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[78.80.27.28]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fb484c71csm185298065e9.8.2025.10.13.03.47.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Oct 2025 03:47:42 -0700 (PDT) Date: Mon, 13 Oct 2025 12:47:40 +0200 From: Vojtech Pavlik To: Oliver Neukum Cc: Alex Henrie , Greg KH , linux-usb@vger.kernel.org, Johan Hovold Subject: Re: ttyACM versus ttyUSB Message-ID: References: <2025101243-ongoing-truce-3aa5@gregkh> <2025101238-mastiff-decibel-4b4f@gregkh> <194258bf-e78c-46b8-8229-3b019f2b8ab0@suse.com> Precedence: bulk X-Mailing-List: linux-usb@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <194258bf-e78c-46b8-8229-3b019f2b8ab0@suse.com> X-Bounce-Cookie: It's a lemon tree, dear Watson! 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