Use of hard-coded /dev/ttyUSB0 in fan drivers can cause conflict

[iainaff]

Problem

The code for the fan drivers, etc, uses a hard-coded device - /dev/ttyUSB0

This is very bad practice, and will cause failures if other USB serial devices are present at boot & device enumeration.

Because the fan serial device is on the USB-C bus - bus 3, device 3 - it will fall to be enumerated AFTER any other USB serial devices present, so will not be numbered as /dev/ttyUSB0, as the devices(s) on the lower buses will get that, so the drivers using the hard-coded /dev/ttyUSB0 device name will open the wrong device , thus breaking both the fan control and the other serial device.

Fix

The simplest fix for this is to use udev rules to cause creation of a symlink to the dynamically assigned /dev/ttyUSB?, and then use that instead of the hard-coded /dev/ttyUSB0.

To get udev to do this, a file /etc/udev/rules.d/20-serial.rules is created, containing:

SUBSYSTEM=="tty",ATTRS{idVendor}=="1a86",ATTRS{idProduct}=="7523",SYMLINK+="ttyFAN0"

(Note than this is a the simplest possible rule; if any other devices of the same vendor:product ID are also present, it will have to be more specific, eg identifying the bus & device, etc, to uniquely identify it, and prevent the same conflict problem happening).

This rule creates a device symlink /dev/ttyFAN0 (pointing to the dynamically assigned /dev/ttyUSB?) which the fan drivers can then safely open and use.

Other necessary changes to the C and Python code from the drivers directory are below, as a git diff:

Patch

diff --git a/drivers/c/fanStop b/drivers/c/fanStop
index f0b09f9..ff18723 100644
--- a/drivers/c/fanStop.c
+++ b/drivers/c/fanStop.c
@@ -10,9 +10,11 @@

 int main(void){
    while(1){
-       int serial_port = open("/dev/ttyUSB0", O_RDWR);
+//         int serial_port = open("/dev/ttyUSB0", O_RDWR);
+       int serial_port = open("/dev/ttyFAN0", O_RDWR);
        if (serial_port < 0){
-           printf("Can not access /dev/ttyUSB0, please check it out.\n");
+//         printf("Can not access /dev/ttyUSB0, please check it out.\n");
+           printf("Can not access /dev/ttyFAN0, please check it out.\n");
            }

    struct termios tty;
diff --git a/drivers/c/pwmControlFan.c b/drivers/c/pwmControlFan.c
index 1f7d08e..56f34aa 100644
--- a/drivers/c/pwmControlFan.c
+++ b/drivers/c/pwmControlFan.c
@@ -15,7 +15,8 @@ int init_serial( char *serial_name)
    serial_port = open(serial_name, O_RDWR);

    if (serial_port < 0){
-       printf("Can not open /dev/ttyUSB0 serial port ErrorCode: %s\n", strerror(errno));
+//     printf("Can not open /dev/ttyUSB0 serial port ErrorCode: %s\n", strerror(errno));
+       printf("Can not open /dev/ttyFAN0 serial port ErrorCode: %s\n", strerror(errno));
        printf("Please check the /boot/config.txt file and add dtoverlay=dwc2, dr_mode=host and reboot RPi \n");
    }

@@ -103,7 +104,8 @@ int main(void){
    char data[8]={0};
    unsigned int conf_info[8];
    unsigned int cpu_temp=0;
-   init_serial("/dev/ttyUSB0");
+// init_serial("/dev/ttyUSB0");
+   init_serial("/dev/ttyFAN0");
     /* default configuration if /etc/deskpi.conf dose not exist */
    conf_info[0]=40;
    conf_info[1]=25;

diff --git a/drivers/c/safecutoffpower.c b/drivers/c/safecutoffpower.c
index f0b09f9..0d0695b 100644
--- a/drivers/c/safecutoffpower.c
+++ b/drivers/c/safecutoffpower.c
@@ -10,9 +10,11 @@

