32 KiB
Quality Function Deployment
Translates what the device must be (user-facing requirements) into what it
must do (engineering functions) and what we must build (components).
Surfaces the few targets that dominate the design and the conflicts between
them. Every decision cell points back to adr.md.
Scope: v0.1 MVP — see
v0.1-mvp-product.md for user-facing scope and
v0.1-mvp-technical.md for implementation —
with the v0.2–v1.0 trajectory (README,
roadmap) in mind so we don't paint into a corner. Terminology
(e.g. Tracked, Local, Save, Publish) follows the project
glossary at ../CONTEXT.md.
Format inspired by the classic House of Quality, kept compact. Strength
weights: 9 strong, 3 medium, 1 weak, blank none. The §3
matrix and §4 roof — plus a guessed competitor perception zone — live
in quality-house.md; this file owns the WHAT/HOW
catalogues (§1, §2), the narrative reading of the numbers (§3, §4), and
the downstream sections (§5–§8).
1. Customer requirements (the WHATs)
What a user (= me) values about the device, with importance weights on a 1–10 scale. Source columns point at the doc the requirement comes from.
| ID | Requirement | Weight | Source |
|---|---|---|---|
| W1 | Sub-second visible response to typing | 10 | product → Write, README → UX |
| W2 | Publishing is one deliberate action away | 9 | product → Publish, CONTEXT → Publish |
| W3 | Pulling power never corrupts the file | 10 | product → Recover, acceptance |
| W4 | Provisioning never interrupts a writing session | 7 | product → Provisioning, roadmap → v0.9 |
| W5 | Quick boot to a writing cursor | 6 | product → acceptance (≤ 5 s) |
| W6 | Long sessions without crash / lag / drift | 9 | product → acceptance (1 h soak) |
| W7 | Nothing on the device competes with prose | 8 | README → vision |
| W8 | The UI never moves except when I move it | 7 | README → UX |
| W9 | Codebase absorbs the planned roadmap without rewrite | 8 | roadmap |
| W10 | I can repair or fork it with hobbyist tools | 5 | README → vision |
| W11 | Multi-day battery life (v0.8 onward) | 4 | roadmap → v0.8 |
| W12 | Local-only file scope coexists with git scope (v0.5+) | 5 | README → scopes, roadmap → v0.5 |
| W13 | Typography sets a writing-tool tone — typewriter or developer editor, never gadget | 7 | roadmap → v1.0, README → UX |
| W14 | I can carry the device and write away from a desk | 8 | roadmap → v0.8, README → hardware |
2. Engineering functions (the HOWs)
Measurable characteristics. Targets are v0.1 unless noted. Direction column shows what "better" looks like (↑ higher, ↓ lower, → fixed).
| ID | Function | Dir | v0.1 target | v1.0 target |
|---|---|---|---|---|
| H1 | Keypress → glyph latency | ↓ | ≤ 200 ms | ≤ 150 ms |
| H2 | Partial-refresh region area per keystroke | ↓ | ≤ 1 text line (~22 px h) | same |
| H3 | Full-refresh cadence (clears ghosting) | → | 1 per 20 partials | tuned by panel temp |
| H4 | Cold boot → cursor ready | ↓ | ≤ 5 s | ≤ 3 s |
| H5 | Continuous-typing endurance (no drop, no leak) | ↑ | ≥ 1 h | ≥ 8 h |
| H6 | Ctrl-G push success rate on healthy Wi-Fi |
↑ | ≥ 95 % | ≥ 99 % |
| H7 | Push end-to-end (one-file commit) | ↓ | ≤ 30 s | ≤ 10 s |
| H8 | Save survives power loss after status confirms | → | 100 % | 100 % |
| H9 | PSRAM heap headroom during push | ↑ | ≥ 1 MB free at peak | same |
| H10 | Firmware binary size | ↓ | ≤ 2 MB | ≤ 1.5 MB |
| H11 | Stack budget across all tasks | ↓ | ≤ 80 KB (sum) | same |
| H12 | Wi-Fi reconnect on transient outage | ↓ | ≤ 30 s | ≤ 10 s |
| H13 | Idle / typing / push current draw | ↓ | measured only | sized for >2 days |
| H14 | Module count / public-API surface (refactor proxy) | → | ≤ 8 modules | same |
| H15 | Build time (clean, release) | ↓ | ≤ 7 min | ≤ 5 min |
3. House of Quality — WHATs × HOWs
The matrix (row × column = how strongly function H advances requirement
W) and its Σ row — the weighted vote Σ(weight × strength) on which
functions deserve the most engineering attention — live in
quality-house.md. The Σ totals quoted below are
from its basement row.
