tcp_bbr: clarify that bbr_bdp() rounds up in comments

This explicitly clarifies that bbr_bdp() returns the rounded-up value of
the bandwidth-delay product and why in the comments.
Signed-off-by: NLuke Hsiao <lukehsiao@google.com>
Acked-by: NSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Acked-by: NPriyaranjan Jha <priyarjha@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_bbr.c

@@ -346,7 +346,7 @@ static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 
 /* Calculate bdp based on min RTT and the estimated bottleneck bandwidth:
  *
- * bdp = bw * min_rtt * gain
+ * bdp = ceil(bw * min_rtt * gain)
  *
  * The key factor, gain, controls the amount of queue. While a small gain
  * builds a smaller queue, it becomes more vulnerable to noise in RTT
@@ -370,7 +370,9 @@ static u32 bbr_bdp(struct sock *sk, u32 bw, int gain)
 
 	w = (u64)bw * bbr->min_rtt_us;
 
-	/* Apply a gain to the given value, then remove the BW_SCALE shift. */
+	/* Apply a gain to the given value, remove the BW_SCALE shift, and
+	 * round the value up to avoid a negative feedback loop.
+	 */
 	bdp = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;
 
 	return bdp;