On Thu, Dec 10, 2020 at 01:05:15PM +0100, Maarten Brock wrote: > 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. Companies/devices lie about vid:pid all the time, wait until your specific vid:pid is repurposed for some other device and then what happens? :) > 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(). O_DIRECT is an interesting hack, has anyone seen if it violates the posix rules for us to use it on a character device like this? thanks, greg k-h
