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Re: primary device changing from boot to boot

On Tue, Sep 5, 2017 at 1:59 PM, Dan Williams <dcbw 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 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 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
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