Support for the `tty` subsystem notifications

[lukipuki]

I'm using USB devices that contain a UART bridge, so they communicate over a serial port bridged via USB.

I've used pyudev previously but I want to make my app run on Windows too. The problem is that the notification from USBMonitor comes too early, before the serial port is initialized. In pyudev, you can specify subsystem=tty to receive notifications when the serial port becomes available.

Benchmarking USBMonitor, which is using subsystem=usb, against pyudev using subsystem=tty gives a fairly long delay (up to 200ms) between those two:

2024-01-23 10:47:39.930515 USBMonitor notification, subsystem=usb
2024-01-23 10:47:39.999694 pyudev notification, subsystem=tty

In order to integrate with USBMonitor I have to put in a hacky sleep for the serial port to be available. Or I can modify the source code in the following way:

Would you be willing to add an option to listen to the tty subsystem notifications?

When I tested USBMonitor on Windows, it looked like this was not needed, as the COM port was part of the name of the device returned by USBMonitor, so I assume it was ready to use. I haven't tested it though, so maybe there needs to be some change in the Windows part too.

[lmbeauplet]

Hi ! This issue interest me too. Dev is minor, i will try to do a dedicated branch and submit a PR. We will wait for a review of @Eric-Canas . While PR is not accepted, i will checkout dev branch.

[Eric-Canas]

Hi!!

Sure! Thanks!

The main problem I have for developing that is that I don't have access to the hardware for testing it. But if you can test it, it should be just a matter of configuration at _constants.py and probably add some new regex.

To keep everything as cross-platform as it's now, it would be also nice to test that output is equivalent for Linux, Windows and macOS, when plugging in the same device.

[lmbeauplet]

Ok thanks for advises, I will check it today. This dev will only be available on Linux (subsystems are os-linked)

[lmbeauplet]

I imagine just a simple solution to change dynamically subsystem from 'usb' to 'tty' after instanciation of USBMonitor. This change will be conditioned by Linux OS like:

# Initializes a USBMonitor instance with a filter for devices matching specific
# vendor and model IDs.
usbmonitor = USBMonitor(
    filter_devices=[
        {"ID_VENDOR_ID": Product.VID_STRING},
        {"ID_MODEL_ID": Product.PID_STRING},
    ]
)

# Assume application run always on Linux
if platform.system() == "Linux":
    usbmonitor.monitor.set_subsystem("tty") # type: ignore

or

usbmonitor = USBMonitor(
    filter_devices=[
        {"ID_VENDOR_ID": TheBoxUsb.VID_STRING},
        {"ID_MODEL_ID": TheBoxUsb.PID_STRING},
    ],
    linux_subsystem="tty"
)

If prefere solution 1 because user have to interact with USBMonitor like a crossplatform interface so it had to be boilerplate

[lmbeauplet]

Also, i think i can't contribute, i have some error message to push my branch. Do you have some blocking things for new contributors ? Do you help me @Eric-Canas ?