linux-kernel - Re: Is it ok if ModemManager process is killed AFTER network-interface is brought up and IP-Address assigned?

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Message-ID: <20160810205346.GB19845@xo-6d-61-c0.localdomain>
Date:	Wed, 10 Aug 2016 22:53:46 +0200
From:	Pavel Machek <pavel@....cz>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Ajay Garg <ajaygargnsit@...il.com>, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: Is it ok if ModemManager process is killed AFTER
 network-interface is brought up and IP-Address assigned?

Hi!

> > We are using Sierra's USB-to-WWAN driver on Ubuntu-14 for Sierra's
> > MC8090 modem, and we have a requirement wherein we need to have access
> > to the modem-serial-port (from our user-application that is).
> > 
> > Right now, we see that /usr/sbin/ModemManager is always connected to
> > /dev/ttyUSB3 (which means we cannot connect to the port from our
> > application at the same time, or even if we can, received-data will be
> > at best inconsistent).
> > 
> > 
> > We are thinking of the following ::
> > 
> > * Initially, let nmcli and ModemManager do their work, and let them
> > bring the WWAN interface up.
> > 
> > * Once this happens, we permanently-down the ModemManager from our
> > application-binary, thereby freeing up /dev/ttyUSB3.
> > 
> > * Thereafter, we are free to connect to /dev/ttyUSB3 from our
> > application, thereby using features like SMS-notification (+CMTI),
> > signal-strength (+CSQ), etc.
> > 
> > 
> > 
> > Does our approach make sense?
> > We will be grateful to any help.
> 
> Why not ask the modem manager team about this?  The kernel doesn't care
> what you do with the device links :)

Arguably, kernel is doing its job pretty badly when it comes to modems.

1) it does not hide differences between different modems (nokia modem is network
protocol, some modems are single tty, some modems are multiple ttys, different AT
command sets...)

2) it does not allow userland applications to share a GSM modem in a reasonable way.
Being able to read signal strength while PPP is connected would be nice, for example.

Anyway, when kernel fails to do its job, there's often userland daemon that can do it.

ofono seems to be the cannonical one there, and it seems to do the job rather well.

So... I guess the original author should take a look at ofono.

										Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html