Tool/software:
I discovered an interesting behavior with the pinctrl of GPIOs in conjunction with internal PU/PD on an AM67A based board (BeaglY-AI).
It started that I wanted to use a GPIO as input via the gpio-keys driver.
To keep it easy I configured the pin as GPIO with internal Pull-Up resistor.
gpio-keys {
compatible = "gpio-keys";
pinctrl-names = "default";
pinctrl-0 = <&hat_36_gpio_pu>;
key {
label = "S1-BTN";
/* HAT 36 (GPIO 16) */
gpios = <&main_gpio1 7 GPIO_ACTIVE_LOW>;
linux,code = <30>;
};
};
The pinctrl definition looks like the following:
hat_36_gpio_pu: hat-36-gpio-pu-pins {
pinctrl-single,pins = <
J722S_IOPAD(0x194, PIN_INPUT_PULLUP, 7) /* (A25) MCASP0_AXR3.GPIO1_7 */
>;
};
But with that I discovered that the pull-up gets lost when the gpio-key driver is probed, so the pin is floating.
I then tried to investigate a bit and it seems that whenever I request the line (kernel- or userspace) via gpiod the pull-up definition is lost and the pin is floating.
That can also be seen, when watching the pinctrl register.
Before requesting line with gpiod:
| Pin state | Pinctrl register | Output gpioinfo |
|---|---|---|
| Pulled up | 0x00060007 | line 7: “GPIO16” unused input active-high |
When line is requested:
Please note that I tested to request the line with and without a defined bias, but this didn't have any effect on the result.
| Pin state | Pinctrl register | Output gpioinfo |
|---|---|---|
| Floating | 0x00050007 | line 7: “GPIO16” “btn-python-example” input active-high [used pull-up] |
When line is released:
| Pin state | Pinctrl register | Output gpioinfo |
|---|---|---|
| Floating | 0x00050007 | line 7: “GPIO16” unused input active-high |
Because I don't have a reference board with the AM67A, I wasn't able to check, if that behavior is there present too.
However I checked it on an AM64x EVM (same Kernel tree and version) and there it doesn't occur.
Here the definition of a internal pull-up / pull-down via pinctrl remains after line request.
The definition of the bias when requesting the line via gpiod doesn't have any effect too.
For the tests I used ti-linux-6.12.y (tag 11.00.12).
Can somebody reproduce that on a reference design board, or does somebody know why that happens?
