On Fri, Aug 25, 2023 at 01:46:15AM +0200, Oliver Neukum wrote: > > > On 24.08.23 20:02, Simon Arlott wrote: > > The ioctl can't be used without opening and closing the tty, which has > > side effects. On open() it'll raise DTR/RTS and activate it. For cdc-acm > > that will indicate to the device that the serial port has been opened > > which will be visible to the software running on the USB device. On > > close() it'll be delayed by the close_delay if any process is currently > > doing a blocking open() and there's no carrier, then the closing_wait > > time if there's been any incomplete transmitted data (by any process). > > And that is an issue of the generic serial layer. Is it feasible to add a sysfs attribute for ttys or the serial layer to control the side effect of opening (avoid raising DTR/RTS)? If that could be done, a program could use the existing ioctl to set close_delay and closing_wait to 0 with no penalties. This would be racy, but for the purposes of udev that shouldn't matter much. Alan Stern
