From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A66032AD32 for ; Sun, 12 Oct 2025 06:07:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760249258; cv=none; b=HnhRwVi6lnhCIh24HrcJfG5etgSkgmzYDhobPjJ/h+rV+TSZUcYoChc7V3X+0BVSfmKo/1/QmC8wxggiCSNUcGKz4NXTBoGXCZTucLmV4w6RYSwDmfjJXX2X/LsrzLlL7gmwzBpJGcIPN3IdiWkuCQ/e2i0sJ0iMl7PPSFTmZug= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760249258; c=relaxed/simple; bh=yqi39mHMtwO4L+BcrOv6Hon8yY7DGmHMmbg6PXrrAbM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=FOdXQ74duehsMTk7pyDOAA2JhqWgNn0f6DkGHrjd4rAkgIRBgrXiKiJQOt01FdrPEkDMOD3197l5/SsL+a+2AiR54PITnPXt3XxrBTvKBuNVeedG5PvljB/V+ZJEEECJNLJOw4WZYqRqpq88PmmLkbsexjmK0d8M8p8VXU467kY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=JYWHLLBL; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="JYWHLLBL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8EB39C4CEE7; Sun, 12 Oct 2025 06:07:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1760249258; bh=yqi39mHMtwO4L+BcrOv6Hon8yY7DGmHMmbg6PXrrAbM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=JYWHLLBL8/tsADZdd55IkkhJ/HXTdBgh4XDeawgKo3igLEGj8rz5lnO920WcO56xo wphS5pS4T7WMYGKa+AVKtWszA/cZ/o8RXqk8CajB71f+WMqZQftCmn0ZjMEOFFEDDU zpSv+owwBGBzEEJudZ8m8AaulpnPJQzsA7weolyg= Date: Sun, 12 Oct 2025 08:07:35 +0200 From: Greg KH To: Alex Henrie Cc: linux-usb@vger.kernel.org, oneukum@suse.com, Johan Hovold , vojtech@suse.cz Subject: Re: ttyACM versus ttyUSB Message-ID: <2025101243-ongoing-truce-3aa5@gregkh> References: 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: On Sat, Oct 11, 2025 at 11:00:00PM -0600, Alex Henrie wrote: > Dear kernel developers, > > I am very curious and I haven't been able to find a definitive answer > anywhere: Why is the cdc-acm driver separate from the general > usbserial driver? There are lots of drivers that use usbserial, each > with its own unique protocol. What makes ACM so special that it needs > to be separated out as /dev/ttyACM* instead of going with everything > else in /dev/ttyUSB*? > > I can think of several possible reasons, but I'd really like to know > what reasons matter to the kernel architects/maintainers. cdc-acm implements the USB specification for that protocol, which is defined by the USB group. All of the usb-serial drivers do NOT follow that protocol and use their own vendor-specific ways of talking to the device. Hope this helps, greg k-h