3 files changed, 18 insertions, 3 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 0d406f82e433..f566eb76aeb2 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -869,6 +869,19 @@ MODULE_PARM_DESC(ccid3_debug, "Enable debug messages");
 
 static __init int ccid3_module_init(void)
 {
+        struct timespec tp;
+
+        /*
+         * Without a fine-grained clock resolution, RTTs/X_recv are not sampled
+         * correctly and feedback is sent either too early or too late.
+         */
+        hrtimer_get_res(CLOCK_MONOTONIC, &tp);
+        if (tp.tv_sec || tp.tv_nsec > DCCP_TIME_RESOLUTION * NSEC_PER_USEC) {
+                printk(KERN_ERR "%s: Timer too coarse (%ld usec), need %u-usec"
+                       " resolution - check your clocksource.\n", __func__,
+                       tp.tv_nsec/NSEC_PER_USEC, DCCP_TIME_RESOLUTION);
+                return -ESOCKTNOSUPPORT;
+        }
         return ccid_register(&ccid3);
 }
 module_init(ccid3_module_init);
diff --git a/net/dccp/dccp.h b/net/dccp/dccp.h
index 1585fa26488c..b63a82ccb2b2 100644
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -86,10 +86,13 @@ extern void dccp_time_wait(struct sock *sk, int state, int timeo);
  */
 #define DCCP_RTO_MAX ((unsigned)(64 * HZ))
 
+/* DCCP base time resolution - 10 microseconds (RFC 4340, 13.1 ... 13.3) */
+#define DCCP_TIME_RESOLUTION    10
+
 /*
  * RTT sampling: sanity bounds and fallback RTT value from RFC 4340, section 3.4
  */
-#define DCCP_SANE_RTT_MIN       100
+#define DCCP_SANE_RTT_MIN       (10 * DCCP_TIME_RESOLUTION)
 #define DCCP_FALLBACK_RTT       (USEC_PER_SEC / 5)
 #define DCCP_SANE_RTT_MAX       (3 * USEC_PER_SEC)
 
diff --git a/net/dccp/timer.c b/net/dccp/timer.c
index e02d5a94f4c0..16359e29e7f5 100644
--- a/net/dccp/timer.c
+++ b/net/dccp/timer.c
@@ -281,8 +281,7 @@ u32 dccp_timestamp(void)
 {
         s64 delta = ktime_us_delta(ktime_get_real(), dccp_timestamp_seed);
 
-        do_div(delta, 10);
-        return delta;
+        return div_u64(delta, DCCP_TIME_RESOLUTION);
 }
 EXPORT_SYMBOL_GPL(dccp_timestamp);