Top engineering priorities (from importance)
- H9 — PSRAM heap during push (193). gitoxide pack + rope + TLS all share the same arena; ADR-001 and ADR-004 trade binary size for ecosystem so this becomes the watched metric. The umbrella typography WHAT (W13) keeps a fixed-size glyph-cache load on top of that arena pressure.
- H2 — partial-refresh region area (177). Bound how many pixels the panel has to flip per keypress; ADR-003 is the hardware-side answer.
- H8 — save durability (156). Atomic-rename + fsync; FAT's weakness is acknowledged in ADR-007 and mitigated, not designed around. H8's voter base spans W3 (power-loss correctness), W6 (long sessions), W12 (file scopes), and W14 (carrying = unclean shutdowns) — the fourth voter is what lifts H8 into the top three by arithmetic alone.
- H12 — Wi-Fi reconnect (153). Mobile use is the chief driver (W14 + W2 + W4 + W6); ADR-005 PAT auth and reconnect backoff own this. Previously below the top six on a stationary v0.1 reading; W14 promotes it.
- H1 — keypress latency (148). The single most user-visible number; ADR-002 and ADR-003 are co-conspirators.
- H3 — full-refresh cadence (144). The ghosting/flash tradeoff; lives in the render layer.
H13 (current draw, 137) sits at #7 — close to the top-six cutoff because W14 promotes the "wall-power for v0.1, measure first" stance from acknowledged tradeoff to watched metric. The v0.1 "measured only" target (§2) is still right; what changes is that bench multimeter readings (§6) gain a second audience — sizing the v0.8 cell against a real portability target, not just informing ADR-008's deferral.
H6 (push success, 134) drops out of the top six. Its ADR ownership (ADR-004 gitoxide + ADR-005 PAT) and spike 7 kill-switch are unchanged — the matrix simply reads W14's mobile-use voter as a louder signal for reconnect (H12) than for the push transport itself.
Why H8 ranks where it does. Pre-W14, HoQ totals rewarded functions that touch many WHATs over functions that absolutely matter for one WHAT. W3 ("Pulling power never corrupts the file", weight 10) was H8's strongest single voter, but H8 still sat at #6 because its base was narrow. W14's "carrying = bumps = unclean shutdowns" widens H8's voter base and pushes it to #3 by arithmetic. §6's "table-stakes correctness" override is no longer the load-bearing argument for H8's prominence — its acceptance-criteria override for H4/H5 still is. See §6.
The bottom three (H7 push time, H15 build time, H10 binary size) are real costs but ones we knowingly took on (ADR-001) and are not in the critical path of user experience. The tightened H15 v0.1 target (≤ 7 min) reflects user preference for faster iteration, not matrix-derived priority; if it pushes back against ADR-001's "+5–10 min" pricing, the target moves before the runtime decision does.
4. Roof — function-vs-function tradeoffs
The roof shows where pushing one function pushes another the wrong way.
Classical QFD single-character symbols: ◎ strong reinforcement,
○ mild reinforcement, × mild conflict, ⊗ strong
conflict. The 15×15 roof matrix lives in
quality-house.md; the cells that actually shape
the design are called out below.
Conflicts that actually shape the design
- H1 latency ↔ H3 refresh cadence (mild). More partial refreshes per second pile up ghosting faster, demanding earlier full refreshes — visible flashes that hurt H8 perception and H1 burst behaviour. The ADR-003 strip aspect is the structural answer: a small framebuffer makes both cheaper, not one at the expense of the other. The runtime answer is render §H3: schedule full refreshes on idle ≥ 1 s (v0.1 tech doc).
