No userspace chardev available when using pwm-ir-tx module - linux

Question

I'm attempted to use a pwm output pin as a IR transmitter using a RAMIPS SoC and the PWM IR TX kernel module. I'm running linux 4.14.37 and have added the following entry to the dts file:
pwm_ir_tx1: pwm-ir-transmitter1 {
compatible = "pwm-ir-tx";
pwms = <&pwm 1 100>;
};
I'm loading the rc-core and pwm-ir-tx kernel modules:
lsmod | grep pwm
pwm_ir_tx 2032 0
pwm_mediatek_ramips 1744 1
rc_core 19348 2 pwm_ir_tx
When the pwm-ir-tx module loads, the kernel logs:
[ 3754.108259] rc rc0: PWM IR Transmitter as /devices/platform/pwm-ir-transmitter1/rc/rc0
The sysfs nodes appear to be loaded correctly:
ls -la /sys/class/rc/
drwxr-xr-x 2 root root 0 May 29 00:18 .
drwxr-xr-x 23 root root 0 Jan 1 1970 ..
lrwxrwxrwx 1 root root 0 May 29 01:16 rc0 -> ../../devices/platform/pwm-ir-transmitter1/rc/rc0
But there is no userspace (chardev) lirc device listed in /dev so I'm not sure how I'm supposed to interact with the device. Ideally I'd like to use the Remote Controller API but this requires a chardev to be present in /dev.
ls /dev
autofs mtd2ro network_throughput
console mtd3 null
cpu_dma_latency mtd3ro port
full mtd4 ptmx
gpiochip0 mtd4ro pts
gpiochip1 mtd5 random
gpiochip2 mtd5ro shm
gpiochip3 mtd6 tty
i2c-0 mtd6ro ttyS0
kmsg mtdblock0 ttyS1
log mtdblock1 ttyS2
memory_bandwidth mtdblock2 urandom
mtd0 mtdblock3 watchdog
mtd0ro mtdblock4 watchdog0
mtd1 mtdblock5 zero
mtd1ro mtdblock6
mtd2 network_latency
I've tried loading the lirc_dev module before and after the pwm-ir-tx module, but nothing appears in /dev still, the following output appears when I load the lirc_dev module:
[ 4775.367966] lirc_dev: IR Remote Control driver registered, major 251
But still no lirc userspace device in /dev... I'm thinking the lirc_dev module is required since it provides the lirc userspace api but there doesn't appear to be any connection between it and the pwm-ir-tx module, and it's not creating any lirc chardevs in /dev.

Answer 1

The pwm_ir_tx module seems to be more or less a piggy-back to the pwm driver. And the pwm driver seems to be available at /sys/class/pwm/. See https://www.kernel.org/doc/Documentation/pwm.txt.
BTW, not having a lirc link in rc0/ is not uncommon - not all drivers implements this.

Answer 2

After reading some of rc source files in the kernel, it became obvious the ir-lirc-codec module was also required.
Basically, the pwm-ir-tx driver is defined as a type of RC_DRIVER_IR_RAW_TX. When the rc-core module registers a driver with a type of RC_DRIVER_IR_RAW or RC_DRIVER_IR_RAW_TX it calls the ir_raw_event_prepare function which in turn tries to load the ir-lirc-codec module. Once this module was available the following kernel logs appear:
[ 10.004460] lirc_dev: IR Remote Control driver registered, major 251
[ 10.131011] IR LIRC bridge handler initialized
[ 10.471561] rc rc0: PWM IR Transmitter as /devices/platform/pwm-ir-transmitter1/rc/rc0
[ 10.487456] rc rc0: lirc_dev: driver ir-lirc-codec (pwm-ir-tx) registered at minor = 0
And in /dev there is a lirc chardev device available:
ls /dev/li*
/dev/lirc0