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Date: Thu, 10 Dec 2020 11:41:24 +0100 From: Maarten Brock <m.brock@...mierlo.com> To: Mychaela Falconia <mychaela.falconia@...il.com> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Johan Hovold <johan@...nel.org>, Jiri Slaby <jirislaby@...nel.org>, "Mychaela N . Falconia" <falcon@...ecalypso.org>, linux-serial@...r.kernel.org, linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2 0/7] tty: add flag to suppress ready signalling on open 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. > If the concern is with the new sysfs interface or the proposed O_DIRECT > alternative, how about deferring those while allowing specific USB ID > support to go in first? Right now there already exists at least one > piece of hardware actively supported by its manufacturer (my gadget) > that has a custom USB ID and requires the quirk - what is wrong with > adding support for this existing specific hw? How about merging > Johan's patch that defines the NORDY flag in tty_port, merging the > ftdi_sio driver patch setting this flag for my custom USB ID, allowing > other hardware engineers in the same boat to submit similar quirk > patches for their affected custom hw with custom USB IDs, while > deferring the sysfs patches until there is a more pressing need for > quirky devices that have no custom USB IDs? > > Sincerely, > Mychaela Again, I agree. Maarten
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