From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AACBCC433FE for ; Fri, 27 May 2022 22:36:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354817AbiE0Wgu (ORCPT ); Fri, 27 May 2022 18:36:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55398 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1354808AbiE0Wgq (ORCPT ); Fri, 27 May 2022 18:36:46 -0400 Received: from freecalypso.org (freecalypso.org [195.154.163.71]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C94CE24BE4; Fri, 27 May 2022 15:36:44 -0700 (PDT) Received: by freecalypso.org (Postfix, from userid 1001) id BA4D3374020E; Fri, 27 May 2022 22:27:03 +0000 (UTC) From: "Mychaela N. Falconia" To: Johan Hovold , Greg Kroah-Hartman , Jiri Slaby Cc: linux-serial@vger.kernel.org, linux-usb@vger.kernel.org, mychaela.falconia@gmail.com Subject: [PATCH 0/6] serial ports: add ability to suppress raising DTR & RTS on open Message-Id: <20220527222703.BA4D3374020E@freecalypso.org> Date: Fri, 27 May 2022 22:27:03 +0000 (UTC) Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org Back in 1970s UNIX a poor design decision was made regarding serial port handling; this bad design has been codified into standards (POSIX, SUS) and persists into present-day Linux. In 2021 FreeBSD 13.0 became the first Unix-style OS to fix the problem; the present patch series aims to implement a similar solution in Linux. The root of the problem is the POSIX/SUS-codified design decision to always automatically assert both DTR and RTS modem control signals on the initial open of a serial port, without giving userspace any ability to say "no, please don't do it". This design is flawed on a fundamental philosophical level: an OS kernel needs to provide a mechanism for applications to operate on hardware, rather than insert its own mind in the middle. If a user wishes to open any kind of hardware device and perform arbitrary operations on that device, the kernel needs to allow this access unobstructed, provided that the user has the necessary permissions. If the user is asking to merely open a hardware device, but the kernel refuses to provide such "simple open" access without mandatorily imposing some specific hardware action along with that open, this behaviour needs to be considered a defect, a design bug. Because the design bug is codified in POSIX, SUS etc, it cannot be simply fixed - a change to default behaviour would violate standards and break bazillion existing applications. Instead the only possible solutions at this point in time have to take the form of creative workarounds - and by necessity, such creative workarounds should NOT be expected to be "pretty" or architecturally clean. The architecturally-clean boat has sailed a few decades ago - in the present time, non-pretty workarounds are required. The solution implemented in FreeBSD relies on a feature of that OS which does not exist in Linux: initial-state devices. In FreeBSD each ttyXX device also has a corresponding ttyXX.init device node; the purpose of this .init device is to set the initial termios state that will apply every time the regular ttyXX device is opened. Furthermore, these .init devices have an additional property: opening an init device does not raise modem control signals, unlike opening of regular tty devices. Having these init devices already available (implemented a long time ago for unrelated reasons) made it possible for FreeBSD to implement the option of suppressing automatic assertion of DTR & RTS on open by way of a simple termios flag: they defined a new termios flag CNO_RTSDTR (added to cflags) which needs to be set on the /dev/ttyXX.init or /dev/cuaXX.init device _before_ opening /dev/ttyXX or /dev/cuaXX for actual serial communication. However, because Linux does not have any equivalent to FreeBSD's /dev/ttyXX.init devices, FreeBSD's CNO_RTSDTR solution cannot be copied to Linux verbatim. A naive approach of copying FreeBSD's termios flag and expecting users to set it on the regular /dev/ttyXX device (lacking /dev/ttyXX.init) would not work: the act of opening /dev/ttyXX for the purpose of setting the flag would in itself assert DTR & RTS, which on some hardware devices can cause irreparable harm to the user. Instead the closest thing that can be implemented in Linux would be a sysfs attribute attached to tty serial devices, and this sysfs approach is what the present patch series implements. With this approach, a serial device control application (typically developed together with the custom hw device in which DTR and RTS are repurposed for non-standard uses) that seeks to support both Linux and FreeBSD can treat both OSes in a unified manner, using an abstraction function to hide the small difference between the two. An application can implement an abstraction function like set_initial_rtsdtr_mode(bool autoraise) with only the guts of this function different between Linux and FreeBSD: the Linux version would open /sys/class/tty/ttyXX/manual_rtsdtr and write into that attribute file, whereas the FreeBSD version would open /dev/ttyXX.init and set or clear CNO_RTSDTR termios flag. It needs to be emphasized that the present patch series is a purely additive change, merely adding a new optional mode of operation that needs to be explicitly invoked by those users who desire it. There is NO change to any existing serial port behaviour or to anything else pre-existing: until and unless a user explicitly writes into the new /sys/class/tty/ttyXX/manual_rtsdtr attribute and sets it to 1, absolutely nothing changes, exactly zero impact. There is also one other possibility: there exist some hardware devices in which a USB-serial converter chip and the application circuit behind that chip (which repurposes DTR & RTS for non-standard uses) are integrated into a single monolithic device, with a custom USB VID:PID identifying the hardware device as a whole. Because they are custom ID codes not known at all to "naive" OS kernels, adding Linux support for any such hw device will necessarily require adding knowledge of that custom VID:PID to the appropriate USB-serial driver - and if it is *known* that this paricular hardware device is wired in such a way that requires the manual_rtsdtr flag to be set, then it makes the most sense for the USB-serial driver to set the flag in the device-specific quirk. The present patch series adds support for one such device. Mychaela N. Falconia (6): tty: add port flag to suppress raising DTR & RTS on open serial: core: add sysfs attribute to suppress raising DTR & RTS on open serial: core: fully suppress raising DTR & RTS on open if manual_rtsdtr USB: serial: add sysfs attribute to suppress raising DTR & RTS on open USB: serial: ftdi_sio: pass port to quirk port_probe functions USB: serial: ftdi_sio: add support for FreeCalypso DUART28C adapter Documentation/ABI/testing/sysfs-tty | 10 +++++++++ drivers/tty/serial/serial_core.c | 30 ++++++++++++++++++++++++- drivers/tty/tty_port.c | 2 +- drivers/usb/serial/bus.c | 36 +++++++++++++++++++++++++++-- drivers/usb/serial/ftdi_sio.c | 45 ++++++++++++++++++++++++++++++------- drivers/usb/serial/ftdi_sio_ids.h | 1 + include/linux/tty_port.h | 11 +++++++++ 7 files changed, 123 insertions(+), 12 deletions(-) -- 2.9.0 base-commit: 25e02ba60f0fbe65ba07553b5b2b8867726273c4