Messages in this thread | | | Date | Thu, 10 Dec 2020 13:05:15 +0100 | From | Maarten Brock <> | Subject | Re: [PATCH v2 0/7] tty: add flag to suppress ready signalling on open |
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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
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