drivers: led_strip: Add driver for software-based WS2812B operation

This driver uses a bit-banging based technique of generating a signal
for the WS2812B LED strip. Since bit-banging is very timing sensitive,
where each CPU cycle counts, the driver uses inline assembly to
perform the most critical operataions. This initial version of the
driver only supports a Cortex-M0 implementation, and can e.g. be used
with the ZIP Halo LED strip for the BBC microbit:

https://www.kitronik.co.uk/5625-zip-halo-for-the-bbc-microbit.html

Signed-off-by: Johan Hedberg <[email protected]>

Author: Johan Hedberg

drivers/led_strip/CMakeLists.txt

@@ -1,3 +1,4 @@
 zephyr_sources_ifdef(CONFIG_LPD880X_STRIP lpd880x.c)
 zephyr_sources_ifdef(CONFIG_WS2812_STRIP  ws2812.c)
+zephyr_sources_ifdef(CONFIG_WS2812B_SW  ws2812b_sw.c)
 zephyr_sources_ifdef(CONFIG_APA102_STRIP  apa102.c)

drivers/led_strip/Kconfig

@@ -1,5 +1,6 @@
 #
 # Copyright (c) 2017 Linaro Limited
+# Copyright (c) 2018 Intel Corporation
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -46,6 +47,8 @@ source "drivers/led_strip/Kconfig.lpd880x"
 
 source "drivers/led_strip/Kconfig.ws2812"
 
+source "drivers/led_strip/Kconfig.ws2812b_sw"
+
 source "drivers/led_strip/Kconfig.apa102"
 
 endif # LED_STRIP

drivers/led_strip/Kconfig.ws2812b_sw

@@ -0,0 +1,39 @@
+#
+# Copyright (c) 2018 Intel Corporation
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+menuconfig WS2812B_SW
+	bool "Enable WS2812B software-based LED strip driver"
+	# Only an Cortex-M0 inline assembly implementation for the nRF51
+	# is supported currently.
+	depends on SOC_SERIES_NRF51X
+	help
+	  A software-based (bit-banging) LED strip driver for daisy
+	  chains of WS2812B devices. This driver implements the signal
+	  sending with software-based bit-banging. If a more efficient
+	  option, such as SPI, is available, another driver is recommended
+	  to be used.
+
+if WS2812B_SW
+
+config WS2812B_SW_NAME
+	string "Driver name"
+	default "ws2812b_sw"
+	help
+	  Device name for WS2812B LED strip.
+
+config WS2812B_SW_GPIO_NAME
+	string "GPIO port that the LED strip is connected to"
+	default GPIO_NRF5_P0_DEV_NAME if SOC_FAMILY_NRF5
+	help
+	  GPIO port name.
+
+config WS2812B_SW_GPIO_PIN
+	int "GPIO pin that the LED strip is connected to"
+	default 3 if BOARD_BBC_MICROBIT # P0
+	help
+	  GPIO pin number that the LED strip is connected to.
+
+endif

