Re: ttyACM versus ttyUSB

[Vojtech Pavlik <vojtech@suse.com>]

On Mon, Oct 13, 2025 at 11:28:59AM +0200, Oliver Neukum wrote:
> On 12.10.25 21:01, Alex Henrie wrote:
> 
> > I did notice that the ACM protocol has no way to set the baud rate or
> > read the CTS line and I wondered if those limitations were the reason
> > for keeping its driver separate. Today there are ttyUSB drivers that
> > likewise have limitations on the baud rate and the flow control lines
> > (some are even based on ACM), but they weren't around in 1999, so I
> > can see how the difference in capabilities would have been another
> > motivation for the difference in naming conventions.
> 
> You could argue that ACM is intended to be a driver for a _modem_ ,
> not a serial line. In principle we are trying to propagate a layering
> violation that is so old that we no longer see it as a violation.

The ttyACM driver is also older than the ttyUSB driver.

And yes, indeed, USB CDC ACM was intended as a class for USB modems, not
for serial ports, so it doesn't directly support pin twiddling.

One would assume that baud rate setting would also be unnecessary in
this usecase, yet, surprisingly enough, many of the early USB modems
absolutely failed to work unless a baud rate was explicitly set.
(USB_CDC_REQ_SET_LINE_CODING)

Today, some USB-serial chips have the ACM protocol, so perhaps it would
make sense to merge the drivers, I certainly would not object.

-- 
Vojtech Pavlik