device: Add simple temperature model for devices

This temperature model has three states:
 * COLD
 * WARM
 * HOT

Device drivers can define the time it requires for the device to get HOT
and COLD. The underlying model assumes an exponential warming and
cooling process and enforces a cool-off time after the device has
reached the HOT state. This cool down period is however shorter than the
specified time in the driver.

libfprint/fp-device-private.h

@@ -22,6 +22,23 @@
 
 #include "fpi-device.h"
 
+/* Chosen so that if we turn on after WARM -> COLD, it takes exactly one time
+ * constant to go from COLD -> HOT.
+ *   TEMP_COLD_THRESH = 1 / (e + 1)
+ */
+#define TEMP_COLD_THRESH (0.26894142136999512075)
+#define TEMP_WARM_HOT_THRESH (1.0 - TEMP_COLD_THRESH)
+#define TEMP_HOT_WARM_THRESH (0.5)
+
+/* Delay updates by 100ms to avoid hitting the border exactly */
+#define TEMP_DELAY_SECONDS 0.1
+
+/* Hopefully 3min is long enough to not get in the way, while also not
+ * properly overheating any devices.
+ */
+#define DEFAULT_TEMP_HOT_SECONDS (3 * 60)
+#define DEFAULT_TEMP_COLD_SECONDS (9 * 60)
+
 typedef struct
 {
   FpDeviceType type;
@@ -58,6 +75,15 @@ typedef struct
   /* State for tasks */
   gboolean            wait_for_finger;
   FpFingerStatusFlags finger_status;
+
+  /* Device temperature model information and state */
+  GSource      *temp_timeout;
+  FpTemperature temp_current;
+  gint32        temp_hot_seconds;
+  gint32        temp_cold_seconds;
+  gint64        temp_last_update;
+  gboolean      temp_last_active;
+  gdouble       temp_current_ratio;
 } FpDevicePrivate;
 
 
@@ -88,3 +114,6 @@ typedef struct
 } FpMatchData;
 
 void match_data_free (FpMatchData *match_data);
+
+void fpi_device_update_temp (FpDevice *device,
+                             gboolean  is_active);