misc: ws2812-pio-rp1: Add pass-through mode

Pass-through mode disables all gamma and brightness processing, sending
the raw pixel data directly to the LEDs. It is enabled by setting the
brightness to zero, either in Device Tree or using the runtime method of
writing a single byte (in this case 0) to the device.

See: https://github.com/raspberrypi/linux/issues/7108

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

arch/arm/boot/dts/overlays/README

@@ -5986,8 +5986,10 @@ Params: brightness              Set the initial brightness for the LEDs. The
                                 a single byte to offset 0 of the device. Note
                                 that brightness is a multiplier for the pixel
                                 values, and only white pixels can reach the
-                                maximum visible brightness. (range 0-255,
-                                default 255)
+                                maximum visible brightness. N.B. Setting
+                                brightness to 0 acctivates pass-through mode,
+                                disabling all brightness and gamma processing.
+                                (range 0-255, default 255)
         dev_name                The name for the /dev/ device entry. Note that
                                 if the name includes '%d' it will be replaced
                                 by the instance number. (default 'leds%d')

drivers/misc/ws2812-pio-rp1.c

@@ -142,8 +142,8 @@ static uint8_t ws2812_apply_gamma(uint8_t brightness, uint8_t val)
 {
 	int bright;
 
-	if (!val)
-		return 0;
+	if (!val || !brightness)
+		return val;
 	bright = (val * brightness) / 255;
 	return ws2812_gamma[bright];
 }