Arria 10 SoC FPGA

From what you've described, it sounds like you're basic device presents itself as a USB serial device based on an FTDI chipset, correct?

You said "the kernel does identify the device as a USB 2.0 but doesn't assign any device". This makes me wonder if your Linux kernel has been compiled with the appropriate drivers or not. I believe the ftdi_sio driver is what is generally used to support an FTDI chipset for USB serial operation, do you know if that is present in your kernel?  I believe at a minimum you need the CONFIG_USB_SERIAL and CONFIG_USB_SERIAL_FTDI_SIO kernel configs enabled in your kernel.  If the ftdi_sio driver is compiled in as a loadable module, you might check to ensure that it is installed on your root filesystem properly.

I don't believe you should require any custom udev or mdev rules to enable a basic USB serial connection. Are you running a defauilt binary image of the GSRD example on your Arria 10 SoC dev kit?  Or are you running a custom software configuration?

The latest GSRD binary image released on releases.rocketboards.org does not appear to have CONFIG_USB_SERIAL enabled by default:
https://releases.rocketboards.org/2025.10/gsrd/a10_gsrd/sdimage.tar.gz

The u-boot-spl banner prints with this build date:
U-Boot SPL 2025.07 (Sep 25 2025 - 01:10:31 +0000)

The u-boot banner prints with this build date:
U-Boot 2025.07 (Sep 25 2025 - 01:10:31 +0000)socfpga_arria10

Once in Linux you can see this kernel build date:
root@arria10:~# uname -a
Linux arria10 6.12.33-altera-g96a033bc584c #1 SMP Thu Sep 25 06:05:32 UTC 2025 armv7l GNU/Linux

Then if you inspect the kernel configuration you see that CONFIG_USB_SERIAL is not enabled by default:
root@arria10:~# gunzip -c /proc/config.gz | grep CONFIG_USB_SERIAL                                                  
# CONFIG_USB_SERIAL is not set

If this is the environment that you are using, you will need to recompile your kernel to enable this functionality.