Re: Moxa UPort 1150 and RS-422/485… what's the "proper" way to switch modes

From: Stuart Longland <stuartl-3e+Fe6x+DsgJbe36r25VNhCuuivNXqWP@public.gmane.org>
To: Oliver Neukum <oneukum-IBi9RG/b67k@public.gmane.org>,
	Johan Hovold <johan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Alan Cox
	<gnomes-qBU/x9rampVanCEyBjwyrvXRex20P6io@public.gmane.org>,
	linux-serial-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	linux-usb-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: Re: Moxa UPort 1150 and RS-422/485… what's the "proper" way to switch modes
Date: Wed, 28 Jun 2017 08:07:54 +1000	[thread overview]
Message-ID: <ae2907c2-cb0f-61e4-c6f2-af3dcec93ace@longlandclan.id.au> (raw)
In-Reply-To: <1498565331.30390.5.camel-IBi9RG/b67k@public.gmane.org>

[-- Attachment #1.1: Type: text/plain, Size: 1862 bytes --]

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.

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]