[RFC] Probing ttys by default

Lars Knudsen larsgk at gmail.com
Fri Mar 21 06:51:18 PDT 2014

If we take a step back and think how users will be best served by the
mechanism, I think there are several things we could consider as components:

*  Could ModemManager perhaps wait X seconds before trying to grab a device
(allowing others to have "first pick")?
*  Could it be possible that if someone plugs in a modem, there may (most
often) be so big human interaction actions already happening that an
interactive mode - potentially with a "remember decision" functionality
would be prefectly fine.  It's very seldom that a modem just does a
plug'n'play and connect without at least some interaction.
*  (already mentioned) - CDC descriptors to be used for detection - at
least having a combination that knowledgable developers can use to 100%
tell ModemManager to either white- or blacklist on the fly?

- Lars

On Fri, Mar 21, 2014 at 2:36 PM, Bjørn Mork <bjorn at mork.no> wrote:

> Sven Köhler <sven.koehler at gmail.com> writes:
> > The value of ATTR{bmCapabilities} you find below may be from the CDC ACM
> > descriptor instead of the CDC Call Management descriptor.
> > At least for my UMTS card that is true, and I see a value of 7 instead
> > of 3. My best guess is, that udev doesn't handle it very well if there
> > are two fields of the same name in different descriptors of the same
> device.
> Ah, you're right.  Thanks for clearing that up.  Then it's not going to
> be that simple to write a rule processing the CDC Call Management
> bmCapabilities.
> This doesn't really have much to do with udev.  It's the driver that
> parses the descriptors and export some functional descriptor fields as
> sysfs attributes.
> The code making this appear in sysfs/udev is:
> static ssize_t show_caps
> (struct device *dev, struct device_attribute *attr, char *buf)
> {
>         struct usb_interface *intf = to_usb_interface(dev);
>         struct acm *acm = usb_get_intfdata(intf);
>         return sprintf(buf, "%d", acm->ctrl_caps);
> }
> static DEVICE_ATTR(bmCapabilities, S_IRUGO, show_caps, NULL);
> ..
>                 case USB_CDC_ACM_TYPE:
>                         ac_management_function = buffer[3];
>                         break;
>                 case USB_CDC_CALL_MANAGEMENT_TYPE:
>                         call_management_function = buffer[3];
>                         call_interface_num = buffer[4];
>                         if ((quirks & NOT_A_MODEM) == 0 &&
> (call_management_function & 3) != 3)
>                                 dev_err(&intf->dev, "This device cannot do
> calls on its own. It is not a modem.\n");
>                         break;
> ..
>         acm->ctrl_caps = ac_management_function;
>         if (quirks & NO_CAP_LINE)
>                 acm->ctrl_caps &= ~USB_CDC_CAP_LINE;
> So the driver parse and use USB_CDC_CALL_MANAGEMENT_TYPE descriptor, but
> it doesn't export any of it.  If MM or other userspace tools want to
> inspect it, then the only option is to parse it again out of either the
> "descriptors" sysfs field or the /dev/bus/usb character device.
> Bjørn
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