misc: Add ws2812-pio-rp1 driver

ws2812-pio-rp1 is a PIO-based driver for WS2812 LEDS. It creates a
character device in /dev, the default name of which is /dev/leds<n>,
where <n> is the instance number. The number of LEDS should be set
in the DT overlay, as should whether it is RGB or RGBW, and the default
brightness.

Write data to the /dev/* entry in a 4 bytes-per-pixel format in RGBW
order:

  RR GG BB WW RR GG BB WW ...

The white values are ignored unless the rgbw flag is set for the device.

To change the brightness, write a single byte to offset 0, 255 being
full brightness and 0 being off.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

drivers/misc/Kconfig

@@ -25,6 +25,16 @@ config RP1_PIO
 		Driver providing control of the Raspberry Pi PIO block, as found in
 		RP1.
 
+config WS2812_PIO_RP1
+	tristate "Raspberry Pi PIO-base WS2812 driver"
+	depends on RP1_PIO || COMPILE_TEST
+	default n
+	help
+		Driver for the WS2812 (NeoPixel) LEDs using the RP1 PIO hardware.
+		The driver creates a character device to which rgbw pixels may be
+		written. Single-byte writes to offset 0 set the brightness at
+		runtime.
+
 config AD525X_DPOT
 	tristate "Analog Devices Digital Potentiometers"
 	depends on (I2C || SPI) && SYSFS

drivers/misc/Makefile

@@ -19,6 +19,7 @@ obj-$(CONFIG_PHANTOM)		+= phantom.o
 obj-$(CONFIG_QCOM_COINCELL)	+= qcom-coincell.o
 obj-$(CONFIG_QCOM_FASTRPC)	+= fastrpc.o
 obj-$(CONFIG_RP1_PIO)		+= rp1-pio.o
+obj-$(CONFIG_WS2812_PIO_RP1)	+= ws2812-pio-rp1.o
 obj-$(CONFIG_SENSORS_BH1770)	+= bh1770glc.o
 obj-$(CONFIG_SENSORS_APDS990X)	+= apds990x.o
 obj-$(CONFIG_ENCLOSURE_SERVICES) += enclosure.o