Ttyjfyk

Why do I want this?

I use the pwm-ir-tx kernel module to blast IR signals from my embedded device. However, when the pwm kernel module is loaded during the boot process, the pin is on high. It takes about 10 seconds or so until I can set it to low with a lirc irsend signal. You can in principle 'overload' the IR-LED to make it brighter, if it is only used in PWM mode and not permanently on. The 10 seconds during the boot process undermine this strategy, however.

What is my system doing so far (e.g. what's working, DT-Overlay file, etc)?

I am using Armbian and modified the pwm-ir-tx driver in the mainline kernel, so that the state after a send is guaranteed low (setting duty cycle to 0, it was randomly 1 or 0 when just disabling the pwm channel on my device).

I am using a device tree overlay that activates the pwm and the pwm-ir-tx.

/dts-v1/;
/plugin/;

/ 
 compatible = "allwinner,sun4i-a10";

 fragment@0 
 target = <&pwm>;
 __overlay__ 
 pinctrl-names = "default";
 pinctrl-0 = <&pwm0_pin>, <&pwm1_pin>;
 status = "okay";
 ;
 ;

 fragment@1 
 target-path = "/";
 __overlay__ 
 pwm-ir-transmitter 
 compatible = "pwm-ir-tx";
 pwms = <&pwm 0 0 0>;
 ;
 ;
 ;

;

When I boot, the pwm-sun4i modules and pwm-ir-tx are loaded and a /dev/lircx character device is available to be used. To turn the LED off, I enabled a systemd service 'lircd-out' with the Unit entry 'After=lircd.service', that turns the led off, but it runs about 10 seconds after the boot process.

Setting modules in the DT Overlay to "disabled" and loading them with modprobe afterwards is not working (not creating pwm or rc devices in sysfs, or a /dev/lircx character device). Maybe since those modules are built in (i.e. configured with 'Y', not 'M' in the .config file), but I must admit my understanding is still a bit fuzzy, here.

What would be ideal?

The ability to control the loading of the modules pwm-sun4i and pwm-ir-tx and thus be able to run a script after pwm-sun4i was loaded that sets the pwm pin to low and then load pwm-ir-tx. But as I mentioned, when I load those modules manually, they are somehow not accessible for the sysfs.

Alternatively, I could pass a parameter in the fragment@0 to the pwm that sets it to low. But I do not know how and do not see anything in the code of pwm-sun4i.c that would allow this. I do not want to modify the kernel source to keep it compatible for updates.

Any suggestions?