Help with device tree overlays on RPI

[Jason Thorpe <thorpej%me.com@localhost>]

I'm trying to use a device tree overlay for a hardware project I'm working on, that will be used with a Raspberry Pi B+ (at least initially).  I'm using a tweaked rpi-mkknlimg.sh from skrll@ that adds the DDTK dynamic device tree tag, and Linux's knlinfo is happy with it:

thorpej-mbp13:thorpej$ perl ../knlinfo netbsd.img 
Kernel trailer found at 5966892/0x5b0c2c:
  DDTK: y
  DTOK: y
  283x: y
thorpej-mbp13:thorpej$ 

/boot/config.txt:

# UART settings, see https://www.raspberrypi.org/documentation/configuration/uart.md
enable_uart=1
force_turbo=0

dtdebug=1
dtoverlay=thorpej-RPI-NixieClock


...and /boot/overlays/thorpej-RPI-NixieClock.dtbo is there.

Basically, the symptom is that the overlay doesn't show up in the converged FDT at all.  I've tried other overlays from 3rd parties, and they don't seem to load, either.  Of course, it's not possible currently to get the VC logs because we don't have the "vcdbg" tool.

Any ideas how I can go about debugging this problem without the VC logs?  Here is my .dts file for the overlay:

// #include <dt-bindings/pinctrl/bcm2835.h>
// #include <dt-bindings/gpio/gpio.h>
// #include <dt-bindings/input/linux-event-codes.h>

/*
 * Device tree overlay for my Nixie clock.
 */
/dts-v1/;
/plugin/;
/ {
        compatible = "raspberrypi,model-b-plus", "brcm,bcm2835";

        /*
         * i2c devices:
         *
         *      - DS3231 real-time clock
         *      - TSL2561 ambient light sensor
         *      - PCA9685 PWM LED controller
         */
        fragment@0 {
                target = <&i2c1>;

                __overlay__ {
                        #address-cells = <1>;
                        #size-cells = <0>;

                        light-sensor@39 {
                                compatible = "amstaos,tsl2561";
                                reg = <0x39>;
                        };

                        pwm@41 {
                                compatible = "nxp,pca9685";
                                reg = <0x41>;
                        };

                        rtc@68 {
                                compatible = "dallas,ds3231";
                                reg = <0x68>;
                        };
                };
        };

        /*
         * Clock controls:
         *
         *      Rotary encoder:
         *              - "Up"                  0
         *              - "Down"                2
         *              - "Enter"               3
         *      - Snooze button                 4
         *      - Alarm-set button              5
         *      - Alarm-off-for-today button    6
         *      - Alarm on/off switch           27
         *      - Setup mode switch             25
         */
        nixie_clock_control_pins: nixie-clock-control-pins {
                brcm,pins = <0 2 3 4 5 6 25 27>;
                brcm,function = <0 /*BCM2835_FSEL_GPIO_IN*/>;
                brcm,pull = <2 /*BCM2835_PUD_UP*/>;
        };

        nixie-clock-controls {
                compatible = "gpio-keys";

                pinctrl-names = "default";
                pinctrl-0 = <&nixie_clock_control_pins>;

                up-button {
                        label = "up";
                        linux,code = <103 /*KEY_UP*/>;
                        gpios = <&gpio 0 1 /*GPIO_ACTIVE_LOW*/>;
                };

                down-button {
                        label = "down";
                        linux,code = <108 /*KEY_DOWN*/>;
                        gpios = <&gpio 2 1 /*GPIO_ACTIVE_LOW*/>;
                };

                enter-button {
                        label = "enter";
                        linux,code = <28 /*KEY_ENTER*/>;
                        gpios = <&gpio 3 1 /*GPIO_ACTIVE_LOW*/>;
                };

                snooze-button {
                        label = "snooze";
                        linux,code = <31 /*KEY_S*/>;
                        gpios = <&gpio 4 1 /*GPIO_ACTIVE_LOW*/>;
                };

                alarm-set-button {
                        label = "alarm-set";
                        linux,code = <0x167 /*KEY_TIME*/>;
                        gpios = <&gpio 5 1 /*GPIO_ACTIVE_LOW*/>;
                };

                awake-button {
                        label = "awake";
                        linux,code = <113 /*KEY_MUTE*/>;
                        gpios = <&gpio 6 1 /*GPIO_ACTIVE_LOW*/>;
                };

                alarm-enable-switch {
                        label = "alarm-on-off";
                        linux,code = <30 /*KEY_A*/>;
                        gpios = <&gpio 27 1 /*GPIO_ACTIVE_LOW*/>;
                };

                setup-mode-switch {
                        label = "setup-mode";
                        linux,code = <141 /*KEY_SETUP*/>;
                        gpios = <&gpio 25 1 /*GPIO_ACTIVE_LOW*/>;
                };
        };
};

-- thorpej