Prolific PL2302 support for high baud rates on HXD chips

[gabor.simon75_gmail.com]

Summary

Added support for formula-based arbitrary baud rates (in contrast with the
current fixed values) which enables use of rates above 1 Mbps.
Improved the detection of HXD chips, and the status flag handling as well.
For details see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=225932

Test Plan

Setting 8 data bits with no parity and writing an endless stream of 0xf0-s
produces 5 high bits followed by 5 low ones, that is, a square wave at a
frequency 1/10 of the baud rate, which can be measured even by a handheld
multimeter.

[• hselasky]

sys/dev/usb/serial/uplcom.c

382 ↗

(On Diff #46447)

bcdDevice is a 16-bit field. Can you use:

UGETW(dd->bcdDevice)

and update the case values to include the most significant byte as well, likely:

case 3 -> case 0x103 I guess, for example

410 ↗

(On Diff #46447)

style:

"case XXX:" should be indented like "switch" keyword.

[gabor.simon75_gmail.com]

Fixed bcdDevice detection, fixed indentation style

[gabor.simon75_gmail.com]

sys/dev/usb/serial/uplcom.c

382 ↗

(On Diff #46447)

Done, and extra thx for pointing out a bug: I was checking the 0th (low) byte of bcdDevice instead of the 1st (high) one.

[• hselasky]

The changes look good to me.

[• hselasky]

sys/dev/usb/serial/uplcom.c

382 ↗

(On Diff #46447)

And the lower one should be zero I guess.

[gabor.simon75_gmail.com]

sys/dev/usb/serial/uplcom.c

382 ↗

(On Diff #46447)

Definitely :), the specifications contain explicitely 0x300, 0x400 and 0x500.

[val_packett.cool]

Since we're at 13-current now, can this be landed?