Messages in this thread | | | Subject | Re: [PATCH 0/5] tty: add flag to suppress ready signalling on open | From | Jiri Slaby <> | Date | Tue, 1 Dec 2020 08:14:07 +0100 |
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On 30. 11. 20, 22:22, Mychaela Falconia wrote: > 2) For situations in which the luxury of a custom USB ID is not > available, e.g., a situation where the device that does not tolerate > automatic DTR/RTS assertion on open is a physical RS-232 device that > can be connected to "any" serial port, the new sysfs attribute comes > to the rescue. > > Johan's patch comments say that the new flag can also be brought out > to termios in the future, similarly to HUPCL,
The difference to other control flags is that open raises DTR/RTS in any case (i.e. including O_NONBLOCK) -- provided baud rate is set (and it is for casual serials). That means you cannot open a port to configure it (using e.g. setserial) without actually raising the DTR/RTS.
> but I question the > usefulness of doing so, as it is a chicken and egg problem: one needs > to open the tty device in order to do termios ioctls on it, and if > that initial open triggers DTR/RTS hardware actions, then the end user > is still screwed. If Johan or someone else can see a potential use > case for manipulating this new flag via termios (as opposed to sysfs > or USB-ID-based driver quirks), perhaps you could elaborate on it?
We would need to (ab)use another open flag (e.g. O_DIRECT). I am not biased to either of solutions.
thanks, -- js
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