- H9 heap ↔ H10 binary size (strong). std + gitoxide + mbedtls inflate both. We chose to spend on these (ADR-001, ADR-004) because 16 MB flash and 8 MB PSRAM make them affordable; the kill-switch is spike 7. If heap during push refuses to come under 1 MB free, ADR-004 flips to libgit2-sys for v0.1.
- H9 heap ↔ H5 soak (strong). A long writing session grows the rope and the glyph cache; pushing on top can OOM. Mitigation: 256 KB file cap (v0.1 tech doc) + glyph cache eviction before push + watching the spike in spike 7.
- H6 push success ↔ H12 Wi-Fi reconnect (reinforcing). Both come from the same network stack; investing in reconnect backoff helps both.
- H10 binary ↔ H15 build time (strong). std builds are slow. Accepted in ADR-001 — refactor leverage (H14) is the long-term payoff, not the per-build seconds.
- H4 boot ↔ H10 binary (mild). Larger binary = slower flash load. Affordable at our size class but worth watching as features land.
- H11 stacks ↔ H13 current draw (mild, future). Idle threads draw little but never zero; a future light-sleep policy (v0.8) wants them parked. W14's portability outcome raises the value of that policy from "battery hygiene" to "the thing that lets the device leave the desk."
- H14 modularity ↔ H15 build time (mild). More small crates = more link work. Boring vs valuable; we lean toward modularity.
- W13 typography ↔ H9 heap + H10 binary (mild, future). Achieving a writing-tool tone needs room for glyph caches and font assets. Not load-bearing in v0.1 (one mono font), but the v1.0 tone goal is why H9 and H10 keep slack rather than being squeezed to the minimum.
- Tightened H15 ↔ ADR-001 (mild). Pulling v0.1 build time from ≤ 10 min to ≤ 7 min eats into ADR-001's accepted "+5–10 min" cost. Worth aiming at via cargo profile / vendor LTO / crate-graph trims; worth giving up before reversing ADR-001.
5. Function → Component mapping (Phase 2)
Which subsystem owns the delivery of each function. Cells are which ADR constrains the choice.
Components (with anchoring ADR):
| ID | Component | ADR |
|---|---|---|
| C1 | ESP32-S3-N16R8 SoC | ADR-001, ADR-008 |
| C2 | esp-idf-rs (std) + ESP-IDF |
ADR-001 |
| C3 | std::thread + crossbeam-channel |
ADR-006 |
| C4 | PSRAM allocator wrapper | ADR-001 |
| C5 | GDEY0579T93 + DESPI-c579 panel | ADR-003 |
| C6 | embedded-graphics + e-paper driver |
ADR-002, ADR-003 |
| C7 | Custom widget / dirty-rect layer | ADR-002 |
| C8 | ropey rope buffer |
ADR-001 (ecosystem) |
| C9 | TinyUSB host (esp-idf bindings) |
ADR-009 |
| C10 | FAT on microSD | ADR-007 |
| C11 | LittleFS on internal flash | ADR-007 |
| C12 | gitoxide (gix-*) |
ADR-004 |
| C13 | mbedtls TLS (via ESP-IDF) | ADR-005 |
| C14 | HTTPS + GitHub PAT auth | ADR-005 |
| C15 | eFuse-derived encryption key | ADR-005, ADR-007 |
| C16 | USB-C wall PSU | ADR-008 |
Function-to-component matrix (9 strong / 3 medium / 1 weak):
| C1 SoC | C2 std | C3 thr | C4 PSR | C5 EPD | C6 eg | C7 wid | C8 rope | C9 USB | C10 SD | C11 LFS | C12 gix | C13 TLS | C14 PAT | C15 efs | C16 PSU | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| H1 lat | 3 | 1 | 9 | 3 | 9 | 9 | 9 | 3 | 9 | |||||||
| H2 area | 9 | 9 | 9 | |||||||||||||
| H3 cad | 9 | 3 | 9 | |||||||||||||
| H4 boot | 3 | 9 | 3 | 1 | 3 | 9 | 3 | |||||||||
| H5 soak | 3 | 3 | 3 | 9 | 1 | 9 | 9 | 3 | 3 | 3 | ||||||
| H6 push% | 3 | 9 | 9 | 9 | ||||||||||||
| H7 push s | 3 | 1 | 3 | 9 | 9 | |||||||||||
| H8 dura | 3 | 9 | 9 | |||||||||||||
| H9 heap | 3 | 3 | 9 | 3 | 9 | 9 | ||||||||||
| H10 bin | 9 | 1 | 3 | 3 | 3 | 3 | 9 | 3 | ||||||||
| H11 stk | 9 | 3 | 3 | |||||||||||||
| H12 wifi | 3 | 9 | 3 | |||||||||||||
| H13 mA | 9 | 1 | 9 | 3 | 3 | 9 | ||||||||||
| H14 mod | 3 | 3 | 3 | 9 | 3 | 9 | ||||||||||
| H15 build | 9 | 9 | 3 |
Read across, not down
- C5/C6/C7 (panel + graphics + widget) are the single most leveraged cluster — they own H1, H2, H3 (the top of the priority list). ADR-002 and ADR-003 are the ADRs to keep most honest as v0.x progresses.
