Arduino: correct the link mode guidance and pin the board core in CI

[psiddh]

Two users hit the same wall within an hour of the library reaching Library
Manager: install it, open an example, Verify, and get

Sketch too big; text section exceeds available space

The README described Dynamic link mode as producing "no serial output at all,
so the board looks dead", which is wrong -- it fails at compile time, so a
user searching for the error they actually saw found nothing. Corrected, and
moved up into the error table with the real message.

Why it happens, since the README had no explanation: Dynamic does a relocatable
link (-r). --gc-sections is passed in both modes but can only work in a final
link, where the linker has an entry point to trace reachability from. Under -r
nothing can be proven unreachable, so every vendored operator survives, and
because a loadable extension is loaded into RAM the retained code is charged
against RAM too. AddModel on core 0.90.0: 507,876 bytes static, 787,508 dynamic.

Recorded that the library cannot work around this. Trimming the vendored
operator sources to only the registered set -- 15 instead of 172 -- moved the
dynamic build 852 bytes. The bulk is the runtime, flatbuffers and CMSIS-NN.

Memory figures were measured on core 0.55.2, which reported a 131,072-byte RAM
ceiling; 0.90.0 reports 262,144, so they were not comparable to anything a
current user sees. Re-measured all three examples on 0.90.0. The KeywordSpotting
arena band is now scoped to the core it was measured on, with the upper bound
marked untested on 0.90.0 -- 40 KB is confirmed, above that is not.

CI pins arduino:zephyr@0.90.0 and Arduino_RouterBridge@0.4.3 rather than
resolving to latest, so a failure means a real incompatibility instead of an
upstream release landing under us. The core moved 0.55.2 -> 0.90.0 unannounced
during this work. Adds the retry and network.connection_timeout hardening the
~1 GB toolchain download needs, since that toolchain is a ~1 GB GitHub release
asset that outruns arduino-cli's default HTTP timeout.

Bumps library.properties to version 0.1.1.

Tracking of dynamic-mode sizes belongs in meta-pytorch/executorch-arduino, on a
nightly, rather than here: it is a property of the published artifact, and three
extra compiles per pull request touching runtime/ or kernels/portable/ is a poor
trade for telemetry this repo cannot act on.

Authored with assistance from Claude Code.

[pytorch-bot]

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 3023c47 with merge base c461421 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  java.lang.IllegalStateException: Cannot access system provider: 'settings' before system providers are installed!

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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[Copilot]

Pull request overview

This PR updates the Arduino library documentation and CI to reflect the Zephyr Uno Q core’s default “Dynamic” link mode behavior (which fails at compile/link time due to lack of effective --gc-sections under relocatable linking), and makes CI more reproducible by pinning the board core and key dependency versions.

Changes: (1) Correct and expand the README’s guidance/error table around link_mode, including updated size/memory measurements on core 0.90.0; (2) bump Arduino library metadata (version/author/maintainer) in library.properties; (3) harden and stabilize CI by pinning arduino:zephyr and Arduino_RouterBridge, adding retry/timeout tuning, and adding an informational Dynamic-mode compile to track the size gap.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
examples/arduino/README.md	Corrects Dynamic-vs-Static guidance, explains why Dynamic overflows, and refreshes memory/size figures for the current core.
examples/arduino/library.properties	Updates published library metadata and bumps version to 0.1.1.
.github/workflows/_test_arduino_library.yml	Pins Arduino core/dependency versions, adds download hardening, and records Dynamic-mode compile outcomes without gating CI.

---

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (4)

examples/arduino/README.md:257

• The last sentence reads as if both Static and Dynamic result in no serial output ("either way"), but only Dynamic fails at compile time; Static builds and can produce serial output. Consider clarifying that the lack of serial output is specific to the Dynamic-mode build failure (nothing gets flashed).

  `Sketch too big; text section exceeds available space`. Same sketch, core
  0.90.0: 507,876 bytes on Static, 787,508 on Dynamic. It fails at compile
  time, so there is nothing to see on the serial port either way.

examples/arduino/library.properties:9

• PR description says library.properties author should be updated to PyTorch Team <packages@pytorch.org>, but the file still lists Meta Platforms. This is a discrepancy that will affect published Library Manager metadata.

version=0.1.1
author=Meta Platforms

.github/workflows/_test_arduino_library.yml:78

• This job explicitly avoids using HOME because it may be unwritable in the container, but arduino-cli config init writes the config under HOME by default. As-is, CI may fail when config init can’t create/write its config directory. Set HOME to a writable path (e.g., ${ARDUINO_CI_DIR}) before running arduino-cli config commands.

        # The Zephyr core pulls a ~1 GB toolchain from a GitHub release, which
        # outruns arduino-cli's default HTTP timeout often enough to matter, and
        # `core install` has no retry of its own.
        arduino-cli config init --overwrite
        arduino-cli config set network.connection_timeout 600s

.github/workflows/_test_arduino_library.yml:94

• This script sources .ci/scripts/utils.sh, which already defines a retry() function. Redefining retry() here silently shadows the shared helper, which is easy to miss and can cause confusing behavior if later code expects the utils.sh semantics. Rename the local helper to avoid the collision.

        retry() {
          local n=1
          until "$@"; do
            if [ "${n}" -ge 3 ]; then
              echo "::error::'$*' failed after ${n} attempts"

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (1)

.github/workflows/_test_arduino_library.yml:105

• The PR description says CI now performs an informational Dynamic (link_mode=dynamic) compile to track the size gap, but this workflow still only compiles sketches with link_mode=static. If Dynamic coverage is intended, add a non-gating Dynamic compile pass (expected to fail today) so the output is recorded and an unexpected success is flagged.

        # link_mode=static is not optional. Dynamic is the board default and
        # does a relocatable link, which makes --gc-sections inert: nothing
        # unused is stripped and the build overflows flash.
        FQBN="arduino:zephyr:unoq:link_mode=static"
        for sketch in arduino_lib/ExecuTorch/examples/*/; do
          echo "::group::compile $(basename "${sketch}")"
          arduino-cli compile --fqbn "${FQBN}" \
            --libraries arduino_lib "${sketch}"
          echo "::endgroup::"
        done

[rascani]

Should we align these numbers with ET release?