 int main(void){
    while(1){
-       int serial_port = open("/dev/ttyUSB0", O_RDWR);
+//         int serial_port = open("/dev/ttyUSB0", O_RDWR);
+       int serial_port = open("/dev/ttyFAN0", O_RDWR);
        if (serial_port < 0){
-           printf("Can not access /dev/ttyUSB0, please check it out.\n");
+//         printf("Can not access /dev/ttyUSB0, please check it out.\n");
+           printf("Can not access /dev/ttyFAN0, please check it out.\n");  
            }

    struct termios tty;
diff --git a/drivers/python/pwmControlFan.py b/drivers/python/pwmControlFan.py
index 193c1c4..f9c551e 100644
--- a/drivers/python/pwmControlFan.py
+++ b/drivers/python/pwmControlFan.py
@@ -5,7 +5,7 @@ import time
 import subprocess 

-ser = serial.Serial("/dev/ttyUSB0", 9600, timeout=30)
+ser = serial.Serial("/dev/ttyFAN0", 9600, timeout=30)

 try: 
     while True:
diff --git a/drivers/python/safecutoffpower.py b/drivers/python/safecutoffpower.py
index c9a927f..134c2b2 100644
--- a/drivers/python/safecutoffpower.py
+++ b/drivers/python/safecutoffpower.py
@@ -5,7 +5,8 @@ import serial
 import time 

-ser = serial.Serial("/dev/ttyUSB0", 9600, timeout=30)
+#ser = serial.Serial("/dev/ttyUSB0", 9600, timeout=30)
+ser = serial.Serial("/dev/ttyFAN0", 9600, timeout=30)

 try: 
     while True:

[mazzystr]

A "healthy" project would merge this immediately. I came from ArgonOne and they neglect their upstream the same way.

The diff looks great. One small knit. I suggest the udev rule and c code use device ttyDPIFAN0 so it's very clear on which fan we're dealing with.

[iainaff]

Yeah, good call - naming things is always hard!

[mraumala-roku]

My DeskPi hosts RPI 4 running 64-bit Ubuntu 22.04.1.

I found the /var/log/syslog inundated with following errors (20 GB of it filling the root filesystem after just one day of uptime)...

Please check out /boot/config.txt file
Can not access /dev/ttyUSB0, please check it out.
serial port can not be accessed

Though the solution provided here seems more robust I solved the /dev/ttyUSB0 issue by just getting rid of brltty...

# systemctl stop brltty-udev.service
# systemctl mask brltty-udev.service
# systemctl stop brltty.service
# systemctl disable brltty.service
# apt purge brltty  # uninstalled it in the end

The fan is working now.

[neuling10]

With your description I was able to solve the fan problem when connecting another device to ttyUSB0. Unfortunately, however, shutdown -h now does not work. I have also done changes in /drivers/c/safecutoffpower.c and /drivers/python/safecutoffpower.py Does that work for you?

My DeskPi hosts RPI 4 running 64-bit Raspberry Pi OS. I have tested install.sh and install-raspbios-64bit.sh

[yoyojacky]

My DeskPi hosts RPI 4 running 64-bit Ubuntu 22.04.1.

I found the /var/log/syslog inundated with following errors (20 GB of it filling the root filesystem after just one day of uptime)...

Please check out /boot/config.txt file
Can not access /dev/ttyUSB0, please check it out.
serial port can not be accessed

Though the solution provided here seems more robust I solved the /dev/ttyUSB0 issue by just getting rid of brltty...

# systemctl stop brltty-udev.service
# systemctl mask brltty-udev.service
# systemctl stop brltty.service
# systemctl disable brltty.service
# apt purge brltty  # uninstalled it in the end

The fan is working now.

This method is good for the OS which can not generate /dev/ttyUSB0 device file, due to this file is generated by this overlay: dtoverlay=dwc2,dr_mode=host, and it will detect the MCU's serial port ,but brltty service will occupy this device file name, so that this is best solution for this issue.