Add underglow feature docs + DT fixes

app/boards/shields/kyria/boards/nice_nano.overlay

@@ -1,6 +1,5 @@
 &spi1 {
 	compatible = "nordic,nrf-spi";
-	/* Cannot be used together with i2c0. */
 	status = "okay";
 	mosi-pin = <6>;
 	// Unused pins, needed for SPI definition, but not used by the ws2812 driver itself.

app/boards/shields/kyria/kyria.conf

@@ -4,3 +4,6 @@
 
 # Uncomment the following line to enable the Kyria OLED Display
 # CONFIG_ZMK_DISPLAY=y
+
+# Uncomment the following lineto enable RGB underglow
+# CONFIG_ZMK_RGB_UNDERGLOW=y

docs/docs/feature/underglow.md

@@ -1,5 +1,119 @@
 ---
 title: RGB Underglow
+sidebar_label: RGB Underglow
 ---
 
-TODO: Documentation on RGB underglow.
+RGB underglow is a feature used to control "strips" of RGB LEDs. Most of the time this is called underglow and creates a glow underneath the board using a ring of LEDs around the edge, hence the name. However, this can be extended to be used to control anything from a single LED to a long string of LEDs anywhere on the keyboard.
+
+ZMK supports all the RGB LEDs supported by Zephyr. Here's the current list supported:
+
+- WS2812-ish (WS2812B, WS2813, SK6812, or compatible)
+- APA102
+- LPD880x (LPD8803, LPD8806, or compatible)
+
+Of the compatible types, the WS2812 LED family is by far the most popular type. Currently each of these types of LEDs are expected to be run using SPI with a couple of exceptions.
+
+Here you can see the RGB underglow feature in action using WS2812 LEDs.
+
+<iframe src="//www.youtube.com/embed/2KJkq8ssDU0" frameborder="0" allowfullscreen width="100%"></iframe>
+
+## Enabling RGB Underglow
+
+To enable RGB underglow on your board or shield, simply enable the `ZMK_RGB_UNDERGLOW` configuration value in the `.conf` file of your user config directory as such:
+
+```
+CONFIG_ZMK_RGB_UNDERGLOW=y
+```
+
+If your board or shield does not have RGB underglow configured, refer to [Adding RGB Underglow to a Board](#adding-rgb-underglow-to-a-board).
+
+## Configuring RGB Underglow
+
+There are various Kconfig options used to configure the RGB underglow feature. These can all be set in the `.conf` file.
+
+| Option                       | Description                                    | Default |
+| ---------------------------- | ---------------------------------------------- | ------- |
+| `ZMK_RGB_UNDERGLOW_HUE_STEP` | Hue step in degrees of 360 used by RGB actions | `10`    |
+| `ZMK_RGB_UNDERGLOW_SAT_STEP` | Saturation step in percent used by RGB actions | `10`    |
+| `ZMK_RGB_UNDERGLOW_BRT_STEP` | Brightness step in percent used by RGB actions | `10`    |
+
+## Adding RGB Underglow to a Board
+
+RGB underglow is always added to a board, not a shield. This is a consequence of needing to configure SPI to control the LEDs.
+If you have a shield with RGB underglow, you must add a `boards/` directory within your shield folder to define the RGB underglow individually for each board that supports the shield.
+Inside the `boards/` folder, you define a `<board>.overlay` for each different board.
+For example, the Kyria shield has a `boards/nice_nano.overlay` file that defines the RGB underglow for the `nice_nano` board specifically.
+
+The first step to adding support for underglow is to select you SPI output. With nRF52 boards, you can just use `&spi1` and define the pins you want to use.
+For other boards, you must select an SPI definition that has the `MOSI` pin as your data pin going to your LED strip.
+
+Here's an example of an nRF52 SPI definition:
+
+```
+&spi1 {
+  compatible = "nordic,nrf-spi";
+  status = "okay";
+  mosi-pin = <6>;
+  // Unused pins, needed for SPI definition, but not used by the ws2812 driver itself.
+  sck-pin = <5>;
+  miso-pin = <7>;
+
+  led_strip: ws2812@0 {
+    compatible = "worldsemi,ws2812-spi";
+    label = "WS2812";
+
+    /* SPI */
+    reg = <0>; /* ignored, but necessary for SPI bindings */
+    spi-max-frequency = <4000000>;
+
+    /* WS2812 */
+    chain-length = <10>; /* number of LEDs */
+    spi-one-frame = <0x70>;
+    spi-zero-frame = <0x40>;
+  };
+};
+```
+
+:::info
+
+If you are configuring SPI for an nRF52840 (or other nRF52) based board, double check that you are using pins that aren't restricted to low frequency I/O.
+Ignoring these restrictions may result in poor wireless performance. You can find the list of low frequency I/O pins [here](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fpin.html&cp=4_0_0_6_0).
+
+:::
+
+Here's another example for a non-nRF52 board on `spi1`:
+
+```
+&spi1 {
+
+  led_strip: ws2812@0 {
+    compatible = "worldsemi,ws2812-spi";
+    label = "WS2812";
+
+    /* SPI */
+    reg = <0>;
+    spi-max-frequency = <5250000>;
+
+    /* WS2812 */
+    chain-length = <10>; /* number of LEDs */
+    spi-one-frame = <0x70>; /* make sure to configure this properly for your SOC */
+    spi-zero-frame = <0x40>; /* make sure to configure this properly for your SOC */
+  };
+};
+```
+
+Once you have your `led_strip` properly defined you need to add it to the root devicetree node `chosen` element:
+
+```
+/ {
+	chosen {
+		zmk,underglow = &led_strip;
+	};
+};
+```
+
+Finally you need to enable the `ZMK_RGB_UNDERGLOW` configuration value in the `.conf` file of your board (or set a default in the `Kconfig.defconfig`):
+
+```
+CONFIG_ZMK_RGB_UNDERGLOW=y
+```