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I need to use the serial ports that i get from a SC16IS752 adapter. I already set them up from /boot/config.txt, and they appear in /dev as ttySC0, ttySC1, ttySC2 and ttySC3 and they work.

The problem is, whenever i boot up the Raspberry, the naming of the ports lacks consistency: sometimes a device is connected to ttySC0, sometimes to ttySC2.

I tried to look around for help, but i found solutions only for USB-UART adapters: it's to create symbolic links to each physical device. The solution i tried to follow is this one:

How to bind USB device under a static name?

I tried to apply the same points, but i noticed several differences in parameters so i got stuck.

So... How do i create symbolic links for a serial port given from a SPI-UART adapter?

Thank you in advance.

Nargacuga
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1 Answers1

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So... I found the solution by myself.

Too bad the solution can't be generalized, since it's "learn how udev and rules.d work".

The main point was to learn every possible detail about the devices, using the command (as they suggested in the article i mentioned in my question):

udevadm info -a -p  $(udevadm info -q path -n /dev/ttySC0)

then to find which parameters you can use to uniquely find the hardware description of that device and create a rule about it in one of the files in /etc/udev/rules.d.

I personally used this guide to learn everything i know about udev and rules.d:

Writing udev rules

To summarize, you have to use the parameters to find the hardware, and then use either NAME or SYMLINK to give it a new name or just a pseudonym.

The most commonly used keywords to find the correct hardware are:

ACTION=="<add/remove>" - "This rule applies if this device is plugged in/plugged out"

KERNEL=="<value>" - "Find the device with <value> in the KERNEL field"

SUBSYSTEM=="<value>" - "Find the device with <value> in the SUBSYSTEM field"

DRIVER=="<value>" - "Find the device with <value> in the DRIVER field"

ATTR{<attribute>}=="<value>" - "Find the device with an attribute <attribute> and the value <value>"

You can also use the plural for all of these keywords except for ACTION (KERNELS, SUBSYSTEMS, DRIVERS, ATTRS) if you want to check a match with either a device or all its parent devices.

You can obviously use more than one parameter per rule, you just have to separate them with a comma and a space.

After correctly finding your device, the last section of your rule should be the naming/symlinking:

NAME="<value>" - "Give the found device this new name (destroying the previous one)"

SYMLINK+="<value>" - "Give the found device an alternative name (keeping the previous one)

As an example, i'll use the rules i had to create:

KERNELS=="spi0.0", SUBSYSTEM=="tty", ATTR{line}=="0", DRIVERS=="sc16is7xx", ACTION=="add", SYMLINK+="plSerial"
KERNELS=="spi0.0", SUBSYSTEM=="tty", ATTR{line}=="2", DRIVERS=="sc16is7xx", ACTION=="add", SYMLINK+="plSerial"
KERNELS=="spi0.0", SUBSYSTEM=="tty", ATTR{line}=="1", DRIVERS=="sc16is7xx", ACTION=="add", SYMLINK+="mdSerial"
KERNELS=="spi0.0", SUBSYSTEM=="tty", ATTR{line}=="3", DRIVERS=="sc16is7xx", ACTION=="add", SYMLINK+="mdSerial"

In human words they say:

"If a tty device uses the spi0.0 port and it's named 0 or 2 (which means it's either ttySC0 or ttySC2) it's now also called plSerial"

"If a tty device uses the spi0.0 port and it's named 1 or 3 (which means it's either ttySC1 or ttySC3) it's now also called mdSerial"

This means the ttySC0/ttySC2 will always be known as plSerial, and the ttySC1/ttySC3 will always be known as mdSerial.

After writing the rules, use udevadm trigger or reboot the system, and your rules will be applied. The new devices will be found in /dev/<your_device_name>.

Nargacuga
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