dccp tfrc/ccid-3: computing the loss rate from the Loss Event Rate

This adds a function to take care of the following, separate cases occurring in
the computation of the Loss Rate p:

* 1/(2^32-1) is mapped into 0% as per RFC 4342, 8.5;
* 1/0 is mapped into 100%, the maximum;
* to avoid that p = 1/x is rounded down to 0 when x is very large, since this
means accidentally re-entering slow-start indicated by p == 0, the minimum
resolution value of p is now returned instead;
* a bug in ccid3_hc_rx_getsockopt is fixed: 1/0 was mapped into ~0U.

Signed-off-by: Gerrit Renker <[e-mail]>

net/dccp/ccids/ccid3.c

···
 
 	/* Update loss event rate (which is scaled by 1e6) */
 	pinv = opt_recv->ccid3or_loss_event_rate;
-	if (pinv == ~0U || pinv == 0)	       /* see RFC 4342, 8.5   */
+	if (pinv == 0)
 		hc->tx_p = 0;
-	else				       /* can not exceed 100% */
-		hc->tx_p = scaled_div(1, pinv);
+	else
+		hc->tx_p = tfrc_invert_loss_event_rate(pinv);
 
 	/*
 	 * Update allowed sending rate X as per draft rfc3448bis-00, 4.2/3
···
 			return -EINVAL;
 		rx_info.tfrcrx_x_recv = hc->rx_x_recv;
 		rx_info.tfrcrx_rtt    = hc->rx_rtt;
-		rx_info.tfrcrx_p      = hc->rx_pinv == 0 ? ~0U :
-					   scaled_div(1, hc->rx_pinv);
+		rx_info.tfrcrx_p      = tfrc_invert_loss_event_rate(hc->rx_pinv);
 		len = sizeof(rx_info);
 		val = &rx_info;
 		break;

net/dccp/ccids/lib/tfrc.h

···
 
 extern u32  tfrc_calc_x(u16 s, u32 R, u32 p);
 extern u32  tfrc_calc_x_reverse_lookup(u32 fvalue);
+extern u32  tfrc_invert_loss_event_rate(u32 loss_event_rate);
 
 extern int  tfrc_tx_packet_history_init(void);
 extern void tfrc_tx_packet_history_exit(void);

net/dccp/ccids/lib/tfrc_equation.c

···
 	index = tfrc_binsearch(fvalue, 0);
 	return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
 }
+
+/**
+ * tfrc_invert_loss_event_rate  -  Compute p so that 10^6 corresponds to 100%
+ * When @loss_event_rate is large, there is a chance that p is truncated to 0.
+ * To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0.
+ */
+u32 tfrc_invert_loss_event_rate(u32 loss_event_rate)
+{
+	if (loss_event_rate == UINT_MAX)		/* see RFC 4342, 8.5 */
+		return 0;
+	if (unlikely(loss_event_rate == 0))		/* map 1/0 into 100% */
+		return 1000000;
+	return max_t(u32, scaled_div(1, loss_event_rate), TFRC_SMALLEST_P);
+}