Querying WWAN iface name for each /dev/cdc-wdm device

[Aleksander Morgado]

On Tue, Jan 27, 2015 at 9:34 AM, Bjørn Mork <bjorn at mork.no> wrote:
> Aleksander Morgado <aleksander at aleksander.es> writes:
>> On Mon, Jan 26, 2015 at 10:04 PM, Bjørn Mork <bjorn at mork.no> wrote:
>>
>>> No objection, but I believe this info is easy to get from sysfs without
>>> any additional tools:
>>>
>>>  bjorn at nemi:~$ ls -l /sys/class/usbmisc/cdc-wdm0/device/net/
>>>  total 0
>>>  drwxr-xr-x 6 root root 0 Jan 26 22:00 wwan0
>>
>> Yeah, maybe for this specific case where we just want to query that
>> device name once and we don't want udev events, it probably is worth
>> to just parse the sysfs path directly... Or will we ever want to get
>> udev events as well? E.g. Could the systemd network interface renames
>> affect the logic?
>
> sysfs will show the current kernel device name, so any renaming should
> Just Work (tm).  Note that this also works the other way round.  So if
> you wonder what the name of the control device is for network device
> "bar", you can do:
>
>  bjorn at nemi:~$ ls -l /sys/class/net/bar/device/usbmisc
>  total 0
>  drwxr-xr-x 3 root root 0 Jan 27 09:20 cdc-wdm0
>
> The only undetected renaming issue I can think of is if someone renames
> the file in /dev so it doesn't match the kernel device name. I don't
> think that is very likely though, and IMHO it should not be supported.
> You can do it, but then you're on your own...
>
> But if you are going to be really, really prepared for anything, you
> could check the major/minor numbers of the /dev file and look up the
> associated kernel devices:
>
>  bjorn at nemi:~$ ls -ln /dev/cdc-wdm0
>  ls: cannot access /dev/cdc-wdm0: No such file or directory
>  bjorn at nemi:~$ ls -ln /dev/foo
>  crw-rw---- 1 0 20 180, 0 Jan 27 08:46 /dev/foo
>  bjorn at nemi:~$ ls -ld /sys/dev/char/180:0/device/{usbmisc,net}/*
>  drwxr-xr-x 6 root root 0 Jan 27 09:23 /sys/dev/char/180:0/device/net/bar
>  drwxr-xr-x 3 root root 0 Jan 27 09:20 /sys/dev/char/180:0/device/usbmisc/cdc-wdm0


It probably isn't worth all the trouble; I think I'll just update it
to use sysfs directly instead. Not that gudev is a big dependency, but
well, if it can be simply avoided by looking at specific sysfs paths,
then it should be ok.

I'll update the branch and let you know, thanks for the comments!

-- 
Aleksander
https://aleksander.es