feat(lilygo-tdeck-max): wire up the EPD frontlight

[Crazypedia]

Closes #629. Builds on #633, which is now merged.

Devicetree only. The panel is reflective, so this lights it from the front, but it is still a BACKLIGHT_TYPE device and shows up as the usual Display brightness slider.

BL_PWM is GPIO41. It has to use LEDC timer/channel 1, because timer/channel 0 is already taken by the keyboard backlight on this board.

The backlight node is left status = "disabled" so Boot starts it and applies the stored brightness, the same handling every other board's backlight gets.

Tested: LilyGO T-Deck Max. The brightness slider appears in Display settings and lights the panel.

Two things worth noting, neither addressed here:

Boot logs W ledc: GPIO 41 is not usable, maybe conflict with others. It is benign: pwm_backlight's start() reserves the pin, then the boot brightness apply reserves it again, and the second esp_gpio_reserve() warns. The panel lights correctly. Happy to look at silencing it separately if you would rather not ship a warning.

This is also the board that motivated your backlight_set_brightness_default() idea on #625: it comes up at the board-agnostic 200/255 default, which lights a panel that is perfectly readable without it. I have deliberately left that out of here since it touches every board.

Summary by CodeRabbit

• New Features
  • Added PWM-based brightness control for the EPD display backlight.
  • Added dedicated backlight configuration support, disabled by default to preserve existing display behavior.
  • The backlight can now be connected to the display and controlled through compatible software or board configurations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: dd7542d4-f7af-4654-a838-6c529b57fbb9

📥 Commits

Reviewing files that changed from the base of the PR and between 5c96f49 and 28c33df.

📒 Files selected for processing (1)

• Devices/lilygo-tdeck-max/lilygo,tdeck-max.dts

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📝 Walkthrough

Walkthrough

The device tree connects the EPD display to a new pwm-backlight node. The backlight uses an ESP32 LEDC PWM channel on GPIO41 with a 33.3 µs period and timer/channel 1. The backlight node is disabled by default.

Merge Risk: ⚪ Minimal · up to 28c33

This change adds the LilyGO T-Deck Max EPD frontlight through its existing display brightness controls without altering broader runtime behavior; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: wiring the LilyGO T-Deck Max EPD frontlight.
Linked Issues check	✅ Passed	The devicetree changes configure the EPD frontlight on GPIO41 with LEDC timer/channel 1 and expose it through the standard pwm-backlight brightness control, matching issue #629.
Out of Scope Changes check	✅ Passed	The pull request contains only the requested devicetree changes for the LilyGO T-Deck Max EPD frontlight. No unrelated changes are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[KenVanHoeylandt]

The order of display_backlight is likely causing the build failure. It needs to be above the device that references it.