Blackberry: Implement buffering

The API provided by QSensor does not fit nicely for what the OS
service API provides. Instead of adding yet another buffer-related
property to QSensor, I added documentation to QSensor describing
the differences in behaviour.

Change-Id: I2d81d9c62ce7e8b50c27631a28aa7fbcaca70373
Reviewed-by: Adam Parco <aparco@rim.com>
Reviewed-by: Lorn Potter <lorn.potter@nokia.com>

2 files changed, 34 insertions, 0 deletions

diff --git a/src/plugins/sensors/blackberry/bbsensorbackend.cpp b/src/plugins/sensors/blackberry/bbsensorbackend.cpp
index 7a8a2933..c13bc585 100644
--- a/src/plugins/sensors/blackberry/bbsensorbackend.cpp
+++ b/src/plugins/sensors/blackberry/bbsensorbackend.cpp
@@ -45,6 +45,7 @@
 #include <fcntl.h>
 
 static const int microSecondsPerSecond = 1000 * 1000;
+static const int defaultBufferSize = 10;
 
 static int microSecondsToHertz(uint microSeconds)
 {
@@ -62,6 +63,10 @@ BbSensorBackendBase::BbSensorBackendBase(const QString &devicePath, sensor_type_
 {
     QCoreApplication::instance()->installEventFilter(this);
     connect(sensor, SIGNAL(alwaysOnChanged()), this, SLOT(applyAlwaysOnProperty()));
+
+    // Set some sensible default values
+    sensor->setProperty("efficientBufferSize", defaultBufferSize);
+    sensor->setProperty("maxBufferSize", defaultBufferSize);
 }
 
 QFile &BbSensorBackendBase::deviceFile()
@@ -169,6 +174,24 @@ void BbSensorBackendBase::start()
         return;
     }
 
+    // Activate event queuing if needed
+    bool ok = false;
+    const int requestedBufferSize = sensor()->property("bufferSize").toInt(&ok);
+    if (ok && requestedBufferSize > 1) {
+        sensor_devctl_queue_u queueControl;
+        queueControl.tx.enable = 1;
+        const int result = devctl(m_deviceFile.handle(), DCMD_SENSOR_QUEUE, &queueControl, sizeof(queueControl), NULL);
+        if (result != EOK) {
+            perror(QString::fromLatin1("Enabling sensor queuing for %1 failed")
+                   .arg(m_deviceFile.fileName()).toLocal8Bit());
+        }
+
+        const int actualBufferSize = queueControl.rx.size;
+        sensor()->setProperty("bufferSize", actualBufferSize);
+        sensor()->setProperty("efficientBufferSize", actualBufferSize);
+        sensor()->setProperty("maxBufferSize", actualBufferSize);
+    }
+
     applyAlwaysOnProperty();
 
     m_socketNotifier.reset(new QSocketNotifier(m_deviceFile.handle(), QSocketNotifier::Read));
diff --git a/src/sensors/qsensor.cpp b/src/sensors/qsensor.cpp
index 2949e771..be0c7862 100644
--- a/src/sensors/qsensor.cpp
+++ b/src/sensors/qsensor.cpp
@@ -776,6 +776,17 @@ int QSensor::error() const
     When the sensor is started with buffering option, values are collected from that
     moment onwards. There is no pre-existing buffer that can be utilized.
 
+    Some backends like Blackberry only support enabling or disabling the buffer and do not give
+    control over the size. In this case, the maxBufferSize and efficientBufferSize properties
+    might not be set at all, even though buffering is supported. Setting the bufferSize property
+    to any value greater than 1 will enable buffering. After the sensor has been started,
+    the bufferSize property will be set to the actual value by the backend.
+
+    On Blackberry, buffering will not wait until the buffer is full to deliver new
+    readings. Instead, the buffer will be used if the backend does not manage to retrieve the readings
+    in time, for example when the event loop is blocked for too long. Without a buffer, these readings
+    would simply be dropped.
+
     The buffer size can only be changed while the sensor is not active.
 
     \sa QSensor::maxBufferSize, QSensor::efficientBufferSize