Re: [PATCH] USB: serial: cp210x: map B0 to B9600

[Alex Henrie <alexh@vpitech.com>]

On Monday, November 28, 2022 11:20:41 AM MST Russell King (Oracle) wrote:
> What exactly do you think should be done when a baud rate of zero is
> requested?

I see two reasonable options: Leave the baud rate alone, or reset it to the 
default (i.e. 9600). In my opinion, either of those options is just fine.

> The fact of the matter is that at hardware level, the UART takes a
> clock, and divides that down. To get to a baud rate of zero, one
> would need an infinitely large divisor, which (a) is impossible to
> program into the hardware and (b) would trigger a divide-by-zero
> error in the kernel. So, we have to choose something.
> 
> That decision was made before my time, when Ted Ts'o was maintaining
> what was the original serial.c 8250-based driver, and the behaviour
> he adopted was to set a baud rate of 9600 when B0 was requested,
> which is reasonable - 9600 baud is the default setting.
> 
> POSIX (which is what we use to define the behaviour of the TTY layer,
> or at least what we _used_ to) doesn't specify the behaviour of B0
> as the output rate, other than it shall cause the model control lines
> to be deasserted. What baud rate you get on the line is unspecified,
> and thus left up to the implementation.
> 
> So basically, 9600 baud for B0 is the behaviour of the 8250 driver
> going back to the very early Linux versions and that has become the
> standard Linux implementation shared by a great many serial drivers.
> In effect, it's almost a de-facto standard.

That is really interesting, thanks for the explanation! I like the idea of 
having consistent behavior across the Linux serial drivers, so it seems to me 
that mapping B0 to B9600 in all drivers is the way to go.

-Alex