- C12 (
gitoxide) is overloaded: H6, H7, H9, H10, H11, H14, H15 all touch it. That's why ADR-004 includes a kill-switch (fall back tolibgit2-sysif spike 7 fails). It's also why H9 sits in the top three priorities —gitoxide's memory profile is the unknown. ADR-010 pins the shape of the publish sequence (thegctflow); C12 is just the library that implements it. Changing ADR-010 doesn't change C12's column, but changing C12 (the kill-switch) does not change ADR-010's user contract. - C11 (LittleFS) is unused in v0.1 — config is build-time. Its non-zero cells in the matrix describe the v0.9+ shape per ADR-007, not v0.1 reality.
- C2 (std runtime) sits underneath almost everything, but it's the enabler (H4 boot, H10 binary, H12 Wi-Fi) rather than the bottleneck. Reversing ADR-001 would force re-deciding ADR-004, ADR-005, ADR-006, ADR-007 all at once — they're a single decision in three drawers.
6. Critical performance budget
A curated rank, drawing from §3 importance and §4 conflicts, with one deliberate override: acceptance-criteria critical paths (H4 boot, H5 soak) move up regardless of weighted-vote spread. (Pre-W14 this list also lifted H8 durability over its narrow voter base; W14 has widened that base, so H8's #3 spot is now arithmetic — see §3.) These are the numbers spikes 2–7 must validate before integration starts.
| Rank | Function | Target | Watched on | If we miss it |
|---|---|---|---|---|
| 1 | H2 region area | ≤ 1 line per keypress | spike 2 + spike 5 | Increase font size to shrink per-glyph dirty rect (ADR-003 consequence) |
| 2 | H9 PSRAM heap | ≥ 1 MB free at push peak | spike 7 | ADR-004 kill-switch → libgit2-sys; cap rope at 128 KB |
| 3 | H8 durability | 100 % survive power yank after status | bench HIL | Re-evaluate ADR-007 (move config to internal NVS only) |
| 4 | H1 latency | ≤ 200 ms keypress→glyph | spike 5 | Larger partial-refresh region; render multi-char bursts |
| 5 | H6 push % | ≥ 95 % on healthy Wi-Fi | spike 6 + spike 7 | TLS cipher trim; reconnect backoff tuning |
| 6 | H3 cadence | full every ~20 partials | spike 2 | Adjust per panel temperature; defer flash to idle ≥ 1 s |
| 7 | H4 boot | ≤ 5 s to cursor | integration smoke | Trim startup logging; lazy-mount SD after splash |
| 8 | H5 soak | 1 h no leak / no drop | 1 h bench soak | Glyph-cache eviction; PSRAM heap-fragmentation review |
The two not-in-MVP rows but already-shaped-by-design:
| — | H13 current | Measured only in v0.1 | bench multimeter | Cell sizing for v0.8 is data-driven, not spec-sheet | | — | H11 stacks | Sum ≤ 80 KB | static analysis | Was off-by-2x in ADR-006 pre-fix — corrected in §7 |
7. Tradeoffs and their why, linked to ADRs
Plain-language summary of what we accepted in exchange for what.
