On 27/06/17 22:08, Oliver Neukum wrote:
> Am Dienstag, den 27.06.2017, 19:43 +1000 schrieb Stuart Longland:
>> Maybe a good start would be a "standard" option (referring to the
>> physical signalling standard, TTL/RS-232/RS-422/RS-485), that lists the
>> available standards when read and shows the "selected" standard in
>> brackets (like the 'trigger' option of the LEDs sysfs interface)… so for
>> this case:
>>
>> # cat /sys/class/tty/ttyUSB0/standard
>> [rs232] rs422 rs485 rs485fd
>>
>> and to select 4-wire ("full duplex") RS-485, one does:
>> # echo rs485fd > /sys/class/tty/ttyUSB0/standard
> This looks like something that people will put into udev.
> So the switch will be done via udev, but the user not necessarily
> started via udev. Looks like a race to me.
It is a race, but unless a daemon is watching for the very moment that a
particular serial interface appears, it's one that init scripts are very
likely to win.
Short of having a flag that blocks reads/writes until a given sysfs file
is written to say "I'm ready, go ahead"… there aren't that many options
I can think of.
> And what do you if the interface is already opened and the sysfs
> interface is used?
We return -EBUSY? If the interface is open, there isn't a lot we can
do, the user either puts up with unpredictable behaviour or we refuse
the switch.
By unpredictable behaviour: if the device is well behaved, there might
be a small signal level glitch… but I'd expect serial devices to be
tolerant of that kind of step change in signal. (Think hot-plugging
serial links.)
This beats the official Moxa solution: their Windows driver wants a
reboot of the entire machine to switch between serial standards.
--
Stuart Longland (aka Redhatter, VK4MSL)
I haven't lost my mind...
...it's backed up on a tape somewhere.
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