primary device changing from boot to boot

[Tim Harvey]

On Tue, Sep 5, 2017 at 1:59 PM, Dan Williams <dcbw at redhat.com> wrote:
> On Tue, 2017-09-05 at 12:32 -0700, Tim Harvey wrote:
>> On Tue, Sep 5, 2017 at 9:53 AM, Dan Williams <dcbw at redhat.com> wrote:
>> > On Tue, 2017-09-05 at 09:38 -0700, Tim Harvey wrote:
>> > > On Fri, Sep 1, 2017 at 1:22 PM, Dan Williams <dcbw at redhat.com>
>> > > wrote:
>> > > > On Fri, 2017-09-01 at 13:01 -0700, Tim Harvey wrote:
>> > > > > Greetings,
>> > > > >
>> > > > > I've got a Sierra Wireless HL7588 modem which exposes three
>> > > > > ttyACM
>> > > > > devs via the cdc_acm driver. The device appears to work fine
>> > > > > with
>> > > > > ModemManager (1.6.8) but occasionally mm detects the primary
>> > > > > device
>> > > > > as
>> > > > > ttyACM2 instead of the usuall ttyACM0. In this case (detected
>> > > > > primary
>> > > > > AT interface of ttyACM2), if I've already configured
>> > > > > NetworkManager
>> > > > > to
>> > > > > use /dev/ttyACM0 I can't bring up the modem.
>> > > >
>> > > > NM has a "DeviceIdentifier" property on GSM connections partly
>> > > > for
>> > > > this
>> > > > reason.  You cannot always guarantee that even if MM *did*
>> > > > detect
>> > > > the
>> > > > first exposed TTY as primary, that it's going to be called
>> > > > ttyACM0.
>> > > > The kernel is free to name these things whatever it wants, and
>> > > > if
>> > > > ttyACM0 is already used it'll pick another one.  I've had this
>> > > > happen
>> > > > when some program holds the TTY open and the modem reboots.
>> > > >
>> > > > So moral of the story is, don't depend on device names.
>> > > >
>> > > > Get the MM device identifier from the modem:
>> > > >
>> > > > dbus-send --print-reply --system --
>> > > > dest=org.freedesktop.ModemManager1
>> > > > /org/freedesktop/ModemManager1/Modem/0
>> > > > org.freedesktop.DBus.Properties.Get
>> > > > string:org.freedesktop.ModemManager1.Modem
>> > > > string:DeviceIdentifier
>> > > >
>> > > > and then set that string as the DeviceIdentifier in the
>> > > > NetworkManager
>> > > > connection for the modem.  That connection will then only ever
>> > > > apply to
>> > > > that specific modem.  You can then do things liek "nmcli con up
>> > > > <connection mame>" and NM will figure out what device it should
>> > > > use.
>> > > >
>> > >
>> > > Dan,
>> > >
>> > > This makes sense, but how do I configure NetworkManager to use
>> > > the
>> > > device-id? I don't see it as an available option for type gsm, so
>> > > perhaps I need to edit the file in
>> > > /etc/NetworkManager/system-connections/mymodem and add it to the
>> > > 'gsm'
>> >
>> > nmcli con mod <connection name> gsm.device-id \
>> >     6909b49a4d44867387a2b09b8095c579e031874c
>> >
>> > Or yeah, drop:
>> >
>> > device-id=6909b49a4d44867387a2b09b8095c579e031874c
>> >
>> > into the [gsm] section of the keyfile in
>> > /etc/NetworkManager/system-
>> > connections.
>> >
>>
>> Using 'device-id' does not appear to not work, at least not in NM
>> 1.2.6:
>
> 1.2.6 should have the right support.  Perhaps I didn't quite understand
> what you were asking though.
>
> device-id is useful if you have multiple modems on a system, or if you
> swap modems between systems and want a connection to apply to a
> specific device.

or if you have a modem where the primary interface mm detects can't
make up it's mind between tty's which is my case.

>
> But your original question was about device naming, and ACM0 vs. ACM2.
> For some modems, it shouldn't matter what the interface name is.  So in
> your case, you wouldn't do anything to "configured NetworkManager to
> use /dev/ttyACM0", you'd simply set the device-id and then use the NM
> connection name to start/stop things.  You wouldn't hardcode "nmcli dev
> connect ttyACM0" or anything like that.
>
> Instead you'd "nmcli con up Verizon" and the device-id would tell NM to
> use the right device, no matter whether ACM0, ACM2, or USB3 or
> whatever.
>
> Basically, you don't want to hardcode tty device names ever, because
> they can and do change.  So if you're doing that somewhere, I'd
> recommend instead using the NM connection name, and device-id to make
> sure the connection always applies to that specific modem.
>

I understood what you meant, but it wasn't working.

I found that in order to use 'device-id' you must also remove the
'interface-name' param from the '[connection]' section as well.

Thanks - it works like a charm now!

Tim