| Tradeoff | Got | Paid | ADR |
|---|---|---|---|
| std (esp-idf-rs) over no_std (esp-hal) | Heap, threads, VFS, mbedtls, gitoxide-compatible | +1 MB binary, +5–10 min builds | ADR-001 |
| Custom widget layer over Ratatui | Dirty-rects aligned to e-ink regions; 200 KB binary back | 500 LoC we own and maintain | ADR-002 |
| e-ink medium over FSTN / memory LCD / OLED | Paper aesthetic; 0 W idle persistence; medium enforces writing posture | ~200–300 ms typing latency; periodic full-refresh flash (scroll worst-case) | ADR-003 |
gitoxide over libgit2-sys |
Pure Rust, modular, no FFI cross-compile pain | Smart-HTTP path is newer; PSRAM profile unproven (spike 7) | ADR-004 |
| HTTPS + PAT over OAuth device-flow or SSH | Simplest auth that gitoxide smart-HTTP already supports |
Long-lived secret on device; in v0.1 the PAT is compiled into the binary (dev-only target user makes this acceptable); v0.9 moves it to encrypted NVS | ADR-005 |
std::thread over embassy or tokio |
Boring, debuggable, real stack traces; no exec to tune | ~76 KB total stack across 5 tasks | ADR-006 |
| FAT-on-SD + LittleFS-on-flash split | Desktop can read SD; config survives SD reformat | Two filesystems to manage; FAT's power-loss weakness mitigated by atomic-rename | ADR-007 |
| Wall power for v0.1, battery deferred | Measure real draw before sizing the cell | Tethered MVP; not the final aesthetic | ADR-008 |
| USB host (TinyUSB) over BLE-HID | No radio contention with Wi-Fi during push; keyboard powered from the device | One more USB connector on enclosure | ADR-009 |
Atomic Ctrl-G + auto-timestamp commit message |
One key, one outcome; matches the user's existing gct workflow; no modal prompt to slow H1 latency |
Commit history is timestamp noise; the device may author merge commits the user never sees; reversal would break muscle memory | ADR-010 |
Conflicts left explicitly unresolved by v0.1
These are the live tensions we are watching, not deciding harder:
- ADR-004 vs H9. If
gitoxidecannot keep ≥ 1 MB PSRAM free at push peak, we are committed to switching transports for v0.1, not absorbing the OOM risk. - ADR-009 vs H6/H13. If TinyUSB host turns out unstable (spike 4), BLE-HID is the documented fallback — at the cost of Wi-Fi radio contention during push (re-checking H6).
- ADR-007 vs H8. Power loss between FAT rename and dir flush yields the previous saved version. We document this as expected behavior; it becomes a real bug only if soak testing shows it triggering on routine saves.
- W13 typography paths. v0.1 ships one mono font; v1.0's writing-tool-tone outcome admits two paths (mono = developer comfort, serif = typewriter feel). Not yet decided whether to ship both or one; decision deferred to the v1.0 design pass. Cost preview per added font: +H9 glyph-cache footprint, +H10 binary for embedded assets.
- ADR-008 vs W11+W14. Wall power in v0.1 is now an explicit disappointment of two WHATs, not one (battery W11 + portability W14). The disappointment is bounded by ADR-008's commitment to measure current draw on real hardware before sizing v0.8's cell — spec the cell against measured numbers, not against the spec sheet. The §3 promotion of H13 (current draw) from #11 to #7 is the matrix registering this: bench multimeter readings now serve portability sizing as well as power profiling.
