2 files changed, 21 insertions, 15 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index b4cc62e2e2bb..eb1bda08eeb2 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -93,19 +93,16 @@ static inline u64 rfc3390_initial_rate(struct sock *sk)
         return scaled_div(w_init << 6, hctx->rtt);
 }
 
-/*
- * Recalculate t_ipi and delta (should be called whenever X changes)
+/**
+ * ccid3_update_send_interval  -  Calculate new t_ipi = s / X_inst
+ * This respects the granularity of X_inst (64 * bytes/second).
  */
 static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hctx)
 {
-        /* Calculate new t_ipi = s / X_inst (X_inst is in 64 * bytes/second) */
         hctx->t_ipi = scaled_div32(((u64)hctx->s) << 6, hctx->x);
 
-        /* Calculate new delta by delta = min(t_ipi / 2, t_gran / 2) */
-        hctx->delta = min_t(u32, hctx->t_ipi / 2, TFRC_OPSYS_HALF_TIME_GRAN);
-
-        ccid3_pr_debug("t_ipi=%u, delta=%u, s=%u, X=%u\n", hctx->t_ipi,
-                       hctx->delta, hctx->s, (unsigned)(hctx->x >> 6));
+        ccid3_pr_debug("t_ipi=%u, s=%u, X=%u\n", hctx->t_ipi,
+                       hctx->s, (unsigned)(hctx->x >> 6));
 }
 
 static u32 ccid3_hc_tx_idle_rtt(struct ccid3_hc_tx_sock *hctx, ktime_t now)
@@ -340,8 +337,8 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
                  * else
                  *       // send the packet in (t_nom - t_now) milliseconds.
                  */
-                if (delay - (s64)hctx->delta >= 1000)
-                        return (u32)delay / 1000L;
+                if (delay >= TFRC_T_DELTA)
+                        return (u32)delay / USEC_PER_MSEC;
 
                 ccid3_hc_tx_update_win_count(hctx, now);
                 break;
diff --git a/net/dccp/ccids/ccid3.h b/net/dccp/ccids/ccid3.h
index 0cfcfff8f5fb..92a5e1688a07 100644
--- a/net/dccp/ccids/ccid3.h
+++ b/net/dccp/ccids/ccid3.h
@@ -47,12 +47,23 @@
 /* Two seconds as per RFC 3448 4.2 */
 #define TFRC_INITIAL_TIMEOUT       (2 * USEC_PER_SEC)
 
-/* In usecs - half the scheduling granularity as per RFC3448 4.6 */
-#define TFRC_OPSYS_HALF_TIME_GRAN  (USEC_PER_SEC / (2 * HZ))
-
 /* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */
 #define TFRC_T_MBI                 64
 
+/*
+ * The t_delta parameter (RFC 3448, 4.6): delays of less than %USEC_PER_MSEC are
+ * rounded down to 0, since sk_reset_timer() here uses millisecond granularity.
+ * Hence we can use a constant t_delta = %USEC_PER_MSEC when HZ >= 500. A coarse
+ * resolution of HZ < 500 means that the error is below one timer tick (t_gran)
+ * when using the constant t_delta  =  t_gran / 2  =  %USEC_PER_SEC / (2 * HZ).
+ */
+#if (HZ >= 500)
+# define TFRC_T_DELTA              USEC_PER_MSEC
+#else
+# define TFRC_T_DELTA              (USEC_PER_SEC / (2 * HZ))
+#warning Coarse CONFIG_HZ resolution -- higher value recommended for TFRC.
+#endif
+
 enum ccid3_options {
         TFRC_OPT_LOSS_EVENT_RATE = 192,
         TFRC_OPT_LOSS_INTERVALS  = 193,
@@ -92,7 +103,6 @@ enum ccid3_hc_tx_states {
  * @no_feedback_timer - Handle to no feedback timer
  * @t_ld - Time last doubled during slow start
  * @t_nom - Nominal send time of next packet
- * @delta - Send timer delta (RFC 3448, 4.6) in usecs
  * @hist - Packet history
  * @options_received - Parsed set of retrieved options
  */
@@ -111,7 +121,6 @@ struct ccid3_hc_tx_sock {
         struct timer_list               no_feedback_timer;
         ktime_t                         t_ld;
         ktime_t                         t_nom;
-        u32                             delta;
         struct tfrc_tx_hist_entry       *hist;
         struct ccid3_options_received   options_received;
 };