netdev - Re: qmi_wwan not using netif_carrier

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Message-ID: <20200617172434.GH205574@lunn.ch>
Date:   Wed, 17 Jun 2020 19:24:34 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Dan Williams <dcbw@...hat.com>
Cc:     Tanjeff-Nicolai Moos <tmoos@...ec.de>, netdev@...r.kernel.org
Subject: Re: qmi_wwan not using netif_carrier_*()

On Wed, Jun 17, 2020 at 11:59:33AM -0500, Dan Williams wrote:
> On Wed, 2020-06-17 at 18:48 +0200, Andrew Lunn wrote:
> > On Wed, Jun 17, 2020 at 03:21:53PM +0200, Tanjeff-Nicolai Moos wrote:
> > > Hi netdevs,
> > > 
> > > Kernel version:
> > > 
> > >   I'm working with kernel 4.14.137 (OpenWRT project). But I looked
> > > at
> > >   the source of kernel 5.7 and found the same situation.
> > > 
> > > Problem:
> > > 
> > >   I'm using the qmi_wwan driver for a Sierra Wireless EM7455 LTE
> > >   modem. This driver does not use
> > >   netif_carrier_on()/netif_carrier_off() to update its link status.
> > >   This confuses ledtrig_netdev which uses netif_carrier_ok() to
> > > obtain
> > >   the link status.
> > > 
> > > My solution:
> > > 
> > >   As a solution (or workaround?) I would try:
> > > 
> > >   1) In drivers/net/usb/qmi_wwan.c, lines 904/913: Add the flag
> > >      FLAG_LINK_INTR.
> > > 
> > >   2) In drivers/net/usb/usbnet.c, functions usbnet_open() and
> > >      usbnet_stop(): Add a call to netif_carrier_*(),
> > >      but only if FLAG_LINK_INTR is set.
> > > 
> > > Question:
> > > 
> > >   Is this the intended way to use FLAG_LINK_INTR and
> > > netif_carrier_*()?
> > >   Or is there another recommended way to obtain the link status of
> > >   network devices (I could change ledtrig_netdev)?
> > 
> > Hi Tanjeff
> > 
> > With Ethernet, having a carrier means there is a link partner, the
> > layer 2 of the OSI 7 layer stack model is working. If the interface
> > is
> > not open()ed, it clearly should not have carrier. However, just
> > because it is open, does not mean it has carrier. The cable could be
> > unplugged, etc.
> > 
> > This is an LTE modem. What does carrier mean here? I don't know if it
> > is well defined, but i would guess it is connected to a base station
> > which is offering service. I'm assuming you are interested in data
> > here, not wanting to make a 911/999/112/$EMERGENCY_SERVICE call which
> > in theory all base stations should accept.
> > 
> > Is there a way to get this state information from the hardware? That
> > would be the correct way to set the carrier.
> 
> There isn't. All the setup that would result in IFF_LOWER_UP (eg
> ability to pass packets to the cellular network) happens over channels
> *other* than the ethernet one. eg CDC-WDM, CDC-ACM, CDC-MBIM, AT
> commands, QMI commands, MBIM commands, etc.
> 
> Something in userspace handles the actual IP-level connection setup and
> once that's done, only then do you really have IFF_LOWER_UP. One way to
> solve this could be to require userspace connection managers to manage
> the carrier state of the device, which is possible for some drivers
> already IIRC.

So Tanjeff, what is you real use case here? I assume you want to
control an LED so it is on when the LTE modem is connected? Could you
export the LED to user space and have a dhclient-enter/exit script change
the state of the LED?

    Andrew