chatter/doc/Chatter_phase1_findings.md

Chatter — Phase 1 On-Device Findings

Date: 2026-06-25 Device: reMarkable Paper Pro Move (RM03A, codename "Chiappa") Method: Read-only SSH probing from Jatke (ssh chatter). No changes made to the device except enabling developer mode / SSH (Phase 0) and adding our SSH key.

This document records what we learned and the implementation decisions those findings support. It feeds the phase plan in Chatter_implementation.md.

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1. Hardware & OS

• SoC / arch: NXP i.MX93, aarch64 (Arm Cortex-A55). Kernel 6.12.49+git-imx93-chiappa.
• OS: Codex Linux 5.7.121 (Yocto scarthgap), image 3.27.1.0. glibc (/lib/ld-linux-aarch64.so.1, libc.so.6).
• RAM: 2 GB (~1.4 GB free) — ample for our app.
• Storage (matters for deployment):
  • / (rootfs): ~435 MB, only ~88 MB free (78% used) — also an OS-managed overlay, likely reset by firmware updates. Do not install here.
  • /home: 46 GB encrypted volume, ~45.8 GB free. Install Chatter under /home (e.g. /home/root/chatter).

2. Input devices

Pen and finger are separate evdev devices — finger-vs-stylus discrimination is free, exactly what Chatter's gesture design needs.

Node	Name	Role	Notes
event0	bbnsm:pwrkey	Power button
event1	Hall effect sensors	Folio open/close	switch events
event2	Elan marker input	Stylus / Marker	on SPI
event3	Elan touch input	Capacitive multitouch (finger)	INPUT_PROP_DIRECT

Pen (event2) — digitizer space 6760 × 11960:

• Buttons: BTN_TOOL_PEN, BTN_TOOL_RUBBER (Marker Plus eraser end — flip-to-erase is detectable), BTN_TOUCH, BTN_STYLUS, BTN_STYLUS2.
• ABS_PRESSURE 0–4096, ABS_DISTANCE (hover) 0–65535, ABS_TILT_X/Y ±9000.

Touch (event3) — grid 1248 × 2208:

• Up to 10 contacts (ABS_MT_SLOT 0–9) with ABS_MT_POSITION_X/Y, ABS_MT_PRESSURE (0–255), ABS_MT_TRACKING_ID, ABS_MT_TOOL_TYPE.

All three coordinate spaces share the screen's portrait aspect (~0.5625): panel 954×1696, pen 6760×11960, touch 1248×2208.

3. Display pipeline

• DRM/KMS only — there is NO /dev/fb* and /sys/class/graphics is empty. Driver is imx-drm; device /dev/dri/card0; connector card0-LVDS-1 ("connected", "disabled" at rest — normal for e-ink, which holds its image without continuous scanout).
• The connector advertises a packed 365×1700 mode (hardware buffer for the E Ink Gallery 3 color subpixel layout); the logical screen is 954×1696. The mapping is handled by the epaper plugin (below) — we don't touch it directly.
• Implication: the mmap /dev/fb0 approach used by rmkit / libreMarkable on older grayscale models does not apply to this device.

4. Graphics / UI stack — the key finding

• The stock app xochitl is a Qt 6.8.2 / Qt Quick (QML) application.
• It renders through a custom Qt platform (QPA) plugin: epaper (/usr/lib/plugins/platforms/libepaper.so, links libdrm). Available QPA plugins on-device: epaper, minimal, offscreen, vnc.
• Qt 6.8.2 runtime libraries and QML modules (QtQuick, QtQuickControls2, etc.) are all present under /usr/lib and /usr/lib/qml. No dev headers on-device (expected — we cross-build).

This means the native path is wide open: build Chatter as a Qt 6 app and run it with -platform epaper to inherit working e-ink display + refresh, instead of reverse-engineering DRM/KMS and waveform handling. This resolves the project's highest-risk unknown in our favor.

5. Stock app & services

• xochitl.service — the main UI app (/usr/bin/xochitl, pid varies).
• marker-manager.service — "Remarkable CSL Marker Manager" (pen support).
• rm-sync.service — document sync to the reMarkable cloud.
• Plus metrics / MDM / crash-uploader services.

Coexistence / toggle implication: on a DRM/KMS device only one process owns the display (DRM master) at a time, and xochitl holds it (and likely grabs the input devices) while running. So the Chatter ↔ standard-GUI toggle is a hand-off (stop or pause xochitl ↔ run Chatter, and a return path), not an overlay. The exact mechanism (systemctl stop/start vs. SIGSTOP/SIGCONT) is to be validated in the Phase 2 spike.

6. Document store (for the Save phase)

Location: /home/root/.local/share/remarkable/xochitl/. Per document:

• UUID.metadata — JSON: visibleName, parent (empty = top level; a folder is a CollectionType document whose UUID is used as children's parent), type (DocumentType), timestamps, pinned, etc.
• UUID.content — JSON: page list under cPages.pages[] (each page has an id, ordering idx, template, scroll position, CRDT-style timestamp fields).
• UUID/ — directory of per-page <pageUUID>.rm files.
• UUID.thumbnails/ — page thumbnails.
• Store-root extras (Codex additions): .tree (binary "rM sync tree" index) and rm-search-index.db (SQLite search index, not the canonical store).

.rm page format: header confirmed as reMarkable .lines file, version=6 — the v6 binary scene-tree format (documented by the community, e.g. rmscene). Authoring valid v6 is possible but non-trivial.

Save options (decide in Phase 5):

1. PDF/PNG-backed document — render Chatter's canvas to PDF/PNG and create a document that references it (xochitl already supports PDF documents). Far simpler and robust; still a real, renamable/syncable library item.
2. Native v6 .rm — author the binary page format so the transcript is an editable notebook. More work; revisit only if option 1 proves insufficient.

Either way, transcripts go under a Chatter CollectionType folder, with filenames derived from date/time.

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7. Decisions supported by Phase 1

1. Stack: C++ / Qt 6.8.2, rendered via the on-device epaper QPA plugin. Cross-build with a Yocto scarthgap aarch64 SDK matching Qt 6.8.2 / the device glibc.
2. Input: pen from event2 (including BTN_TOOL_RUBBER for flip-to-erase), finger from event3 (10-pt). Confirm in Phase 2 whether Qt+epaper already delivers these or we read evdev directly for custom gestures.
3. Toggle = display/input hand-off with xochitl, not an overlay.
4. Save = PDF-backed document (leaning), in a Chatter folder.
5. Deploy under /home, never the rootfs.

8. Open items for the Phase 2 spike

• Stand up the cross-SDK and build a trivial Qt app; run it with -platform epaper after stopping xochitl; confirm it draws and refreshes.
• Verify how input arrives in a Qt app under epaper (Qt event stream vs. raw evdev) and how reliably finger and pen separate at the Qt layer.
• Validate the toggle hand-off (stop/start vs. stop/cont) and a return path.