rc80211-pid: add sharpening factor

This patch introduces a PID sharpening factor for faster response after
association and low activity events.
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mac80211/rc80211_pid.c

@@ -23,13 +23,16 @@
  *
  * The controller basically computes the following:
  *
- * adj = CP * err + CI * err_avg + CD * (err - last_err)
+ * adj = CP * err + CI * err_avg + CD * (err - last_err) * (1 + sharpening)
  *
  * where
  * 	adj	adjustment value that is used to switch TX rate (see below)
  * 	err	current error: target vs. current failed frames percentage
  * 	last_err	last error
  * 	err_avg	average (i.e. poor man's integral) of recent errors
+ *	sharpening	non-zero when fast response is needed (i.e. right after
+ *			association or no frames sent for a long time), heading
+ * 			to zero over time
  * 	CP	Proportional coefficient
  * 	CI	Integral coefficient
  * 	CD	Derivative coefficient
@@ -65,6 +68,10 @@
 #define RC_PID_SMOOTHING_SHIFT 3
 #define RC_PID_SMOOTHING (1 << RC_PID_SMOOTHING_SHIFT)
 
+/* Sharpening factor (used for D part of PID controller) */
+#define RC_PID_SHARPENING_FACTOR 0
+#define RC_PID_SHARPENING_DURATION 0
+
 /* Fixed point arithmetic shifting amount. */
 #define RC_PID_ARITH_SHIFT 8
 
@@ -131,8 +138,11 @@ struct rc_pid_sta_info {
 	 */
 	s32 err_avg_sc;
 
-	/* Last framed failes percentage sample */
+	/* Last framed failes percentage sample. */
 	u32 last_pf;
+
+	/* Sharpening needed. */
+	u8 sharp_cnt;
 };
 
 /* Algorithm parameters. We keep them on a per-algorithm approach, so they can
@@ -267,20 +277,26 @@ static void rate_control_pid_sample(struct rc_pid_info *pinfo,
 
 	mode = local->oper_hw_mode;
 	spinfo = sta->rate_ctrl_priv;
+
+	/* In case nothing happened during the previous control interval, turn
+	 * the sharpening factor on. */
+	if (jiffies - spinfo->last_sample > 2 * RC_PID_INTERVAL)
+		spinfo->sharp_cnt = RC_PID_SHARPENING_DURATION;
+
 	spinfo->last_sample = jiffies;
 
-	/* If no frames were transmitted, we assume the old sample is
+	/* This should never happen, but in case, we assume the old sample is
 	 * still a good measurement and copy it. */
-	if (spinfo->tx_num_xmit == 0)
+	if (unlikely(spinfo->tx_num_xmit == 0))
 		pf = spinfo->last_pf;
 	else {
 		pf = spinfo->tx_num_failed * 100 / spinfo->tx_num_xmit;
 		pf <<= RC_PID_ARITH_SHIFT;
-
-		spinfo->tx_num_xmit = 0;
-		spinfo->tx_num_failed = 0;
 	}
 
+	spinfo->tx_num_xmit = 0;
+	spinfo->tx_num_failed = 0;
+
 	/* If we just switched rate, update the rate behaviour info. */
 	if (pinfo->oldrate != sta->txrate) {
 
@@ -302,8 +318,11 @@ static void rate_control_pid_sample(struct rc_pid_info *pinfo,
 	spinfo->err_avg_sc = spinfo->err_avg_sc - err_avg + err_prop;
 	err_int = spinfo->err_avg_sc >> RC_PID_SMOOTHING_SHIFT;
 
-	err_der = pf - spinfo->last_pf;
+	err_der = pf - spinfo->last_pf
+		  * (1 + RC_PID_SHARPENING_FACTOR * spinfo->sharp_cnt);
 	spinfo->last_pf = pf;
+	if (spinfo->sharp_cnt)
+			spinfo->sharp_cnt--;
 
 	/* Compute the controller output. */
 	adj = (err_prop * pinfo->coeff_p + err_int * pinfo->coeff_i