From a5911e69c7635c5575814b5172e78a752ea0c6dd Mon Sep 17 00:00:00 2001
From: Richard Cochran <richardcochran@gmail.com>
Date: Sat, 1 Nov 2014 12:27:14 +0100
Subject: [PATCH] Use SO_SELECT_ERR_QUEUE when available.

The current implementation fetches a transmit time stamp by polling on the
socket with pollfd.events set to zero, and then checking if POLLERR has
been returned by the kernel in pollfd.revents. This has the unfortunate
side effect of sleeping in poll() for the entire time out duration,
regardless of when the error queue becomes readable.

Linux kernel version 3.10 introduced a new socket option that allows
polling for transmit time stamps explicitly, waking the process as soon as
a time stamp becomes available. This patch enables the socket option,
falling back to the old behavior if necessary.

Suggested-by: Joe Schaack <jschaack@xes-inc.com>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
---
 missing.h |  4 ++++
 sk.c      | 16 ++++++++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/missing.h b/missing.h
index 43ac6cf..f7efd92 100644
--- a/missing.h
+++ b/missing.h
@@ -58,6 +58,10 @@ enum _missing_hwtstamp_tx_types {
 #define SIOCGHWTSTAMP 0x89b1
 #endif
 
+#ifndef SO_SELECT_ERR_QUEUE
+#define SO_SELECT_ERR_QUEUE 45
+#endif
+
 #ifndef HAVE_CLOCK_ADJTIME
 static inline int clock_adjtime(clockid_t id, struct timex *tx)
 {
diff --git a/sk.c b/sk.c
index c48cf45..847855a 100644
--- a/sk.c
+++ b/sk.c
@@ -32,6 +32,7 @@
 
 #include "address.h"
 #include "ether.h"
+#include "missing.h"
 #include "print.h"
 #include "sk.h"
 
@@ -208,6 +209,9 @@ int sk_interface_addr(const char *name, int family, struct address *addr)
 	return result;
 }
 
+static short sk_events = POLLPRI;
+static short sk_revents = POLLPRI;
+
 int sk_receive(int fd, void *buf, int buflen,
 	       struct address *addr, struct hw_timestamp *hwts, int flags)
 {
@@ -230,7 +234,7 @@ int sk_receive(int fd, void *buf, int buflen,
 	msg.msg_controllen = sizeof(control);
 
 	if (flags == MSG_ERRQUEUE) {
-		struct pollfd pfd = { fd, 0, 0 };
+		struct pollfd pfd = { fd, sk_events, 0 };
 		res = poll(&pfd, 1, sk_tx_timeout);
 		if (res < 1) {
 			pr_err(res ? "poll for tx timestamp failed: %m" :
@@ -238,7 +242,7 @@ int sk_receive(int fd, void *buf, int buflen,
 			pr_err("increasing tx_timestamp_timeout may correct "
 			       "this issue, but it is likely caused by a driver bug");
 			return res;
-		} else if (!(pfd.revents & POLLERR)) {
+		} else if (!(pfd.revents & sk_revents)) {
 			pr_err("poll for tx timestamp woke up on non ERR event");
 			return -1;
 		}
@@ -352,6 +356,14 @@ int sk_timestamping_init(int fd, const char *device, enum timestamp_type type,
 		return -1;
 	}
 
+	flags = 1;
+	if (setsockopt(fd, SOL_SOCKET, SO_SELECT_ERR_QUEUE,
+		       &flags, sizeof(flags)) < 0) {
+		pr_warning("%s: SO_SELECT_ERR_QUEUE: %m", device);
+		sk_events = 0;
+		sk_revents = POLLERR;
+	}
+
 	/* Enable the sk_check_fupsync option, perhaps. */
 	if (sk_general_init(fd)) {
 		return -1;