CircuitPython `Matrix`; per-pixel I²C is too slow to animate), the **polymeter scheduler** driven by
`track-format::schedule::lane_durs` (the cross-impl contract) with per-lane step clocks + ramp +
gap-trainer, **4-button input** (A tap=play/stop, hold=cycle view; B tap=next track, hold=next set
list; X/Y=tempo ∓ with auto-repeat), the **built-in set lists**, and three LED views:
- **Ticker** (default): track name infinite-scrolls across the left (cols 0–10, full height); BPM is
  pinned right, **rotated 90° CCW** — a vertical hundreds **dot-bar** in col 11 (one dot per 100) +
  the last two digits rotated into cols 12–16 (tens bottom, units top). So `130` → 1 dot + rotated
  "30". This is the user-designed landscape readout. Rotation/geometry verified off-bench with an
  ASCII replica of `draw_ticker`.
- **Grid** (lanes×steps + playhead) and **Pendulum** (bouncing arm + beat ticks) — ports of
  `_render_grid` / `_render_pendulum`.
Boot splash scrolls "PM-G1 GRID" (liveness + pixel-map check).

**Deferred to the next milestone** (matches pm-kit's order — none built yet): **USB-MIDI** note-out
(the Scroll Pack has NO speaker, so this is the real audio path) + MIDI clock, **live-sync SysEx**
(0x40-0x43 + version query), firmware push (0x10/0x21-0x23), on-device practice log, settings.json,
and playback-flow auto-advance (`rep`/`end`/continue). An optional piezo on a free GPIO is also TODO.

### Stage 4 — native A/B + secure boot
Replace the `.mpy`-level A/B hack (`code.py` loads `app.mpy`, rolls back to `app.bak`) with the
**RP2350 bootrom's native** partition-table A/B + signed boot, configured via `picotool` (the
chip already provides this — see the earlier hardware discussion). The Rust app is the image in
the slot; rollback and version selection move into silicon.

## What you keep / lose

- **Gain:** memory safety, native A/B + secure boot, performance headroom, one typed model instead
  of three hand-written parsers.
- **Lose:** the live one-click `.mpy` push (Rust is compile→flash→reboot). The editor's *data*
  live-sync (tempo/pattern/setlist mirroring) still works — it's a data protocol. Only live
  *logic* edits go away, and an embedded `wasm3`/script module could buy those back if wanted.

## Acceptance gate

Every codec/engine change must pass `tests/fixtures/track-format.json`. The Rust crate joins
`js`/`py` as a runner adapter, so "same groove on web, device, and the Rust build" is enforced,
not hoped for.

## Recommendation

Do **Stage 1 in a container next** — it's small, testable today (given a toolchain), reuses the
suite, and produces a real artifact to judge the Rust path on before committing to drivers or a
firmware rewrite. Defer Stages 3–4 until Stage 1–2 are green and you've decided the live-push
tradeoff is acceptable.