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? Sincerely, Mychaela Falconia (she/her) Custom hardware design engineer, designing hw specifically for use with FOSS operating systems
