rt2x00: Fix rounding errors in RSSI average calculation

Small changes in signal level was not detected up by the
MOVING_AVERAGE() due to a rounding error, using 'int' type.

rt2x00lib_antenna_diversity_eval:
rssi:	  -62 -62 -62 -62 -62 -62 -62 -62 -62 -62 -62 -62 -62 -62 -62
rssi_avg: -57 -57 -57 -57 -57 -57 -57 -57 -57 -57 -57 -57 -57 -57 -57

The signal level reported back could be significantly (5dBm)
different from the actual value. A level +3dBm is the same as
double the AP output power.

Signed-off-by: Lars Ericsson <Lars_Ericsson@telia.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

2 files changed, 52 insertions, 28 deletions

diff --git a/drivers/net/wireless/rt2x00/rt2x00.h b/drivers/net/wireless/rt2x00/rt2x00.h
index 704e94474b1d..e6e73be31e37 100644
--- a/drivers/net/wireless/rt2x00/rt2x00.h
+++ b/drivers/net/wireless/rt2x00/rt2x00.h
@@ -134,6 +134,17 @@
                                   GET_DURATION(IEEE80211_HEADER + ACK_SIZE, 10) )
 
 /*
+ * Structure for average calculation
+ * The avg field contains the actual average value,
+ * but avg_weight is internally used during calculations
+ * to prevent rounding errors.
+ */
+struct avg_val {
+        int avg;
+        int avg_weight;
+};
+
+/*
  * Chipset identification
  * The chipset on the device is composed of a RT and RF chip.
  * The chipset combination is important for determining device capabilities.
@@ -256,7 +267,7 @@ struct link_ant {
          * Similar to the avg_rssi in the link_qual structure
          * this value is updated by using the walking average.
          */
-        int rssi_ant;
+        struct avg_val rssi_ant;
 };
 
 /*
@@ -285,7 +296,7 @@ struct link {
         /*
          * Currently active average RSSI value
          */
-        int avg_rssi;
+        struct avg_val avg_rssi;
 
         /*
          * Currently precalculated percentages of successful
diff --git a/drivers/net/wireless/rt2x00/rt2x00link.c b/drivers/net/wireless/rt2x00/rt2x00link.c
index dd68f3a57b6d..c64db0ba7f40 100644
--- a/drivers/net/wireless/rt2x00/rt2x00link.c
+++ b/drivers/net/wireless/rt2x00/rt2x00link.c
@@ -46,7 +46,15 @@
 #define DEFAULT_PERCENTAGE      50
 
 /*
- * Small helper macro to work with moving/walking averages.
+ * Small helper macro for percentage calculation
+ * This is a very simple macro with the only catch that it will
+ * produce a default value in case no total value was provided.
+ */
+#define PERCENTAGE(__value, __total) \
+        ( (__total) ? (((__value) * 100) / (__total)) : (DEFAULT_PERCENTAGE) )
+
+/*
+ * Helper struct and macro to work with moving/walking averages.
  * When adding a value to the average value the following calculation
  * is needed:
  *
@@ -60,18 +68,28 @@
  * for a few minutes but when the device is moved away from the AP
  * the average will not decrease fast enough to compensate.
  * The walking average compensates this and will move towards
- * the new values correctly allowing a effective link tuning.
+ * the new values correctly allowing a effective link tuning,
+ * the speed of the average moving towards other values depends
+ * on the value for the number of samples. The higher the number
+ * of samples, the slower the average will move.
+ * We use two variables to keep track of the average value to
+ * compensate for the rounding errors. This can be a significant
+ * error (>5dBm) if the factor is too low.
  */
-#define MOVING_AVERAGE(__avg, __val, __samples) \
-        ( (((__avg) * ((__samples) - 1)) + (__val)) / (__samples) )
-
-/*
- * Small helper macro for percentage calculation
- * This is a very simple macro with the only catch that it will
- * produce a default value in case no total value was provided.
- */
-#define PERCENTAGE(__value, __total) \
-        ( (__total) ? (((__value) * 100) / (__total)) : (DEFAULT_PERCENTAGE) )
+#define AVG_SAMPLES     8
+#define AVG_FACTOR      1000
+#define MOVING_AVERAGE(__avg, __val) \
+({ \
+        struct avg_val __new; \
+        __new.avg_weight = \
+            (__avg).avg_weight  ? \
+                ((((__avg).avg_weight * ((AVG_SAMPLES) - 1)) + \
+                  ((__val) * (AVG_FACTOR))) / \
+                 (AVG_SAMPLES) ) : \
+                ((__val) * (AVG_FACTOR)); \
+        __new.avg = __new.avg_weight / (AVG_FACTOR); \
+        __new; \
+})
 
