On Thu, May 26, 2022 at 12:26:03AM -0800, Mychaela Falconia wrote: > Hello Linux serial and usb-serial maintainers, > > I am the hardware engineer who fought here unsuccessfully a year and a > half ago to add Linux support for serial hardware devices in which DTR > and/or RTS modem control signals have been repurposed for non-standard > uses. On such hw devices it is very often the case that DTR and RTS > may be asserted ONLY when explicitly requested by specialized > userspace applications that go with the hw (by way of TIOCMBIS ioctl, > often followed by a delay and TIOCMBIC to produce a pulse), and NOT at > any other times - in particular, it must be possible to open the > serial port for byte Rx/Tx communication with the target device > _without_ that open syscall unstoppably asserting DTR & RTS right > there and then. Here is the previous round of discussion: > > https://lore.kernel.org/linux-serial/X8iuCXYhOBVMGvXv@localhost/T/ > > I am now revisiting this issue and making another attempt to get this > capability added to Linux. The new development is that I have tested > the corresponding feature in FreeBSD, and found it to work correctly - > thus FreeBSD is now the first (and so far only) Unix-style OS in the > world that features an actually working, usable fix for the 1970s UNIX > design bug of unstoppably asserting DTR & RTS on serial port open. > Wouldn't it be good for Linux to follow suit? > > The feature in question was added to FreeBSD quite recently: > > https://reviews.freebsd.org/D20031 > > The above link was provided by Johan here a year and a half ago, in > the previous round of this battle. However, only now I have had a > chance to test FreeBSD's implementation on actual hardware, and > confirm that it actually works as required for signal-repurposing hw > applications. > > The diff that appears on the FreeBSD review page above only adds one > new termios flag, CNO_RTSDTR added to cflags. Johan was proposing > implementing a similar new termios flag in Linux - however, there is a > crucial difference between what Johan was proposing for Linux vs what > is actually implemented in FreeBSD, and it's the difference between > usable and unusable. > > FreeBSD's counterpart to Linux ttyUSBx is ttyUx or cuaUx devices. > However, FreeBSD also has .init and .lock devices which don't exist in > Linux, and it's the .init device that makes their newly added > CNO_RTSDTR feature actually work in a usable manner. If you have a > serial device that absolutely does not tolerate unwanted assertions of > DTR/RTS, not even for one nanosecond, under FreeBSD you can make it > work as follows (using cuaU0 as example here): > > Step 1: open /dev/cuaU0.init (the .init suffix is crucial) and perform > the ioctl setting CNO_RTSDTR on this initial-state device. This step > can be done with stty command. These .init devices have the special > property that opening one does NOT cause modem control lines to be > asserted, unlike the regular ttyXX or cuaXX device. > > Step 2: once the previous step is done, you can open the regular > /dev/cuaU0 device for communication, and no unwanted DTR/RTS assertion > will happen. > > Johan's proposal for Linux was superficially similar: he proposed > adding a similar new termios flag. But because Linux does not have > any counterpart to FreeBSD's .init devices, a termios flag won't work > for this purpose in Linux, it would create a chicken-and-egg problem: > one would need to open the serial port first in order to set the > termios flag, and this very act would cause DTR & RTS to be asserted, > causing irreparable damage: electrocution, setting off explosives, use > your imagination for how these signals could be wired to do highly > damaging actions. > > Out of various methods that were discussed here a year and a half ago, > only the sysfs approach would produce a user capability match to what > FreeBSD now provides. The termios flag idea is a dead end, unless > this community feels that FreeBSD's ttyXX.init and ttyXX.lock should > also be replicated in Linux in full. Another sensible way would be to > define a new open flag, such as O_NODTR, or reuse/abuse an existing > flag like O_DIRECT which currently does nothing for tty devices - but > people have objected that this approach would be limited to custom > userspace programs, precluding the use of echo, cat etc. > > I now argue for the sysfs attribute approach. Johan had a patch (he > made it, but didn't fight for it because he really preferred the > termios flag instead) adding a /sys/class/tty/ttyXXX/nordy attribute - > setting this attribute to 1 would suppress automatic assertion of DTR > and RTS on serial port open. One could argue that a better name for > this new sysfs attribute would be something like > /sys/class/tty/ttyXXX/manual_dtr_rts - but I'll be happy no matter how > it's named, as long as the essential functionality remains. > > This sysfs attribute would produce the same fundamental workflow as > currently exists in FreeBSD. In FreeBSD one needs to open > /dev/cuaXX.init, do the necessary ioctl, then close that fd and open > another path (/dev/cuaXX) to do the actual serial communication. With > the proposed sysfs attribute, the same fundamental workflow will apply > here: open the sysfs path, do the necessary attribute write, then > close the sysfs fd and open /dev/ttyXXX for the actual communication. > In both cases the preliminary step (/dev/cuaXX.init in FreeBSD, sysfs > path in my proposal for Linux) can be done from the shell, or it can > be incorporated into custom userspace sw that works with the custom hw > device. > > I can dig up Johan's old patch adding the nordy attribute, update it > for current HEAD, and formally resubmit it - would the maintainers be > agreeable to such course? Please rebase and resubmit it and we will be glad to consider it. thanks, greg k-h
