git: 4f2cc73f34eb - main - tcp: Refactor tcp_get_srtt()

The branch main has been updated by jtl:

URL: https://cgit.FreeBSD.org/src/commit/?id=4f2cc73f34eb5f4bd6e66afda1c367676547f516

commit 4f2cc73f34eb5f4bd6e66afda1c367676547f516
Author:     Jonathan T. Looney <jtl@FreeBSD.org>
AuthorDate: 2023-05-31 19:16:20 +0000
Commit:     Jonathan T. Looney <jtl@FreeBSD.org>
CommitDate: 2023-05-31 19:16:20 +0000

    tcp: Refactor tcp_get_srtt()
    
    Refactor tcp_get_srtt() into its two component operations: unit
    conversion and shifting. No functional change is intended.
    
    Reviewed by:    cc, tuexen
    Sponsored by:   Netflix
    Differential Revision:  https://reviews.freebsd.org/D40304
---
 sys/netinet/tcp_subr.c | 49 +++++++++++++++++++++++++++----------------------
 1 file changed, 27 insertions(+), 22 deletions(-)

diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index 52222d7c1634..fe065fd08737 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -4647,28 +4647,33 @@ tcp_get_srtt(struct tcpcb *tp, int granularity)
 {
 	uint32_t srtt;
 
-	if (tp->t_tmr_granularity == TCP_TMR_GRANULARITY_USEC)
-		srtt = tp->t_srtt;
-	else if (tp->t_tmr_granularity == TCP_TMR_GRANULARITY_TICKS) {
-		/* TICKS are stored shifted; unshift for the real TICKS */
-		srtt = tp->t_srtt >> TCP_RTT_SHIFT;
-	}
-	if (tp->t_tmr_granularity == granularity)
-		return (srtt);
-	/* If we reach here they are oppsite what the caller wants */
-	if (granularity == TCP_TMR_GRANULARITY_USEC) {
-		/*
-		 * The user wants useconds and internally
-		 * its kept in ticks, convert to useconds.
-		 * Put unshift at last improves precision.
-		 */
-		srtt = TICKS_2_USEC(tp->t_srtt) >> TCP_RTT_SHIFT;
-	} else if (granularity == TCP_TMR_GRANULARITY_TICKS) {
-		/*
-		 * The user wants ticks and internally its
-		 * kept in useconds, convert to ticks.
-		 */
-		srtt = USEC_2_TICKS(srtt);
+	KASSERT(granularity == TCP_TMR_GRANULARITY_USEC ||
+	    granularity == TCP_TMR_GRANULARITY_TICKS,
+	    ("%s: called with unexpected granularity %d", __func__,
+	    granularity));
+
+	srtt = tp->t_srtt;
+
+	/*
+	 * We only support two granularities. If the stored granularity
+	 * does not match the granularity requested by the caller,
+	 * convert the stored value to the requested unit of granularity.
+	 */
+	if (tp->t_tmr_granularity != granularity) {
+		if (granularity == TCP_TMR_GRANULARITY_USEC)
+			srtt = TICKS_2_USEC(srtt);
+		else
+			srtt = USEC_2_TICKS(srtt);
 	}
+
+	/*
+	 * If the srtt is stored with ticks granularity, we need to
+	 * unshift to get the actual value. We do this after the
+	 * conversion above (if one was necessary) in order to maximize
+	 * precision.
+	 */
+	if (tp->t_tmr_granularity == TCP_TMR_GRANULARITY_TICKS)
+		srtt = srtt >> TCP_RTT_SHIFT;
+
 	return (srtt);
 }