 /*
  * For calculating the Signal quality we have determined
@@ -98,8 +116,8 @@ static int rt2x00link_antenna_get_link_rssi(struct rt2x00_dev *rt2x00dev)
 {
         struct link_ant *ant = &rt2x00dev->link.ant;
 
-        if (ant->rssi_ant && rt2x00dev->link.qual.rx_success)
-                return ant->rssi_ant;
+        if (ant->rssi_ant.avg && rt2x00dev->link.qual.rx_success)
+                return ant->rssi_ant.avg;
         return DEFAULT_RSSI;
 }
 
@@ -121,7 +139,8 @@ static void rt2x00link_antenna_update_rssi_history(struct rt2x00_dev *rt2x00dev,
 
 static void rt2x00link_antenna_reset(struct rt2x00_dev *rt2x00dev)
 {
-        rt2x00dev->link.ant.rssi_ant = 0;
+        rt2x00dev->link.ant.rssi_ant.avg = 0;
+        rt2x00dev->link.ant.rssi_ant.avg_weight = 0;
 }
 
 static void rt2x00lib_antenna_diversity_sample(struct rt2x00_dev *rt2x00dev)
@@ -258,8 +277,6 @@ void rt2x00link_update_stats(struct rt2x00_dev *rt2x00dev,
         struct link_qual *qual = &rt2x00dev->link.qual;
         struct link_ant *ant = &rt2x00dev->link.ant;
         struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)skb->data;
-        int avg_rssi = rxdesc->rssi;
-        int ant_rssi = rxdesc->rssi;
 
         /*
          * Frame was received successfully since non-succesfull
@@ -279,16 +296,12 @@ void rt2x00link_update_stats(struct rt2x00_dev *rt2x00dev,
         /*
          * Update global RSSI
          */
-        if (link->avg_rssi)
-                avg_rssi = MOVING_AVERAGE(link->avg_rssi, rxdesc->rssi, 8);
-        link->avg_rssi = avg_rssi;
+        link->avg_rssi = MOVING_AVERAGE(link->avg_rssi, rxdesc->rssi);
 
         /*
          * Update antenna RSSI
          */
-        if (ant->rssi_ant)
-                ant_rssi = MOVING_AVERAGE(ant->rssi_ant, rxdesc->rssi, 8);
-        ant->rssi_ant = ant_rssi;
+        ant->rssi_ant = MOVING_AVERAGE(ant->rssi_ant, rxdesc->rssi);
 }
 
 static void rt2x00link_precalculate_signal(struct rt2x00_dev *rt2x00dev)
@@ -421,10 +434,10 @@ static void rt2x00link_tuner(struct work_struct *work)
          * collect the RSSI data we could use this. Otherwise we
          * must fallback to the default RSSI value.
          */
-        if (!link->avg_rssi || !qual->rx_success)
+        if (!link->avg_rssi.avg || !qual->rx_success)
                 qual->rssi = DEFAULT_RSSI;
         else
-                qual->rssi = link->avg_rssi;
+                qual->rssi = link->avg_rssi.avg;
 
         /*
          * Only perform the link tuning when Link tuning
@@ -442,7 +455,7 @@ static void rt2x00link_tuner(struct work_struct *work)
         /*
          * Send a signal to the led to update the led signal strength.
          */
-        rt2x00leds_led_quality(rt2x00dev, link->avg_rssi);
+        rt2x00leds_led_quality(rt2x00dev, qual->rssi);
 
         /*
          * Evaluate antenna setup, make this the last step when