An ESP32 firmware, built by PlatformIO

cpp_host proves the C ABI on a desktop, with CMake. This proves the thing a firmware author actually has to do: get Rust compiled by PlatformIO, linked into an ESP32 image, and running beside the C++ that was already there. The port changed no Rust at all; what a laptop and a device disagree about is the six files under cpp/ — main.cpp, four host_*.cpp, and one translation unit that gives the header-only SDK a home.

Using it

export FREEINK_SDK_DIR=/path/to/freeink-sdk   # once

pio run -e simulator_x3    # a window, on this machine
pio run -e default         # Xteink X3, ESP32-C3
pio run -e sticky          # Seeed Sticky, ESP32-S3

All three build. The two device environments produce flashable images — 265 kB for the C3, 280 kB for the S3 — and the simulator one produces the same desktop binary CMake does, self-test and all. There is no panel driver: flush() in cpp/main.cpp counts the ink and logs it, which is where one goes.

Homebrew's pio may lack the littlefs module the espressif32 builder wants. If a device build stops with ModuleNotFoundError: No module named 'littlefs', use ~/.platformio/penv/bin/pio.

Checking it

No gate invokes this directory: ./build-and-test.sh builds the desktop host through CMake, so a break here surfaces when somebody runs pio run. Build all three environments before pushing a change to cpp_host or to the boundary.

Where next

docs/boundary.md

what porting to a device cost, why the runtime lives in cpp_host, the heap figures that become true on a device, the panel-driver seam, and what is kept in step with CrossPoint by hand