rgbled-fb

framebuffer device driver that implements a simple framebuffer device for:

• ws2812b - tested with adafruit-arc - 15 pixel and 32x8 panel)
• ws2812 - not tested due to lack of device
• apa102 - not tested due to lack of device

hw-setup/level translation

Basically you have to set it up with a 74HCT125 as a level translator

• ws2812b (and ws2812):
• SPI-CS is connected to 1/OE
• SPI-MOSI is connected to 1A
• WS2812-DI is connected to 1Y (possibly with a pulldown)
• apa102
• SPI-CS is connected to 1/OE and 2/OE
• SPI-SCK is connected to 1A
• SPI-MOSI is connected to 2A
• APA102-CI is connected to 1Y (possibly with pulldowns)
• APA102-DI is connected to 1Y (possibly with pulldowns)

device-tree

You need to configure the framebuffer via the device-tree

Here an example for a ws2812b string that creates a 40x16 pixel framebuffer from 2 matrix 8x32 plus 2 matrix 8x8:

fb2: fb@0 {
	reg = <0>;
	status = "okay";

	compatible = "worldsemi,ws2812b";
	/* spi speed needs to be 3 x the 800kHz pixel rate of the ws2812b */
	spi-max-frequency = <2400000>;
	/* limit current for the whole panel to 4A
	 * - depending on power supply */
	current-limit = <4000>;
	/*
	 * define the panels
	 * note that if they overlap they will produce identical content
	 * for the defined portion
	 */
	panel@0 {
		reg = <0>;
		compatible = "adafruit,neopixel,matrix,32x8";
	};
	panel@1 {
		reg = <1>;
		compatible = "adafruit,neopixel,matrix,32x8";
		x = <0>;
		y = <8>;
	};
	panel@1 {
		reg = <2>;
		compatible = "adafruit,neopixel,matrix,8x8";
		/* set the coordinates */
		x = <32>;
		y = <0>;
		multiple = <2>;
	};
};

sysfs

Lots of values are exposed in /sys/class/graphics/fbX/:

• led_count - number of LED in the "strip"
• led_max_current_blue - mAmper that a single led consumes at full power blue
• led_max_current_green - mAmper that a single led consumes at full power green
• led_max_current_red - mAmper that a single led consumes at full power red
• current - estimated mAmper that the led string consumes
• current_max - estimated maximum mAmper that the led string consumed
• current_limit - current limit in mAmper that triggers a reduction in overall brigthness to stay below this value
• brightness - overall display brightness (scaled automatically to limit current)

Missing/todo:

• better documentation
• upstreaming to official kernel
• merge with foundation kernels
• mixing different densities
  • allowing "regional averaging"
  • allowing "radial averaging" (for "Ambilight(tm)" like approaches)
• panel rotation