drivers/led_strip/ws2812b_sw.c

@@ -0,0 +1,148 @@
+/*
+ * Copyright (c) 2018 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <led_strip.h>
+
+#include <string.h>
+
+#define SYS_LOG_LEVEL CONFIG_SYS_LOG_LED_STRIP_LEVEL
+#include <logging/sys_log.h>
+
+#include <zephyr.h>
+#include <board.h>
+#include <gpio.h>
+#include <device.h>
+#include <clock_control.h>
+
+#define BLOCKING ((void *)1)
+
+static int send_buf(u8_t *buf, size_t len)
+{
+	/* Address of OUTSET. OUTCLR is OUTSET + 4 */
+	volatile u32_t *base = (u32_t *)(NRF_GPIO_BASE + 0x508);
+	u32_t pin = BIT(CONFIG_WS2812B_SW_GPIO_PIN);
+	struct device *clock;
+	unsigned int key;
+	/* Initilization of i is strictly not needed, but it avoids an
+	 * uninitialized warning with the inline assembly.
+	 */
+	u32_t i = 0;
+
+	clock = device_get_binding(CONFIG_CLOCK_CONTROL_NRF5_M16SRC_DRV_NAME);
+	if (!clock) {
+		SYS_LOG_ERR("Unable to get HF clock");
+		return -EIO;
+	}
+
+	/* The inline assembly further below is designed to work only with
+	 * the 16 MHz clock enabled.
+	 */
+	clock_control_on(clock, BLOCKING);
+	key = irq_lock();
+
+	while (len--) {
+		u32_t b = *buf++;
+
+		/* Generate signal out of the bits, MSB. 1-bit should be
+		 * roughly 0.85us high, 0.4us low, whereas a 0-bit should be
+		 * roughly 0.4us high, 0.85us low.
+		 */
+		__asm volatile ("movs %[i], #8\n" /* i = 8 */
+				".start_bit:\n"
+
+				/* OUTSET = BIT(LED_PIN) */
+				"strb %[p], [%[r], #0]\n"
+
+				/* if (b & 0x80) goto .long */
+				"tst %[b], %[m]\n"
+				"bne .long\n"
+
+				/* 0-bit */
+				"nop\nnop\n"
+				/* OUTCLR = BIT(LED_PIN) */
+				"strb %[p], [%[r], #4]\n"
+				"nop\nnop\nnop\n"
+				"b .next_bit\n"
+
+				/* 1-bit */
+				".long:\n"
+				"nop\nnop\nnop\nnop\nnop\nnop\nnop\n"
+				/* OUTCLR = BIT(LED_PIN) */
+				"strb %[p], [%[r], #4]\n"
+
+				".next_bit:\n"
+				/* b <<= 1 */
+				"lsl %[b], #1\n"
+				/* i-- */
+				"sub %[i], #1\n"
+				/* if (i > 0) goto .start_bit */
+				"bne .start_bit\n"
+				:
+				[i] "+r" (i)
+				:
+				[b] "l" (b),
+				[m] "l" (0x80),
+				[r] "l" (base),
+				[p] "r" (pin)
+				:);
+	}
+
+	irq_unlock(key);
+	clock_control_off(clock, NULL);
+
+	return 0;
+}
+
+static int ws2812b_sw_update_rgb(struct device *dev, struct led_rgb *pixels,
+				 size_t num_pixels)
+{
+	u8_t *ptr = (u8_t *)pixels;
+	size_t i;
+
+	/* Convert from RGB to GRB format */
+	for (i = 0; i < num_pixels; i++) {
+		u8_t r = pixels[i].r;
+		u8_t b = pixels[i].b;
+		u8_t g = pixels[i].g;
+
+		*ptr++ = g;
+		*ptr++ = r;
+		*ptr++ = b;
+	}
+
+	return send_buf((u8_t *)pixels, num_pixels * 3);
+}
+
+static int ws2812b_sw_update_channels(struct device *dev, u8_t *channels,
+				      size_t num_channels)
+{
+	SYS_LOG_ERR("update_channels not implemented");
+	return -ENOSYS;
+}
+
+static int ws2812b_sw_init(struct device *dev)
+{
+	struct device *gpio;
+
+	gpio = device_get_binding(CONFIG_WS2812B_SW_GPIO_NAME);
+	if (!gpio) {
+		SYS_LOG_ERR("Unable to find %s", CONFIG_WS2812B_SW_GPIO_NAME);
+		return -ENODEV;
+	}
+
+	gpio_pin_configure(gpio, CONFIG_WS2812B_SW_GPIO_PIN, GPIO_DIR_OUT);
+
+	return 0;
+}
+
+static const struct led_strip_driver_api ws2812b_sw_api = {
+	.update_rgb = ws2812b_sw_update_rgb,
+	.update_channels = ws2812b_sw_update_channels,
+};
+
+DEVICE_AND_API_INIT(ws2812b_sw, CONFIG_WS2812B_SW_NAME, ws2812b_sw_init, NULL,
+		    NULL, POST_KERNEL, CONFIG_LED_STRIP_INIT_PRIORITY,
+		    &ws2812b_sw_api);