As support for my upcoming gadget (see the μArt on Crowd Supply), I have submitted kernel patches for two different but common USB-UART bridges:

For the CP2102N (Silicon Labs)
- The driver can now handle all baudrates supported by the chip
- The driver now supports the GPIOs of the chip
- Will be part of 4.19 (already accepted into mainline)

For the FT230X, FT231X, and FT234X (FTDI)
- The driver now supports CBUS GPIOs of the chip
- Will be probably part of 4.20 (submitted, under review process)

It was about time, the former IC is 2yrs old, the latter chip is 6 years old. Just letting you know. And of course, check out the campaign too, you might be interested.