Why has USBSerialDriverKit disappeared?

Question

At WWDC 2019, Apple announced a new driver architecture. More and more types of drivers are supposed to run in userspace instead of in the kernel. Among the first affected drivers types – according to the announcement – are USB-to-serial drivers.

This type of drivers is supposed to be implemented using the USBSerialDriverKit, and Apple published early documentation for it. However, the pages have disappeared. There are still indexed in Google, e.g.: https://developer.apple.com/documentation/usbserialdriverkit. But they all lead to a 404. Related driver kits, e.g. USBDriverKit, are still online.

Why have the documentation and most likely also the related APIs disappeared – both from the web site as well as from macOS Catalina beta 3? Will they still be part of the final macOS Catalina?

Answer 1

No reason was ever given why it disappeared. I guess it just wasn't ready yet.

Anyhow, it has reappeared with macOS Catalina 10.15.4 or 10.15.5.

I'm not sure how relevant it is for third parties. There is a limited number of USB-to-Serial chips available and many are already covered:

• USB CDC ACM: The official USB standard for USB-to-serial communication. Driver provided by Apple out of the box. If you have any control over the hardware, this should be your preferred solution as it works on all modern operating systems without further driver installation. This is the protocol usually implemented when the USB-to-serial conversion is implemented in software (in a MCU with a USB peripheral).

• FTDI chips (such as FT232x, FT2232, FT4232): Popular chip for USB-to-serial conversion. Chinese clones exist as well. Driver is provided by Apple out-of-the-box.

• Silicon Labs CP210x chips: Popular chip for USB-to-serial conversion. Driver is provided by Apple out-of-the-box.

• WCH CH34xx chips: Popular chip for USB-to-serial conversion from the Chinese manufacturer WCH. Driver is provided by Apple out-of-the-box.

• Prolific PL230x chips: Chips for USB-to-serial conversion from a Taiwanese manufacturer. No user-space driver available yet.

So the Prolific driver is the only one missing. With the up-coming Apple Silicon based Macs, they will probably have pressure to deliver it by the end of 2020. Looking forward to it.