Beaglebone devm_pwm_get returns ENODEV - linux

Question

I am experimenting with a pwm-driver for the Beaglebone black, based of this one.
As I am using Yocto with the meta-bbb layer, I had to rewrite the .dtsi:
&am33xx_pinmux {
bbb_pwm_P8_13_pins: bbb_pwm_P8_13_pins {
pinctrl-single,pins = <0x024 0x4>; /* P8_13 (ZCZ ball T10) | MODE 4 */
};
};
/ {
bbb-pwm#123 {
compatible = "tfe,bbb_pwm-1.00.a";
pwms = <&ehrpwm2 1 0 1>;
pwm-names = "PWM_P8_13";
pinctrl-names = "default";
pinctrl-0 = <&bbb_pwm_P8_13_pins>;
enabled = <0>;
duty = <0>;
status = "okay";
};
};
However, during the driver-probe function, the call
pwm_test->pwm = devm_pwm_get(&pdev->dev, NULL);
returns ENODEV:
[ 7.538249] pinctrl-single 44e10800.pinmux: found group selector 15 for bbb_pwm_P8_13_pins
[ 7.538278] pinctrl-single 44e10800.pinmux: request pin 9 (44e10824.0) for bbb-pwm#123
[ 7.538291] pinctrl-single 44e10800.pinmux: enabling bbb_pwm_P8_13_pins function15
[ 7.538366] Loading bbb_pwm
[ 7.541304] bbb-pwm bbb-pwm#123: obtain a copy of previously claimed pinctrl
[ 7.541321] bbb-pwm bbb-pwm#123: Unable to request PWM (err = -19)
I found that the error-code is returned by a sub-call of devm_pwm_get:
static int pwm_device_request(struct pwm_device *pwm, const char *label)
{
/* .... */
if (!try_module_get(pwm->chip->ops->owner))
return -ENODEV;
/* ... */
}
However, since I am fairly new to Linux-drivers, I do not understand why this happens. Any clues?

Answer 1

It turned out that the lower-level PWM-driver (EHRPWM) was disabled in the kernel. Enabling it using menuconfig and ensuring that EHRPWM and EPWMSS was enabled in the device tree solved my problem:
Using the meta-bbb layer, I simply accessed menuconfig through bitbake:
bitbake virtual/kernel -c menuconfig
and loaded the defconfig located /meta-bbb/recipes-kernel/linux/linux-stable-4.4/beaglebone/defconfig
I also added the following line to my local.conf
PREFERRED_VERSION_linux-stable = "4.4"
Here is my dtsi:
&am33xx_pinmux {
bbb_pwm_P8_13_pins: bbb_pwm_P8_13_pins {
pinctrl-single,pins = <0x024 0x4>; /* P8_13 (ZCZ ball T10) | MODE 4 */
};
};
&ehrpwm2 {
status = "okay";
};
&epwmss2 {
status = "okay";
};
/ {
bbb-pwm#123 {
compatible = "tfe,bbb_pwm-1.00.a";
pwms = <&ehrpwm2 1 0 1>;
pwm-names = "PWM_P8_13";
pinctrl-names = "default";
pinctrl-0 = <&bbb_pwm_P8_13_pins>;
enabled = <0>;
duty = <0>;
status = "okay";
};
};