AM625: "echo 0 > export", but got "-sh: echo: write error: Device or resource busy"

Andrew Tan

Part Number: AM625

I refer to this thread Linux/AM5728: PWM on Timer15, and after "echo 0 > export", I get "-sh: echo: write error: Device or resource busy".

I want to use TIMER_IO3 and GPIO0_43 to drive the pwm signal to make the buzzer sound.

How can I fix this error?

step 1:

I add following node to k3-am62x-sk-common.dtsi

/{
	buzzer1: buzzer {
		status = "okay";
		compatible = "pwm-beeper";
		pwms = <&main_pwm3 0 322580 0>; // 3.1khz
	};

	main_pwm3: dmtimer-main-pwm-3 {
		compatible = "ti,omap-dmtimer-pwm";
		#pwm-cells = <3>;
		ti,timers = <&main_timer3>;
	};
};

&main_timer3 {
 status = "okay";
 pinctrl-names = "default";
 pinctrl-0 = <&buzzer_pwm_pins_default>;
};

&main_pmx0 {
    buzzer_pwm_pins_default: buzzer-pwm-pins-default {
		pinctrl-single,pins = <
			AM62X_IOPAD(0x00b0, PIN_OUTPUT, 7) /* (K22) GPIO0_43*/ // Buzzer GPIO
		>;
	};
};

&cbass_main {
	status = "okay";
};

step 2:

dmesg | grep "pwm"

[    1.027597] omap-dmtimer-pwm dmtimer-main-pwm-3: Please implement the .get_state() callback
[    1.286667] pwm-beeper buzzer: supply amp not found, using dummy regulator
[    1.293804] input: pwm-beeper as /devices/platform/buzzer/input/input0

step 3:

dmesg | grep "timer"

[    0.000000] arch_timer: cp15 timer(s) running at 200.00MHz (phys).
[    0.013110] Calibrating delay loop (skipped), value calculated using timer frequency.. 400.00 BogoMIPS (lpj=800000)
[    1.027597] omap-dmtimer-pwm dmtimer-main-pwm-3: Please implement the .get_state() callback

step 4:

cat /sys/kernel/debug/pwm

platform/dmtimer-main-pwm-3, 1 PWM device
 pwm-0   (buzzer              ): requested period: 322581 ns duty: 0 ns polarity: normal

step 5:

cd /sys/class/pwm/

ls -al

total 0
drwxr-xr-x  2 root root 0 Feb 24 04:51 .
drwxr-xr-x 68 root root 0 Feb 24 04:51 ..
lrwxrwxrwx  1 root root 0 Feb 24 04:51 pwmchip0 -> ../../devices/platform/dmtimer-main-pwm-3/pwm/pwmchip0

step 6:

cd pwmchip0

ls -al

total 0
drwxr-xr-x 3 root root    0 Feb 24 04:51 .
drwxr-xr-x 3 root root    0 Feb 24 04:51 ..
lrwxrwxrwx 1 root root    0 Feb 24 04:52 device -> ../../../dmtimer-main-pwm-3
--w------- 1 root root 4096 Feb 24 04:52 export
-r--r--r-- 1 root root 4096 Feb 24 04:52 npwm
drwxr-xr-x 2 root root    0 Feb 24 04:52 power
lrwxrwxrwx 1 root root    0 Feb 24 04:51 subsystem -> ../../../../../class/pwm
-rw-r--r-- 1 root root 4096 Feb 24 04:51 uevent
--w------- 1 root root 4096 Feb 24 04:52 unexport

step 7:

echo 0 > export

-sh: echo: write error: Device or resource busy

Thanks,

Andrew

over 2 years ago

0 Nick Saulnier over 2 years ago

TI__Guru** 100540 points

7/24/2023: The part of this response talking about timer PWM functionality is incorrect. Please see the response from July 24 2023 with corrections.

Hello Andrew,

At least as of AM62x SDK 8.5, there was no timer PWM functionality planned for implementation in software: https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/08_05_00_21/exports/docs/build_sheet/am62x-sw-buildsheet.html

What functionality are you looking for? You can use both the PWM and ECAP modules to drive a PWM signal, or implement a PWM signal from PRU using either the PRU Subsystem's hardware PWM, or manually toggling a PWM signal with the PRU cores in software.

Regards,

Nick

0 Andrew Tan over 2 years ago in reply to Nick Saulnier

Prodigy 240 points

Hi Nick,

Thanks for your reply.

We are using version 08.06.00.42.

May I ask if we want to use TIMER_IO3 to generate a 3.1kHz PWM signal, is this feasible?

We need this information since the same happens with our PWM LCD backlight.

Thanks,

Andrew

+1 Nick Saulnier over 2 years ago in reply to Andrew Tan

TI__Guru** 100540 points

7/24/2023: The part of this response talking about timer PWM functionality is incorrect. Please see the response from July 24 2023 with corrections.

Hello Andrew,

To clarify, the TIMER functionality of the timer peripheral was added in SDK 8.6. Not the PWM functionality of the timer peripheral.

What is the functionality that you are looking for? Like I said, there are multiple other PWM options on the processor that have been enabled in software.

Reference https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/08_06_00_42/exports/docs/linux/Foundational_Components/Kernel/Kernel_Drivers/PWM.html

Regards,

Nick

0 Andrew Tan over 2 years ago in reply to Nick Saulnier

Prodigy 240 points

Hi Nick,

Thanks for your help, I have successfully driven the pwm-backlight and pwm-beeper after referring to this thread.

AM623: PWM config in device tree

Thanks,

Andrew

0 Nick Saulnier over 2 years ago in reply to Andrew Tan

TI__Guru** 100540 points

Hello Andrew,

Glad to hear that things are working!

Regards,

Nick

0 Nick Saulnier over 2 years ago in reply to Nick Saulnier

TI__Guru** 100540 points

Hello Andrew,

Just wanted to followup here for you and future customers to say that I was wrong about timer as PWM support.

Support for DM timer as PWM was added starting in AM62x SDK 8.5 (the clue is that in SDK 8.5, arch/arm64/boot/dts/ti/k3-am62x-sk-common.dtsi adds dmtimer-main-pwm-7 with compatible = "ti,omap-dmtimer-pwm"). The support continues into SDK 9.0 and future releases. I am taking some followup actions on this side to try to prevent future confusion between the developers and the apps team going forward.

Regards,

Nick

Processors

Processors forum

AM625: "echo 0 > export", but got "-sh: echo: write error: Device or resource busy"