led_strip: add a APA102 driver.

The APA102 is a RGB LED with integrated controller.  LEDs can be
daisy-chained and use SPI for communication.  The SPI port is
configured via Device Tree.

Tested on the Adafruit Trinket M0.

Signed-off-by: Michael Hope <mlhx@google.com>

drivers/led_strip/CMakeLists.txt

@@ -1,2 +1,3 @@
 zephyr_sources_ifdef(CONFIG_LPD880X_STRIP lpd880x.c)
 zephyr_sources_ifdef(CONFIG_WS2812_STRIP  ws2812.c)
+zephyr_sources_ifdef(CONFIG_APA102_STRIP  apa102.c)

drivers/led_strip/Kconfig

@@ -46,4 +46,6 @@ source "drivers/led_strip/Kconfig.lpd880x"
 
 source "drivers/led_strip/Kconfig.ws2812"
 
+source "drivers/led_strip/Kconfig.apa102"
+
 endif # LED_STRIP

drivers/led_strip/Kconfig.apa102

@@ -0,0 +1,15 @@
+#
+# Copyright (c) 2018 Google LLC.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+menuconfig APA102_STRIP
+	bool "APA102 SPI LED strip driver"
+	depends on SPI
+	depends on !SPI_LEGACY_API
+	select LED_STRIP_RGB_SCRATCH
+	default n
+	help
+	 Enable the LED strip driver for a chain of APA102 RGB LEDs.
+	 These are sold as DotStar by Adafruit and Superled by others.

drivers/led_strip/apa102.c

@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2018 Google LLC.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <errno.h>
+#include <led_strip.h>
+#include <spi.h>
+
+struct apa102_data {
+	struct spi_config cfg;
+};
+
+static int apa102_update(struct device *dev, void *buf, size_t size)
+{
+	struct apa102_data *data = dev->driver_data;
+	static const u8_t zeros[] = {0, 0, 0, 0};
+	static const u8_t ones[] = {0xFF, 0xFF, 0xFF, 0xFF};
+	struct spi_buf tx[] = {
+		{
+			/* Start frame: at least 32 zeros */
+			.buf = (u8_t *)zeros,
+			.len = sizeof(zeros),
+		},
+		{
+			/* LED data itself */
+			.buf = buf,
+			.len = size,
+		},
+		{
+			/* End frame: at least 32 ones to clock the
+			 * remaining bits to the LEDs at the end of
+			 * the strip.
+			 */
+			.buf = (u8_t *)ones,
+			.len = sizeof(ones),
+		},
+	};
+
+	return spi_write(&data->cfg, tx, ARRAY_SIZE(tx));
+}
+
+static int apa102_update_rgb(struct device *dev, struct led_rgb *pixels,
+			     size_t count)
+{
+	u8_t *p = (u8_t *)pixels;
+	size_t i;
+	/* SOF (3 bits) followed by the 0 to 31 global dimming level */
+	u8_t prefix = 0xE0 | 31;
+
+	/* Rewrite to the on-wire format */
+	for (i = 0; i < count; i++) {
+		u8_t r = pixels[i].r;
+		u8_t g = pixels[i].g;
+		u8_t b = pixels[i].b;
+
+		*p++ = prefix;
+		*p++ = b;
+		*p++ = g;
+		*p++ = r;
+	}
+
+	BUILD_ASSERT(sizeof(struct led_rgb) == 4);
+	return apa102_update(dev, pixels, sizeof(struct led_rgb) * count);
+}
+
+static int apa102_update_channels(struct device *dev, u8_t *channels,
+				  size_t num_channels)
+{
+	/* Not implemented */
+	return -EINVAL;
+}
+
+static int apa102_init(struct device *dev)
+{
+	struct apa102_data *data = dev->driver_data;
+	struct device *spi;
+
+	spi = device_get_binding(CONFIG_APA102_STRIP_BUS_NAME);
+	if (!spi) {
+		return -ENODEV;
+	}
+
+	data->cfg.dev = spi;
+	data->cfg.frequency = CONFIG_APA102_STRIP_FREQUENCY;
+	data->cfg.operation =
+		SPI_OP_MODE_MASTER | SPI_TRANSFER_MSB | SPI_WORD_SET(8);
+
+	return 0;
+}
+
+static struct apa102_data apa102_data_0;
+
+static const struct led_strip_driver_api apa102_api = {
+	.update_rgb = apa102_update_rgb,
+	.update_channels = apa102_update_channels,
+};
+
+DEVICE_AND_API_INIT(apa102_0, CONFIG_APA102_STRIP_NAME, apa102_init,
+		    &apa102_data_0, NULL, POST_KERNEL,
+		    CONFIG_LED_STRIP_INIT_PRIORITY, &apa102_api);

dts/bindings/led_strip/apa,apa-102.yaml

@@ -0,0 +1,17 @@
+---
+title: APA102 SPI LED strip
+id: apa,apa102
+version: 0.1
+
+description: APA102 SPI LED strip binding
+
+inherits:
+    !include spi-device.yaml
+
+properties:
+    compatible:
+      type: string
+      category: required
+      description: compatible strings
+      constraint: "apa,apa102"
+...