Re: tap device support (for iphone)?

[Dan Williams <dcbw redhat com>]

On Mon, 2009-11-30 at 16:11 +1100, Bradley Baetz wrote:
> Hi,
> 
> I'm working on a userspace driver for the iphone's tethering
> functionality via USB, which I've reverse engineered (my carrier
> allows tethering - this isn't a jailbroken iPhone). After a bit of
> setup, this ends up basically just bridging a tap device (userspace L2
> ethernet) to the appropriate USB pipes.

Any particular reason it's not done as a real ethernet driver in the
kernel?  Most drivers should be in the kernel, because then we get
access to a lot more information about the device.  If it's a userspace
driver then we can't follow the device tree, find out what device the
interface is associated with, we can't respond to events when the device
is plugged/unplugged, and we can't figure out anything about the device
since it's not hooked into the normal sysfs mechanisms (AFAIK).

> I have this working, and can run dhclient on the tap interface
> manually, however I'm having trouble getting NetworkManager to
> recognise the device. Even if I call the interface 'eth1' rather than
> 'iphone0' NM (NetworkManager-0.7.996-6.git20091021.fc12.x86_64 from
> F12) logs:
> 
> NetworkManager: <WARN>  device_creator():
> /sys/devices/virtual/net/eth1: couldn't determine device driver;
> ignoring...
> 
> which is sort of correct, since there isn't a kernel driver for it.

Exactly; see above.  There's quite a lot of benefit to having a kernel
driver for the device.

> What I'd like to do is have the iphone interface made available to NM,
> but at a lower priority than ethernet or wifi interfaces, ideally with
> the 'mobile broadband' icon appearing in the applet, and without DHCP

Yeah, though since it's just a tap device that's no different than a VPN
tap device really, we can't tell that it's a mobile broadband device.
We could potentially do that if it were a kernel driver and the devtype
was set to 'wwan' like Marcel Holtman has sent patches for.

> running unless the interface is the top choice (I don't want to send
> data over the mobile interface unless we have to, due to cost). There
> may later be a way to get a signal strength out of the interface, but
> for the moment the only status messages I can decode are the
> linkup/linkdown toggles (which my code currently ignores)

Ugh, you want to handle those.  If it were an in-kernel driver, you'd
call netif_carrier_on/netif_carrier_off and we'd also get normal netlink
events out of the kernel when the link went up/down as we do for normal
ethernet devices.

> Is there any way to do this? I took a look at ModemManager, but that
> looks like its designed around serial interfaces, and the iPhone
> presents as ethernet.

Right, MM is for modems.  The iPhone isn't presenting itself as a modem,
but as an ethernet device.  Thus it wouldn't use MM, just like Bluetooth
PAN doesn't use NM, and just like my SonyEricsson TM-506 via USB cable
shows up as a CDC-Ether device and thus doesn't use NM.

In the end, I think there are some pretty good benefits to an in-kernel
driver, and you'd get those benefits mainly for free.  Some of them can
be faked with a tun/tap device, but that seems hackish.  Not to diminish
the work that you've done of course...

Dan