Re: [PATCH v2 0/7] tty: add flag to suppress ready signalling on open

[Maarten Brock <m.brock@vanmierlo.com>]

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