base-modem: setup a maximum number of timeouts also for USB modems

We have a maximum number of timeouts that could be enabled for TTY
modems, e.g. to detect whether the modem is available in the RS232
port or has been unplugged. This was required because RS232 modems
aren't notified via udev, so there was no other way to check presence
of the modem.

But, the same kind of checks may be used in order to detect failed
broken modems. If we have a modem that gets into an unstable state
where no AT port command goes through and all get timed out, we can
definitely flag the modem as invalid and get rid of it. If we don't do
this, we would end up leaving the modem exposed and available for
users of the API, but in a non-working state.

    ....
    <info>  Modem /org/freedesktop/ModemManager1/Modem/0: state changed (registered -> connecting)
    <debug> Launching 3GPP connection attempt with APN 'telefonica.es'
    <debug> Looking for best CID...
    <debug> (ttyACM2) device open count is 2 (open)
    <debug> (ttyACM2): --> 'AT+CGDCONT?<CR>'
    <debug> Unexpected +CGDCONT? error: 'Serial command timed out'
    <warn>  (tty/ttyACM2) at port timed out 9 consecutive times
    <debug> Using empty CID 1 with PDP type 'ipv4'
    <debug> (ttyACM2) device open count is 3 (open)
    <debug> (ttyACM2) device open count is 2 (close)
    <debug> (ttyACM2): --> 'AT+CGDCONT=1,"IP","telefonica.es"<CR>'
    <warn>  Couldn't initialize PDP context with our APN: 'Serial command timed out'
    <debug> Couldn't connect bearer '/org/freedesktop/ModemManager1/Bearer/0': 'Serial command timed out'
    <info>  Modem /org/freedesktop/ModemManager1/Modem/0: state changed (connecting -> registered)
    <debug> Couldn't connect bearer: 'Serial command timed out'
    <debug> (ttyACM2) device open count is 1 (close)
    <error> (tty/ttyACM2) at port timed out 10 consecutive times, marking modem '/org/freedesktop/ModemManager1/Modem/0' as invalid
    <debug> Removing from DBus bearer at '/org/freedesktop/ModemManager1/Bearer/0'
    <debug> [device /sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.5] unexported modem from path '/org/freedesktop/ModemManager1/Modem/0'
    <debug> Periodic signal checks disabled
    <debug> (ttyACM2) device open count is 0 (close)
    <debug> (ttyACM2) closing serial port...
    <debug> (ttyACM2) serial port closed
    <debug> (ttyACM2) forced to close port
    <debug> Modem (u-blox) '/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.5' completely disposed

src/mm-base-modem.c

@@ -36,6 +36,10 @@
 
 G_DEFINE_ABSTRACT_TYPE (MMBaseModem, mm_base_modem, MM_GDBUS_TYPE_OBJECT_SKELETON);
 
+/* If we get 10 consecutive timeouts in a serial port, we consider the modem
+ * invalid and we request re-probing. */
+#define DEFAULT_MAX_TIMEOUTS 10
+
 enum {
     PROP_0,
     PROP_VALID,
@@ -1318,6 +1322,8 @@ mm_base_modem_init (MMBaseModem *self)
                                                g_str_equal,
                                                g_free,
                                                g_object_unref);
+
+    self->priv->max_timeouts = DEFAULT_MAX_TIMEOUTS;
 }
 
 static void
@@ -1497,7 +1503,7 @@ mm_base_modem_class_init (MMBaseModemClass *klass)
                            "Max timeouts",
                            "Maximum number of consecutive timed out commands sent to "
                            "the modem before disabling it. If 0, this feature is disabled.",
-                           0, G_MAXUINT, 0,
+                           0, G_MAXUINT, DEFAULT_MAX_TIMEOUTS,
                            G_PARAM_READWRITE);
     g_object_class_install_property (object_class, PROP_MAX_TIMEOUTS, properties[PROP_MAX_TIMEOUTS]);