overlays: gpio-led: new overlay

arch/arm/boot/dts/overlays/Makefile

@@ -54,6 +54,7 @@ dtbo-$(CONFIG_ARCH_BCM2835) += \
 	gpio-ir.dtbo \
 	gpio-ir-tx.dtbo \
 	gpio-key.dtbo \
+	gpio-led.dtbo \
 	gpio-no-bank0-irq.dtbo \
 	gpio-no-irq.dtbo \
 	gpio-poweroff.dtbo \

arch/arm/boot/dts/overlays/README

@@ -914,6 +914,62 @@ Params: gpio                    GPIO pin to trigger on (default 3)
         keycode                 Set the key code for the button
 
 
+
+Name:   gpio-led
+Info:   This is a generic overlay for activating LEDs (or any other component)
+        by a GPIO pin. Multiple LEDs can be set up using multiple calls to the
+        overlay. While there are many existing methods to activate LEDs on the
+        RPi, this method offers some advantages:
+        1) Does not require any userspace programs.
+        2) LEDs can be connected to the kernel's led-trigger framework,
+           and drive the LED based on triggers such as cpu load, heartbeat,
+           kernel panic, key input, timers and others.
+        3) LED can be tied to the input state of another GPIO pin.
+        4) The LED is setup early during the kernel boot process (useful
+           for cpu/heartbeat/panic triggers).
+
+        Typical electrical connection is:
+           RPI-GPIO.19  ->  LED  -> 300ohm resister  -> RPI-GND
+        The GPIO pin number can be changed with the 'gpio=' parameter.
+
+        To control an LED from userspace, write a 0 or 1 value:
+           echo 1 > /sys/class/leds/myled1/brightness
+        The 'myled1' name can be changed with the 'label=' parameter.
+
+        To connect the LED to a kernel trigger from userspace:
+           echo cpu > /sys/class/leds/myled1/trigger
+           echo heartbeat > /sys/class/leds/myled1/trigger
+           echo none > /sys/class/leds/myled1/trigger
+        To connect the LED to GPIO.26 pin (physical pin 37):
+           echo gpio > /sys/class/leds/myled1/trigger
+           echo 26 > /sys/class/leds/myled1/gpio
+        Available triggers:
+           cat /sys/class/leds/myled1/trigger
+
+        More information about the Linux kernel LED/Trigger system:
+           https://www.kernel.org/doc/Documentation/leds/leds-class.rst
+           https://www.kernel.org/doc/Documentation/leds/ledtrig-oneshot.rst
+Load:   dtoverlay=gpio-led,<param>=<val>
+Params: gpio                    GPIO pin connected to the LED (default 19)
+        label                   The label for this LED. It will appear under
+                                /sys/class/leds/<label> . Default 'myled1'.
+        trigger                 Set the led-trigger to connect to this LED.
+                                default 'none' (LED is user-controlled).
+                                Some possible triggers:
+                                 cpu - CPU load (all CPUs)
+                                 cpu0 - CPU load of first CPU.
+                                 mmc - disk activity (all disks)
+                                 panic - turn on on kernel panic
+                                 heartbeat - indicate system health
+                                 gpio - connect to a GPIO input pin (note:
+                                        currently the GPIO PIN can not be set
+                                        using overlay parameters, must be
+                                        done in userspace, see examples above.
+        active_low              Set to 1 to turn invert the LED control
+                                (writing 0 to /sys/class/leds/XXX/brightness
+                                will turn on the GPIO/LED). Default '0'.
+
+
 Name:   gpio-no-bank0-irq
 Info:   Use this overlay to disable GPIO interrupts for GPIOs in bank 0 (0-27),
         which can be useful for UIO drivers.

arch/arm/boot/dts/overlays/gpio-led-overlay.dts

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * gpio-led - generic connection of kernel's LED framework to the RPI's GPIO.
+ * Copyright (C) 2021 House Gordon Software Company Ltd. <[email protected]>
+ *
+ * Based on information from:
+ *   https://mjoldfield.com/atelier/2017/03/rpi-devicetree.html
+ *   https://www.raspberrypi.org/documentation/configuration/device-tree.md
+ *   https://www.kernel.org/doc/html/latest/leds/index.html
+ *
+ * compile with:
+ *   dtc -@ -Hepapr -I dts -O dtb -o gpio-led.dtbo gpio-led-overlay.dts
+ *
+ * There will be some warnings (can be ignored):
+ *  Warning (label_is_string): /__overrides__:label: property is not a string
+ *  Warning (unit_address_vs_reg): /fragment@0/__overlay__/led_pins@0:
+ *                                 node has a unit name, but no reg property
+ *  Warning (unit_address_vs_reg): /fragment@1/__overlay__/leds@0:
+ *                                 node has a unit name, but no reg property
+ *  Warning (gpios_property): /__overrides__: Missing property
+ *                 '#gpio-cells' in node /fragment@1/__overlay__/leds@0/led
+ *                  or bad phandle (referred from gpio[0])
+ *
+ * Typical electrical connection is:
+ *    RPI-GPIO.19  ->  LED  -> 300ohm resister  -> RPI-GND
+ *    The GPIO pin number can be changed with the 'gpio=' parameter.
+ *
+ * Test from user-space with:
+ *   # if nothing is shown, the overlay file isn't found in /boot/overlays
+ *   dtoverlay -a | grep gpio-led
+ *
+ *   # Load the overlay
+ *   dtoverlay gpio-led label=moo gpio=19
+ *
+ *   # if nothing is shown, the overlay wasn't loaded successfully
+ *   dtoverlay -l | grep gpio-led
+ *
+ *   echo 1 > /sys/class/leds/moo/brightness
+ *   echo 0 > /sys/class/leds/moo/brightness
+ *   echo cpu > /sys/class/leds/moo/trigger
+ *   echo heartbeat > /sys/class/leds/moo/trigger
+ *
+ *   # unload the overlay
+ *   dtoverlay -r gpio-led
+ *
+ * To load in /boot/config.txt add lines such as:
+ *   dtoverlay=gpio-led,gpio=19,label=heart,trigger=heartbeat
+ *   dtoverlay=gpio-led,gpio=26,label=brain,trigger=cpu
+ */
+
+/dts-v1/;
+/plugin/;
+
+/ {
+	compatible = "brcm,bcm2835";
+
+	fragment@0 {
+		// Configure the gpio pin controller
+		target = <&gpio>;
+		__overlay__ {
+			led_pin: led_pins@19 {
+				brcm,pins = <19>; // gpio number
+				brcm,function = <1>; // 0 = input, 1 = output
+				brcm,pull = <0>; // 0 = none, 1 = pull down, 2 = pull up
+			};
+		};
+	};
+	fragment@1 {
+		target-path = "/";
+		__overlay__ {
+			leds: leds@0 {
+				compatible = "gpio-leds";
+				pinctrl-names = "default";
+				pinctrl-0 = <&led_pin>;
+				status = "okay";
+
+				led: led {
+			                label = "myled1";
+					gpios = <&gpio 19 0>;
+			                linux,default-trigger = "none";
+				};
+			};
+		};
+	};
+
+	__overrides__ {
+		gpio =       <&led>,"gpios:4",
+		             <&leds>,"reg:0",
+		             <&led_pin>,"brcm,pins:0",
+		             <&led_pin>,"reg:0";
+		label =      <&led>,"label";
+		active_low = <&led>,"gpios:8";
+		trigger =    <&led>,"linux,default-trigger";
+	};
+
+};
+