Simple GPIO Device Tree Example for Beaglebone Black Deb 10.3 - gpio

Question

My goal is to write a simple .dts file (to be compiled to .dtbo using DT 1.4.4) to configure a GPIO output on boot on a Beaglebone Black Rev C running Debian 10.3
I intend to place the .dtbo in /lib/firmware and then specify it in /boot/uEnv.txt
I understand some parts of the .dts file and have tried decompiling exisiting .dtbo files in /lib/firmware/ for guidance but none of them are a simple GPIO output example. A lot of online resources involve make and make install but I believe DT should be able to handle it by now right?
I was able to get the following to compile but with issue:
/* dtc -O dtb -o BB-P8_13-LED.dtbo -b 0 -# BB-P8_13-LED-00A0.dts */
/dts-v1/;
/plugin/;
/ {
compatible = "ti,beaglebone-black";
/* identification */
part-number = "BB-P8_13-LED";
version = "00A0";
/* state the resources this cape uses */
exclusive-use =
/* the pin header uses */
"P8.13", /* GPIO_23 */
/* the hardware ip uses */
"gpio23";
fragment#0 {
target = <&am33xx_pinmux>;
__overlay__ {
bb_gpio23_pin: pinmux_bb_gpio23_pin {
pinctrl-single,pins = < 0x024 0x07 >; /*P8_13 GPIO23 MODE7*/
};
};
};
fragment#1 {
target = <&gpio23>;
__overlay__ {
leds {
status = "okay";
pinctrl-names = "default";
pinctrl-0 = <&bb_gpio23_pin>;
compatible = "gpio-leds";
P8_13 {
label = "P8_13";
default-state = "on";
};
};
};
};
};
Q: Why does loading this .dtbo in /boot/uEnv.txt cause all other GPIOs to disappear from /sys/class/gpio/? I thought fragment0 was for excluding a single gpio, not all of them.
###Additional custom capes
uboot_overlay_addr4=/lib/firmware/BB-P8_13-LED-00A0.dtbo
Q: Where are the files for controlling the GPIO (for testing) or rather what can I add to my .dts file so the gpio23 still appears in /sys/class/gpio or even /sys/class/leds? Ultimately I want to be able to control this GPIO with Node-RED.
Q: Do I need to be consistent with my use of P8.13 vs. P8_13? I think I'm mixing up terminology used in .dts files that get compiled with make vs DT.
Q: I think my fragment#1 P8_13 child node is missing something to specify the gpio bank and active high/low setting. Something like "gpios = <&gpio3 19 GPIO_ACTIVE_HIGH>;" Where can I look to research which bank GPIO23 is in? What does the '19' mean in that statement?

Answer 1

So Dr. Derek Molly did a really nice job of laying this out and I was able to use the example in his repo. Here is a page he made for explaining how to configure GPIO at boot using Device Tree Overlays:
http://derekmolloy.ie/beaglebone/beaglebone-gpio-programming-on-arm-embedded-linux/
Even though his solution is for kernel 3.8 I was able to get the following to compile on 4.19
/* dtc -O dtb -o BB-P8_13-LED-00A0.dtbo -b 0 -# BB-P8_13-LED-00A0.dts */
/dts-v1/;
/plugin/;
/{
compatible = "ti,beaglebone-black";
part-number = "BB-P8_13-LED";
version = "00A0";
fragment#0 {
target = <&am33xx_pinmux>;
__overlay__ {
pinctrl_test: BB-P8_13-LED {
pinctrl-single,pins = <
0x024 0x27 /* P8_13 9 PULLUP ENABLED OUTPUT MODE7 - The LED Output */
>;
};
};
};
fragment#1 {
target = <&ocp>;
__overlay__ {
test_helper: helper {
compatible = "bone-pinmux-helper";
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_test>;
status = "okay";
};
};
};
};
All that needs to be edited for a different GPIO pin is the "0x024" (address offset) and the "0x27" to set various aspects of the GPIO like pullup vs. pulldown and pinmux mode. Derek Molly has an older version of his guide which has the table for building the pinmux binary values (that need to be converted to hex): http://derekmolloy.ie/gpios-on-the-beaglebone-black-using-device-tree-overlays/
Most of the information is available in the Beaglebone SRM which I should probably read at some point.

Answer 2

The .dts file in my first answer did not fix my problem. GPIO P8_13 still boots as an input. After more digging and testing I have discovered it is NOT possible to make a GPIO direction survive a reboot. It will always boot to the default and the best you can do is enable a pullup or pulldown resistor to keep the pin high or low until a custom service file (or program) can write to /sys/class/gpio/gpioXXX/direction. I even tried decompiling am335x-boneblack.dtb, editing it, and re-compiling with no luck.
This is sad and incredibly frustrating. What good is an output that flickers during reboot? Guess I'll have to compensate with fancy external circuitry.

Answer 3

Per the original question, you can author a dts file where the GPIO remains in /sys/gpio/class/. See the article I wrote below for an example that works this way.
https://takeofftechnical.com/beaglebone-black-led-control/