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[23.128.96.18]) by mx.google.com with ESMTP id b21si2469582ejk.520.2020.12.10.04.19.56; Thu, 10 Dec 2020 04:20:19 -0800 (PST) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731133AbgLJMGf (ORCPT + 99 others); Thu, 10 Dec 2020 07:06:35 -0500 Received: from fieber.vanmierlo.com ([84.243.197.177]:38130 "EHLO kerio9.vanmierlo.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726147AbgLJMGf (ORCPT ); Thu, 10 Dec 2020 07:06:35 -0500 X-Footer: dmFubWllcmxvLmNvbQ== Received: from roundcube.vanmierlo.com ([192.168.37.37]) (authenticated user m.brock@vanmierlo.com) by kerio9.vanmierlo.com (Kerio Connect 9.3.0 patch 1) with ESMTPA; Thu, 10 Dec 2020 13:05:15 +0100 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Thu, 10 Dec 2020 13:05:15 +0100 From: Maarten Brock To: Greg Kroah-Hartman Cc: Mychaela Falconia , Johan Hovold , Jiri Slaby , "Mychaela N . Falconia" , linux-serial@vger.kernel.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 0/7] tty: add flag to suppress ready signalling on open In-Reply-To: References: <20201202113942.27024-1-johan@kernel.org> <6b81cca21561305b55ba8f019b78da28@vanmierlo.com> Message-ID: <3fc3097ce1d35ce1e45fa5a3c7173666@vanmierlo.com> X-Sender: m.brock@vanmierlo.com User-Agent: Roundcube Webmail/1.3.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-12-10 11:50, Greg Kroah-Hartman wrote: > On Thu, Dec 10, 2020 at 11:41:24AM +0100, Maarten Brock wrote: >> Hello Mychaela, >> >> On 2020-12-09 23:49, Mychaela Falconia wrote: >> > Greg K-H wrote: >> > >> > > I think we need more review for the rest of the series. This does >> > > change the way serial ports work in a non-traditional way (i.e. using >> > > sysfs instead of terminal settings). >> > >> > But the problem is that the current status quo is fundamentally broken >> > for those hardware devices in which DTR and/or RTS have been repurposed >> > for something other than modem and flow control. Right now whenever a >> > "cold" (never previously opened) serial port is opened for the first >> > time, that open action immediately and unstoppably asserts both DTR >> > and RTS hardware outputs, without giving userspace any opportunity to >> > say "no, please don't do it". Yes, this behaviour is codified in a >> > bunch of standards that ultimately trace back to 1970s Original UNIX, >> > but just because it is a standard does not make it right - this >> > Unix/POSIX/Linux "standard" serial port behaviour is a bug, not a >> > feature. >> >> I agree. And an application not configuring the required handshakes, >> but >> still relying on them is an equal bug. >> >> > But if there exist some custom hw devices out there that are in the >> > same predicament as my DUART28 adapter, but are different in that they >> > are classic old-fashioned RS-232 rather than integrated USB-serial, >> > with no place to assign a custom USB ID, *then* we need a non-USB-ID- >> > dependent solution such as Johan's sysfs attribute or O_DIRECT. >> >> Any device with a classic old-fashioned RS-232 has probably already >> solved this in another way or is accepted as not working on Linux. >> >> And then there is also the device tree (overlay?) through which a >> quirk >> like this can be communicated to the kernel driver. Not sure if this >> could help for a plug-and-play device like on USB. >> >> > > So I want to get a bunch of people >> > > to agree that this is ok to do things this way now before taking this >> > > new user-visible api. >> >> Personally, I would prefer the VID:PID to enforce the quirk and an >> O_DIRECT (or other) flag used on open() as general backup plan. To >> me a sysfs solution seems illogical. > > The "problem" of a vid:pid is that for usb-serial devices, that only > describes the device that does the conversion itself, NOT the serial > device the converter is plugged into that cares about these types of > line-wiggling. > > Just like you would not want to classify all devices that met the PCI > serial class signature for this type of thing either, there is nothing > special about USB here other than it happens to be a common transport > for these signals these days. > > thanks, > > greg k-h This is true for a generic USB-UART board or cable, but not for a dedicated PCB where both the USB-UART chip and the special connection are implemented and which has a dedicated VID:PID different from any generic one. In this case the VID:PID describes the whole board. If the line-wiggling requirement is created behind some sort of connector (real RS-232 DB9/DB25 or CMOS pin header or whatever) then the problem is the same as for an 8250 on any other bus. For this situation I would prefer the O_DIRECT flag on open(). Maarten