Use a specific device ?

[Jean-Christian de Rivaz]

Le 28. 05. 15 21:54, Dan Williams a écrit :
> On Thu, 2015-05-28 at 18:39 +0200, Aleksander Morgado wrote:
>> On Thu, May 28, 2015 at 4:41 PM, Jean-Christian de Rivaz <jc at eclis.ch> wrote:
>>
>>> My proposition got that way:
>>> 1) By default ModemManager identify the modem on D-Bus using the device name
>>> (e.g. ttyACMx).
>>> 2) If ID_MM_DEVICE_TAG is provided from udev rules, then ModemManager use
>>> ID_MM_DEVICE_TAG value to identify the modem on D-Bus instead of 1).
>>> 3) If (choose a method here) then ModemManager use the modem ID / SIM ID to
>>> identify the modem on D-Bus instead of 1) and 2).
>> I still think that the ideal thing would be to have NetworkManager
>> settings tied to specific devices; but not identified by a
>> user-provided tag or a specific /dev/ttyX name. From my point of view,
>> NetworkManager should allow to either:
>>   a) Have connection settings not tied to any modem (possible nowadays).
>>   b) Have connection settings tied to a very specific modem (e.g. to a
>> specific IMEI, MEID...) or even better, SIM card (e.g. IMSI, SIM
>> ICCID...).
>>
>> The fact that ModemManager exports the primary port in DBus is really
>> just a convenience for NetworkManager to provide a descriptive modem
>> interface device name in the logs (isn't it?). The device names are of
>> course not ensured to be the same across reboots, and therefore
>> identifying a modem via the exposed device name is never a good idea.
>> Hence, the suggestion to identify it directly by IMEI, MEID,... or via
>> the "Equipment ID" property.
>>
>> Don't take me wrong, the idea of exposing in DBus the value of a given
>> property is feasible. But then, why does ModemManager have to do that?
>> If you want to have specific udev tags tied to a given modem, you can
>> still do that, and as ModemManager already exposes the full list of
>> ports of a modem, you could traverse each looking for the specific
>> udev tag. Don't know, I just don't see the value on complicating the
>> logic this way :/
> How about just reading g_udev_device_get_device_file_symlinks () and
> exposing that information as an "Aliases(as)" property on the MMModem
> object?
>
> # cat /etc/udev/rules.d/90-huawei-symlink.rules
> ACTION=="add", ATTRS{idVendor}=="12d1", ATTRS{idProduct}=="1001", SUBSYSTEM=="tty", SYMLINK+="gsmmodem1_%n"
>
> # udevadm info --query=all /dev/ttyUSB2
> ...
> S: gsmmodem1_2
> S: serial/by-id/usb-ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ_HUAWEI_Mobile-if02-port0
> S: serial/by-path/pci-0000:00:14.0-usb-0:1:1.2-port0
> ...

Good advance, thanks. Certainly a working idea for modem that expose a 
single serial device, but how this will work with a modem that expose 
multiple serial devices?

For example I have a modem that expose no less than 7 serial devices. I 
can use the ENV{ID_USB_INTERFACE_NUM}=="xx" to select a specific serial 
device of the modem and assign a fixed symbolic link to it like this:

ls -al /dev/gsm*
lrwxrwxrwx 1 root root 7 May 28 22:05 /dev/gsm0 -> ttyACM1
lrwxrwxrwx 1 root root 7 May 28 22:05 /dev/gsm1 -> ttyACM2
lrwxrwxrwx 1 root root 7 May 28 22:05 /dev/gsm2 -> ttyACM3
lrwxrwxrwx 1 root root 7 May 28 22:05 /dev/gsm3 -> ttyACM4
lrwxrwxrwx 1 root root 7 May 28 22:05 /dev/gsm4 -> ttyACM5
lrwxrwxrwx 1 root root 7 May 28 22:05 /dev/gsm5 -> ttyACM6
lrwxrwxrwx 1 root root 7 May 28 22:05 /dev/gsm6 -> ttyACM7

Guessing what symbolic link name will be selected by ModemManager is not 
trivial . From experience this is sometime the first one, sometime the 
3th, I think this depend on some timings on the modem, or the USB bus, 
or into the kernel. From a user point of view I don't see an advantage 
over the real device name as it's still not a fixed one.

Obviously a single symbolic link can't point to multiple serial devices. 
And an user don't have to study the modem technical details to select 
himself one of the specific serial device as he expect that ModemManager 
will automatically probe the serial devices and select the appropriate one.

I still think that the ID_MM_DEVICE_TAG give more advantage as the user 
only have to give a single udev rule to tag all the serial devices of a 
particular modem with the same tag and let ModemManager do his auto 
probing. Whenever the serial device is selected by ModemManager is a 
internal detail not in the direct interest of the user. What the user 
want is just a fixed name to refer to the modem.

> I suppose we could also do this on the NM side instead of ModemManager,
> but that would prevent other non-udev-based schemed in ModemManager in
> the future.  I guess I'm fine with either way, either a new MM property
> or having NM look this up.
>

I you follow the previous paragraph, you will understand that it's 
clearly the job of ModemManager. This just plain logic as this is the 
only entity in the chain that receive the udev event about the modem.

Jean-Christian