[systemd-devel] Use a specific device ?

[Jean-Christian de Rivaz]

Le 05. 06. 15 15:41, poma a écrit :
> On 05.06.2015 15:29, Harald Hoyer wrote:
>> On 05.06.2015 15:28, Mantas Mikulėnas wrote:
>>> On Fri, Jun 5, 2015 at 4:22 PM, Harald Hoyer <harald.hoyer at gmail.com
>>> <mailto:harald.hoyer at gmail.com>> wrote:
>>>
>>>      On 05.06.2015 15:09, poma wrote:
>>>      > On 05.06.2015 14:14, Jean-Christian de Rivaz wrote:
>>>      >> Le 05. 06. 15 13:18, Aleksander Morgado a écrit :
>>>      >>> On Tue, May 26, 2015 at 12:19 AM, Jean-Christian de Rivaz <jc at eclis.ch
>>>      <mailto:jc at eclis.ch>> wrote:
>>>      >>>> I have a system where the modem have multiple /dev/ttyACMx ports where
>>>      x is
>>>      >>>> not constant because of the dynamic nature of others serial devices.
>>>      >>> It may be worth noting that a very similar issue with the one faced
>>>      >>> here is the one with network interface names, where interface names
>>>      >>> were created as kernel drivers probed the different interfaces, ending
>>>      >>> up with "eth0", "eth1" and so on. Then, there would be network
>>>      >>> interface configurations for each network interface based on the name,
>>>      >>> but no one really ensured that the name was the same upon reboots. The
>>>      >>> solution provided by systemd to ensure that the proper configuration
>>>      >>> is applied always to the proper interface is to make the device names
>>>      >>> "predictable", see:
>>>      >>>
>>>      http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/
>>>      >>>
>>>      >>> This solution avoids the need of any other udev rules to e.g. create
>>>      >>> network interface names containing the device MAC address or what not.
>>>      >>>
>>>      >>> I'm wondering whether the same could be applied not only to network
>>>      >>> interfaces, but also to ttyACMs, ttyUSBs and cdc-wdms, and end up
>>>      >>> having predictable tty names like e.g. /dev/ttyACMp0s20u4i0. Sure,
>>>      >>> those names are a nightmare to type, but they are predictable (e.g. in
>>>      >>> this case by including the physical location of the connector of the
>>>      >>> hardware).
>>>      >>>
>>>      >>
>>>      >> This would be a wonderful solution. The only problem is when will this
>>>      >> feature be available in a stable Linux kernel widely used by all majors
>>>      >> distributions? Until this dream happens (probably not before severals
>>>      >> years I guess), an other option must be implemented.
>>>      >>
>>>      >> Jean-Christian
>>>      >
>>>      >
>>>      > Face your broadband modem, live your dreams?
>>>      >
>>>      > Kay, when this would happen - Predictable Broadband Modem Interface Names?
>>>      >
>>>
>>>      Wouldn't it be nicer to have symlinks like in /dev/disk ?
>>>      /dev/tty/by-path/....
>>>
>>>
>>> 60-serial.rules:16:ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="",
>>> SYMLINK+="serial/by-path/$env{ID_PATH}"
>>> 60-serial.rules:17:ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="?*",
>>> SYMLINK+="serial/by-path/$env{ID_PATH}-port$env{.ID_PORT}"
>>>
>>> -- 
>>> Mantas Mikulėnas <grawity at gmail.com <mailto:grawity at gmail.com>>
>> There we go.. already implemented :)
>

This is only a symbolic link and ModemManager don't use symbolic link. 
In addition this will not work for modems with multiples serial ports 
where ModemManager select a unpredictable ports because of device or bus 
or kernel or udev timing that affect the order when the ports are added 
(I don't known the exact cause but I have a system where this exists).

Jean-Christian