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

[Alex Henrie <alexh@vpitech.com>]

On Monday, November 28, 2022 7:41:59 AM MST Johan Hovold wrote:
> On Fri, Nov 25, 2022 at 08:58:25PM -0700, Alex Henrie wrote:
> > When a baud rate of 0 is requested, both the 8250 driver and the FTDI
> > driver reset the baud rate to the default of 9600 (see the comment above
> > the uart_get_baud_rate function). Some old versions of the NXP blhost
> > utility depend on this behavior.
> 
> What exactly do you mean by "depend on" here? Setting B0 is supposed to
> hang up a modem connection by deasserting the modem control lines, but
> there's nothing mandating any particular line speed to be set in
> hardware. Why would that even matter?
> 
> If the user space tool is thrown off by the fact that B0 isn't
> implemented, perhaps that's what should be addressed.

Oh, it's definitely a bug in blhost. The program sets the baud rate to 0, then 
tries to communicate over the UART assuming that the baud rate is 9600. It's 
been fixed in the latest version of blhost, but I'm stuck on an old version and 
there's nothing I can do about that.

I don't know why the 8250 and FTDI drivers map B0 to B9600, however it's very 
old behavior that must have had a purpose. Maybe Russell knows? Alternatively, 
leaving the baud rate unchanged seems like reasonable behavior and would also 
work with the old blhost. But mapping B0 to B300 makes even less sense than 
mapping it to B9600.

-Alex