8. Inconsistencies spotted and fixed
- ADR-006 stack figure. ADR-006 previously said "~40 KB of stack
space for task stacks" — but the v0.1 technical design's task table
(
usb 8 + wifi 8 + ui 16 + render 12 + git 32) sums to 76 KB. Updated ADR-006's Consequences section to reflect the actual budget and cross-reference the tech doc. The 76 KB figure still fits comfortably in the ESP32-S3's 512 KB internal SRAM, so no design change — just documentation accuracy. - Commit-message format triple-mismatch. README said
git commit -m "wip", the v0.1 product doc said"wip <timestamp>", and the user's actual shell alias (gct/git-commit-timestamp) uses a pure ISO-8601 timestamp with nowipprefix. Resolved by aligning all docs ongctand recording the decision as ADR-010. Pulled the v0.7 roadmap item "Commit message prompt instead of hard-codedwip" — it's now contradicted by ADR-010 and removed. - First-run flow vs. target user. The v0.1 product doc described a captive-portal first-run, but the same doc names the v0.1 target user as the dev themselves ("Me. Solo."). Provisioning a solo-dev device through a captive portal is ceremony without a user. Resolved by switching v0.1 to build-time env-var config (no NVS, no LittleFS, no AP mode); on-device provisioning is the v0.9 release that introduces non-dev users. Touches ADR-005, ADR-007, the v0.1 product + technical docs, and the v0.9 roadmap entry.
- Vocabulary leak. Earlier docs used "commit" and "push" as if they
were distinct user actions; the gct/ADR-010 model collapses them into a
single user-facing Publish. Resolved by introducing
CONTEXT.mdas the canonical glossary; user-facing text now uses Save and Publish only. - House of Quality column sums recomputed. Earlier Σ row drifted from the matrix arithmetic — H1 listed 138 but sums to 148; H8 147 vs 132; H9 162 vs 172; H13 74 vs 65; smaller deltas elsewhere. Recomputed all sums from the cells. H8 dropped from #3 to #6 — a "fewer WHAT voters" artifact, not a signal that durability matters less to the design.
- W13 reframed, W14 removed. Earlier W13/W14 rows named solutions ("beautiful monospace", "beautiful serif") inside the requirements column, conflating what the user values with which asset delivers it. Replaced with one outcome WHAT — typography sets a writing-tool tone — and moved the mono+serif option to §7 as a v1.0 unresolved tension. Σ shifted (H9 205→193, H2 198→177, H1 155→148) because the prior W13/W14 cells were scoring solution-fit rather than outcome-fit.
- WHATs swept for solution-shape phrasing. Following the W13 reframe,
the same drift was found in W2 (named the key
Ctrl-G), W4 (named the process shape "one-shot"), W7 (named the hardware "surface"), W8 (named the medium "e-ink"), W10 (named the deliverable "BOM"), and W9 ("nine releases" — brittle vs roadmap reshuffles). All rephrased as outcomes; the named solutions remain documented in §7 tradeoffs and the relevant ADRs where they belong. Matrix cell strengths held — each cell scored the function against the underlying outcome, not the surface phrasing — so no Σ recompute. - §3 vs §6 priority lists clarified. The two were giving different orderings without saying why. §6 now states explicitly that it is a curated rank with two named overrides over §3's pure arithmetic: acceptance-criteria critical paths (H4, H5) and table-stakes correctness (H8) get manual lifts. §3 now names the HoQ structural bias that makes the curation necessary — reward for spread, penalty for narrow-but- critical functions — using H8/W3 as the canonical example.
- W14 added — portability outcome. Captures "I can carry the device and write away from a desk" as a distinct WHAT from W11 (multi-day battery), weight 8. Recomputed basement Σ; H8 lifted from #6 to #3 in the §3 priority list as its voter base widened from W3+W6+W12 to also include W14, and H12 entered the top six at #4; H6 dropped out. The ID "W14" was previously held by a deprecated typography row (see the "W13 reframed, W14 removed" bullet above); the slot is now repurposed. §6's "(b) narrow voter base" override for H8 no longer applies and has been retired in the §6 preamble.
The minor variance between README's "~12 lines" and product/ADR-003's "~11 lines" of edit area is within rounding for a 14 px glyph in a 240 px tall edit region and is not load-bearing.
How to keep this document honest
- When a new ADR lands, add its components to §5 and re-score any function-row whose dominant component changed.
- When a spike returns numbers, update §6's "Target" or "Watched on" columns — this is the doc that should feel out of date if measured reality drifts from estimates.
- The WHATs (§1) change rarely; the HOWs (§2) change with each release.
When either changes, re-score the matrix and recompute the basement Σ
in
quality-house.md; then check the §3 priority list and §4 conflict list here still match the new picture.