inhibit network interface over slow link

[Giacinto Cifelli]

Dear all,

I have a problem with some modems that don't support USB <2.00 for the
network interface.
In this case, PPP must be used.

It is a modem-specific limitation, and maybe it should have a udev flag:
MM_USE_PPP_OVER_USB1, or something more creative.

The question is how to recognize the overall USB version . The udev USB
version property for the device  will be >=2.00, but along the way, either
the root hub or an intermediate one, will bring the effective version down.
The speed property is "cumulative": if a hub along the way is 12Mbps, then
the final speed for the device is shown as 12Mbps:

  looking at device
'/devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2.4/3-2.4.3/3-2.4.3.2/3-2.4.3.2:1.0/tty/ttyACM4':
    KERNEL=="ttyACM4"
    SUBSYSTEM=="tty"
    DRIVER==""

  looking at parent device
'/devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2.4/3-2.4.3/3-2.4.3.2/3-2.4.3.2:1.0':
    KERNELS=="3-2.4.3.2:1.0"
    SUBSYSTEMS=="usb"
    DRIVERS=="cdc_acm"
    ATTRS{authorized}=="1"
    ATTRS{bAlternateSetting}==" 0"
    ATTRS{bInterfaceClass}=="02"
    ATTRS{bInterfaceNumber}=="00"
    ATTRS{bInterfaceProtocol}=="01"
    ATTRS{bInterfaceSubClass}=="02"
    ATTRS{bNumEndpoints}=="01"
    ATTRS{bmCapabilities}=="2"
    ATTRS{iad_bFirstInterface}=="00"
    ATTRS{iad_bFunctionClass}=="02"
    ATTRS{iad_bFunctionProtocol}=="01"
    ATTRS{iad_bFunctionSubClass}=="02"
    ATTRS{iad_bInterfaceCount}=="02"
    ATTRS{interface}=="CDC Abstract Control Model (ACM)"
    ATTRS{supports_autosuspend}=="1"

  looking at parent device
'/devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2.4/3-2.4.3/3-2.4.3.2':
    KERNELS=="3-2.4.3.2"
    SUBSYSTEMS=="usb"
    DRIVERS=="usb"
    ATTRS{authorized}=="1"
    ATTRS{avoid_reset_quirk}=="0"
    ATTRS{bConfigurationValue}=="1"
    ATTRS{bDeviceClass}=="ef"
    ATTRS{bDeviceProtocol}=="01"
    ATTRS{bDeviceSubClass}=="02"
    ATTRS{bMaxPacketSize0}=="64"
    ATTRS{bMaxPower}=="500mA"
    ATTRS{bNumConfigurations}=="1"
    ATTRS{bNumInterfaces}=="12"
    ATTRS{bcdDevice}=="0318"
    ATTRS{bmAttributes}=="a0"
    ATTRS{busnum}=="3"
    ATTRS{configuration}==""
    ATTRS{devnum}=="79"
    ATTRS{devpath}=="2.4.3.2"
    ATTRS{idProduct}=="0065"
    ATTRS{idVendor}=="1e2d"
    ATTRS{ltm_capable}=="no"
    ATTRS{manufacturer}=="Cinterion"
    ATTRS{maxchild}=="0"
    ATTRS{product}=="LTE Modem"
    ATTRS{quirks}=="0x0"
    ATTRS{removable}=="unknown"
    ATTRS{serial}=="8d8f52c8"
    ATTRS{speed}=="12"
    ATTRS{urbnum}=="463"
    ATTRS{version}==" 2.10"


It is possible to cross the entire parent chain, and then get the minimum
version. I don't have a root hub USB 1.x at hand, but I have found an old
external USB hub 1.1, and connected this way: device - hub1.1 - hub2.0 -
roothub2.0:
name: ttyACM4, version:       NULL, minversion: NULL
name: ttyACM4, version:       NULL, minversion: NULL
name: ttyACM4, version:       2.10. minversion:  2.10
name: ttyACM4, version:       1.10. minversion:  1.10
name: ttyACM4, version:       2.00. minversion:  1.10
name: ttyACM4, version:       2.00. minversion:  1.10
name: ttyACM4, version:       2.00. minversion:  1.10
name: ttyACM4, version:       NULL, minversion:  1.10
name: ttyACM4, version:       NULL, minversion:  1.10

I have placed some code in mm-kernel-device-udev.c to obtain the chain
above.

The question is whether there is a better way to do this, possibly without
touching the udev implementation.
Or whether it is acceptable to read the speed property instead of
rebuilding the effective usb version: is it ok to replace a variable for
another here for all foreseeable cases?

Thank you all in advance,
Kind Regards,
Giacinto
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