Add Read Cache and Stats

- baro.cache_ttl_ms avoids multiple reads
- by default, cache is off (cache_ttl_ms=0)
- stats are updated (similar to how prometheus-sensors does this)

include/mgos_barometer.h

@@ -30,11 +30,23 @@ enum mgos_barometer_type {
 
 struct mgos_barometer;
 
+struct mgos_barometer_stats {
+  double   last_read_time;       // value of mg_time() upon last call to _read()
+  uint32_t read;                 // calls to _read()
+  uint32_t read_success;         // successful _read()
+  uint32_t read_success_cached;  // calls to _read() which were cached
+  // Note: read_errors := read - read_success - read_success_cached
+  double   read_success_usecs;   // time spent in successful uncached _read()
+};
+
 struct mgos_barometer *mgos_barometer_create_i2c(struct mgos_i2c *i2c, uint8_t i2caddr, enum mgos_barometer_type type);
 void mgos_barometer_destroy(struct mgos_barometer **sensor);
 bool mgos_barometer_has_thermometer(struct mgos_barometer *sensor);
 bool mgos_barometer_has_barometer(struct mgos_barometer *sensor);
 
+/* Set cache TTL -- will limit reads and return cached data. Set msecs=0 to turn off */
+bool mgos_barometer_set_cache_ttl(struct mgos_barometer *sensor, uint16_t msecs);
+
 /* Read all available sensor data from the IMU */
 bool mgos_barometer_read(struct mgos_barometer *sensor);
 
@@ -44,6 +56,12 @@ bool mgos_barometer_get_pressure(struct mgos_barometer *sensor, float *p);
 /* Return temperature data in units of Celcius */
 bool mgos_barometer_get_temperature(struct mgos_barometer *sensor, float *t);
 
+/*
+ * Return statistics on the sensor.
+ */
+bool mgos_barometer_get_stats(struct mgos_barometer *sensor, struct mgos_barometer_stats *stats);
+
+
 /*
  * Initialization function for MGOS -- currently a noop.
  */

src/main.c

@@ -217,6 +217,8 @@ int main() {
   */
   if (!(baro=mgos_barometer_create_i2c(i2c, 0x60, BARO_MPL115))) {
     LOG(LL_ERROR, ("Cannot create MPL115 barometer"));
+  } else {
+    mgos_barometer_set_cache_ttl(baro, 5000);
   }
 
   for (;;) {

src/mgos_barometer.c

@@ -87,9 +87,25 @@ bool mgos_barometer_has_barometer(struct mgos_barometer *sensor) {
 }
 
 bool mgos_barometer_read(struct mgos_barometer *sensor) {
+  double start = mg_time();
+  bool ret = false;
+
   if (!sensor) return false;
   if (!sensor->read) return false;
-  return sensor->read(sensor);
+
+  sensor->stats.read++;
+  if (1000*(start - sensor->stats.last_read_time) < sensor->cache_ttl_ms) {
+    sensor->stats.read_success_cached++;
+    return true;
+  }
+
+  ret = sensor->read(sensor);
+  if (ret) {
+    sensor->stats.read_success++;
+    sensor->stats.read_success_usecs += 1000000 * (mg_time() - start);
+    sensor->stats.last_read_time      = start;
+  }
+  return ret;
 }
 
 bool mgos_barometer_get_pressure(struct mgos_barometer *sensor, float *p) {
@@ -106,6 +122,22 @@ bool mgos_barometer_get_temperature(struct mgos_barometer *sensor, float *t) {
   return true;
 }
 
+bool mgos_barometer_set_cache_ttl(struct mgos_barometer *sensor, uint16_t msecs) {
+  if (!sensor) return false;
+  sensor->cache_ttl_ms = msecs;
+  return true;
+}
+
+bool mgos_barometer_get_stats(struct mgos_barometer *sensor, struct mgos_barometer_stats *stats) {
+  if (!sensor || !stats) {
+    return false;
+  }
+
+  memcpy((void *)stats, (const void *)&sensor->stats, sizeof(struct mgos_barometer_stats));
+  return true;
+}
+
+
 bool mgos_barometer_init(void) {
   return true;
 }

src/mgos_barometer_internal.h

@@ -32,6 +32,7 @@ typedef bool (*mgos_barometer_mag_read_fn)(struct mgos_barometer *dev);
 struct mgos_barometer {
   struct mgos_i2c *      i2c;
   uint8_t                i2caddr;
+  uint16_t          cache_ttl_ms;
   enum mgos_barometer_type type;
   bool has_thermometer;
   bool has_barometer;
@@ -45,6 +46,8 @@ struct mgos_barometer {
 
   float pressure; // in Pascals
   float temperature; // in Celcius
+
+  struct mgos_barometer_stats stats;
 };
 
 #ifdef __cplusplus

src/mgos_barometer_mpl115.c

@@ -12,6 +12,7 @@ bool mgos_barometer_mpl115_create(struct mgos_barometer *dev) {
   mpl115_data=calloc(1, sizeof(struct mgos_barometer_mpl115_data));
   if (!mpl115_data) return false;
 
+  // TODO(pim): fix this -- math is wrong.
   mpl115_data->a0=(float)(((uint16_t) data[0] << 8) | data[1]) / 8.0;
   mpl115_data->b1=(float)(((uint16_t) data[2] << 8) | data[3]) / 8192.0;
   mpl115_data->b2=(float)(((uint16_t) data[4] << 8) | data[5]) / 16384.0;