api,signal: new signal threshold based setup

A new SetupThreshold() method is added to configure and control the
device signal notifications sent to the host. RSSI and error rate
based thresholds are initially assumed, but the interface allows
growing the method with additional threshold types.

Includes updates by Aleksander Morgado to improve documentation of the
new methods and properties.

introspection/org.freedesktop.ModemManager1.Modem.Signal.xml

@@ -3,7 +3,8 @@
 <!--
  ModemManager 0.8 Interface Specification
 
-   Copyright (C) 2013 Aleksander Morgado <aleksander@gnu.org>
+   Copyright (C) 2013-2021 Aleksander Morgado <aleksander@aleksander.es>
+   Copyright (C) 2021 Intel Corporation
 -->
 
 <node name="/" xmlns:doc="http://www.freedesktop.org/dbus/1.0/doc.dtd">
@@ -18,14 +19,27 @@
       registered in the cellular network. 3GPP devices will require a valid
       unlocked SIM card before any of the features in the interface can be
       used.
+
+      There are two different ways defined to use the interface: either enabling
+      peridic polling (with the Setup() method), or configuring the device with
+      thresholds so that it reports the value updates automatically (with the
+      SetupThresholds() method, since 1.20).
+
+      Both Setup() and SetupThresholds() can also be used at the same time if
+      required, e.g. if they report different signal quality measurement types.
   -->
   <interface name="org.freedesktop.ModemManager1.Modem.Signal">
 
     <!--
         Setup:
-        @rate: refresh rate to set, in seconds. 0 to disable retrieval.
+        @rate: refresh rate to set, in seconds. Use 0 to disable periodic polling.
 
-        Setup extended signal quality information retrieval.
+        Enable or disable the extended signal quality information retrieval via
+        periodic polling.
+
+        Polling is less than optimal; a better way to be notified of extended
+        signal quality updates is to configure the modem to trigger the reports
+        when the signal changes, i.e. with SetupThresholds().
 
         Since: 1.2
     -->
@@ -33,16 +47,89 @@
       <arg name="rate" type="u" direction="in" />
     </method>
 
+    <!--
+        SetupThresholds:
+        @settings: threshold values to set.
+
+        Setup thresholds so that the device itself decides when to report the
+        extended signal quality information updates.
+
+        The thresholds configured via this method specify the delta between
+        specific signal quality measurements that would trigger a report by the
+        modem. For example, the user may want to be notified every time the
+        signal RSSI changes more than 10dBm, so a value of 10 would be
+        configured as <literal>"rssi-threshold"</literal>.
+
+        The device may not support this kind of threshold setting, and instead
+        support fixed signal levels as thresholds (e.g. trigger reports when
+        signal RSSI crosses -90dBm). On these devices, the threshold configured
+        by the user as a difference between measurements is converted to fixed
+        signal levels automatically, depending on the expected range for each of
+        the configured values. E.g. if the user configures 10dBm as
+        <literal>"rssi-threshold"</literal>, the fixed signal levels could be
+        automatically set to -100dBm, -90dBm, -80dBm, -70dBm and -60dBm.
+
+        <variablelist>
+          <varlistentry><term><literal>"rssi-threshold"</literal></term>
+            <listitem>
+              The difference of signal RSSI measurements, in dBm, that should
+              trigger a signal quality report update, given as an unsigned
+              integer (signature <literal>"u"</literal>). Use 0 to disable this
+              threshold.
+            </listitem>
+          </varlistentry>
+          <varlistentry><term><literal>"error-rate-threshold"</literal></term>
+            <listitem>
+              A boolean value, indicating whether signal quality report updates
+              should be triggered when error rate measurements change (signature
+              <literal>"b"</literal>).
+            </listitem>
+          </varlistentry>
+        </variablelist>
+
+        If any of the settings is not given as input, the corresponding threshold
+        will be considered disabled, and device defaults will apply.
+
+        Since: 1.20
+    -->
+    <method name="SetupThresholds">
+      <arg name="settings" type="a{sv}" direction="in" />
+    </method>
+
     <!--
         Rate:
 
-        Refresh rate for the extended signal quality information updates,
-        in seconds. A value of 0 disables the retrieval of the values.
+        Refresh rate, in seconds, for the extended signal quality information
+        periodic polling, as configured via the Setup() method.
+
+        A value of 0 indicates the periodic polling is disabled.
 
         Since: 1.2
     -->
     <property name="Rate" type="u" access="read" />
 
+    <!--
+        RssiThreshold:
+
+        The difference of signal RSSI measurements, in dBm, that should trigger
+        a signal quality report update.
+
+        A value of 0 indicates the threshold is disabled.
+
+        Since: 1.20
+    -->
+    <property name="RssiThreshold" type="u" access="read" />
+
+    <!--
+        ErrorRateThreshold:
+
+        Flag indicating whether signal quality report updates should be triggered
+        on error rate measurement changes.
+
+        Since: 1.20
+    -->
+    <property name="ErrorRateThreshold" type="b" access="read" />
+
     <!--
         Cdma: