USB: serial: ch341: Add support for limited chips (was: Add support for HL340 devices)

A subset of CH341 devices does not support all features, namely the prescaler is limited to a reduced precision and there is no support for sending a RS232 break condition.

These devices can usually be identified by an imprint of "340" on the turquoise-colored plug. They're also sometimes called "HL340", but not all such devices are affected.

This series takes multiple steps to support the limited chips:

* Remove special cases from clock prescaler computation
* Detect limited devices by trying to read a register related to sending a break condition
* Amend clock prescaler computation to only use working values on limited chips
* Simulate an RS232 break condition by temporarily lowering the baud rate and sending a NUL byte

Michael Hanselmann (6):
USB: serial: ch341: Reduce special cases in clock calculation
USB: serial: ch341: Add basis for quirk detection
USB: serial: ch341: Limit prescaler on quirky chips
USB: serial: ch341: Name prescaler, divisor registers
USB: serial: ch341: Compute minimum baud rate
USB: serial: ch341: Simulate break condition if not supported

 drivers/usb/serial/ch341.c | 294 ++++++++++++++++++++++++++++++-------
 1 file changed, 244 insertions(+), 50 deletions(-)

2.20.1