Fix signal calculation for WEXT-based drivers

This patch introduces a behavior change. Previously the
'SignalStrength' role was interpreted as dBm. This was working
fine for nl80211 based drivers, for WEXT-based drivers it
contained garbage values. Now the 'SignalStrength' role will
always be interpreted as percentage.

The wpa_supplicant passes back only the 'level' of the scanned
BSS, which with nl80211-based driver is almost always dBm (val < 0).
WEXT-based driver has its own interpretation of 'level' - it
can contain dBm, percentage [0-100] (and some other values for
historical reasons, but these are converted by the wpa_supplicant
to dBm). This is what I have concluded from looking at NetworkManager
and wpa_supplicant sources and git history. Even the official GUI tool
wpa_gui for the wpa_supplicant is out-of-date and can not be used as
a referance.

Change-Id: I70d15e26e0a1700ad0f7d19d23cbade2d3d46054
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@theqtcompany.com>

3 files changed, 22 insertions, 15 deletions

diff --git a/src/imports/wifi/pluginmain.cpp b/src/imports/wifi/pluginmain.cpp
index 7d129d2..b498b66 100644
--- a/src/imports/wifi/pluginmain.cpp
+++ b/src/imports/wifi/pluginmain.cpp
@@ -93,13 +93,13 @@ QT_BEGIN_NAMESPACE
     interface, accessed with the following roles:
 
     \list
-    \li \e ssid - informal (human) name of a Wifi network
-    \li \e bssid - basic service set identification of a network, used to uniquely identify BSS
-    \li \e signalStrength - strength of a Wifi signal, measured in dBm
-    \li \e supportsWPA - holds whether network access point supports WPA security protocol
-    \li \e supportsWPA2 - holds whether network access point supports WPA2 security protocol
-    \li \e supportsWEP - holds whether network access point supports WEP security protocol
-    \li \e supportsWPS - holds whether network access point supports WPS security protocol
+    \li \e ssid - informal (human) name of a Wifi network (string)
+    \li \e bssid - basic service set identification of a network, used to uniquely identify BSS (string)
+    \li \e signalStrength - strength of a Wifi signal represented as percentage (0-100) (int)
+    \li \e supportsWPA - holds whether network access point supports WPA security protocol (bool)
+    \li \e supportsWPA2 - holds whether network access point supports WPA2 security protocol (bool)
+    \li \e supportsWEP - holds whether network access point supports WEP security protocol (bool)
+    \li \e supportsWPS - holds whether network access point supports WPS security protocol (bool)
     \endlist
  */
 
diff --git a/src/wifi/qwifimanager.cpp b/src/wifi/qwifimanager.cpp
index 7585cf2..2a5cfff 100644
--- a/src/wifi/qwifimanager.cpp
+++ b/src/wifi/qwifimanager.cpp
@@ -212,11 +212,11 @@ void QWifiManagerPrivate::updateLastError(const QString &error)
 
     \value SSID informal (human) name of a Wifi network (QString)
     \value BSSID basic service set identification of a network, used to uniquely identify BSS (QString)
-    \value SignalStrength strength of a Wifi signal, measured in dBm (int)
-    \value WPASupported holds whether network access point supports WPA security protocol (QString)
-    \value WPA2Supported holds whether network access point supports WPA2 security protocol (QString)
-    \value WEPSupported holds whether network access point supports WEP security protocol (QString)
-    \value WPSSupported holds whether network access point supports WPS security protocol (QString)
+    \value SignalStrength strength of a Wifi signal represented as percentage (0-100) (int)
+    \value WPASupported holds whether network access point supports WPA security protocol (bool)
+    \value WPA2Supported holds whether network access point supports WPA2 security protocol (bool)
+    \value WEPSupported holds whether network access point supports WEP security protocol (bool)
+    \value WPSSupported holds whether network access point supports WPS security protocol (bool)
 */
 
 QWifiManager* QWifiManager::m_instance = 0;
diff --git a/src/wifi/qwifinetworklistmodel.cpp b/src/wifi/qwifinetworklistmodel.cpp
index 7ed4be4..9d6202f 100644
--- a/src/wifi/qwifinetworklistmodel.cpp
+++ b/src/wifi/qwifinetworklistmodel.cpp
@@ -132,9 +132,16 @@ void QWifiNetworkListModel::parseScanResults(const QString &results)
         QWifiNetwork *knownNetwork = networkForSSID(ssid, &pos);
         if (!knownNetwork)
             knownNetwork = outOfRangeListContains(ssid);
-        // signal strength is in dBm. Deprecated, but still widely used "wext"
-        // wifi driver reports positive values for signal strength, we workaround that.
-        int signalStrength = qAbs(info.at(2).trimmed().toInt()) * -1;
+
+        int signalStrength = info.at(2).trimmed().toInt();
+        if (signalStrength < 0) {
+            // signal is reported in dBm, rough conversion: best = -40, worst = -100
+            int val = qAbs(qMax(-100, qMin(signalStrength, -40)) + 40); // clamp and normalize to 0
+            signalStrength = 100 - (int) ((100.0 * (double) val) / 60.0);
+        } else if (signalStrength > 100) {
+            qCWarning(B2QT_WIFI) << "unexpected value for a signal level: " << signalStrength;
+        }
+
         if (!knownNetwork) {
             QWifiNetwork *network = new QWifiNetwork();
             network->setOutOfRange(false);