ptp4l: Use poll() instead of a try-again loop

This patch modifies sk_receive in order to use poll() on POLLERR instead of the
tryagain loop as this resolves issues with drivers who do not return timestamps
quickly enough. It also resolves the issue of wasting time repeating every
microsecond. It lets the kernel sleep our application until the data or timeout arrives.

This change also replaces the old tx_timestamp_retries config value with
tx_timestamp_timeout specified in milliseconds (the smallest length of time poll
accepts). This does have the side effect of increasing the minimum delay before
missing a timestamp by up to 1ms, but the poll should return sooner in the
normal case where a packet timestamp was not dropped.

This change vastly improves some devices and cleans the code up by simplifying a
race condition window due to drivers returning tx timestamp on the error queue.

[ RC - removed the unused 'try_again' variable. ]

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

config.c

@@ -270,10 +270,10 @@ static enum parser_result parse_global_setting(const char *option,
 			return BAD_VALUE;
 		*cfg->assume_two_step = val ? 1 : 0;
 
-	} else if (!strcmp(option, "tx_timestamp_retries")) {
+	} else if (!strcmp(option, "tx_timestamp_timeout")) {
 		if (1 != sscanf(value, "%u", &val) || !(val > 0))
 			return BAD_VALUE;
-		*cfg->tx_timestamp_retries = val;
+		*cfg->tx_timestamp_timeout = val;
 
 	} else if (!strcmp(option, "pi_proportional_const")) {
 		if (1 != sscanf(value, "%lf", &df) || !(df >= 0.0 && df < 1.0))

config.h

@@ -61,7 +61,7 @@ struct config {
 	struct default_ds dds;
 	struct port_defaults pod;
 	int *assume_two_step;
-	int *tx_timestamp_retries;
+	int *tx_timestamp_timeout;
 
 	enum servo_type clock_servo;
 

default.cfg

@@ -30,7 +30,7 @@ assume_two_step		0
 logging_level		6
 path_trace_enabled	0
 follow_up_info		0
-tx_timestamp_retries	100
+tx_timestamp_timeout	1
 use_syslog		1
 verbose			0
 summary_interval	0

gPTP.cfg

@@ -29,7 +29,7 @@ assume_two_step		1
 logging_level		6
 path_trace_enabled	1
 follow_up_info		1
-tx_timestamp_retries	100
+tx_timestamp_timeout	1
 use_syslog		1
 verbose			0
 summary_interval	0

ptp4l.8

@@ -264,10 +264,10 @@ Treat one-step responses as two-step if enabled. It is used to work around
 buggy 802.1AS switches.
 The default is 0 (disabled).
 .TP
-.B tx_timestamp_retries
-The number of retries to fetch the tx time stamp from the kernel when a message
-is sent.
-The default is 100.
+.B tx_timestamp_timeout
+The number of milliseconds to poll waiting for the tx time stamp from the kernel
+when a message has recently been sent.
+The default is 1.
 .TP
 .B clock_servo
 The servo which is used to synchronize the local clock. Currently only one

ptp4l.c

@@ -89,7 +89,7 @@ static struct config cfg_settings = {
 	.transport = TRANS_UDP_IPV4,
 
 	.assume_two_step = &assume_two_step,
-	.tx_timestamp_retries = &sk_tx_retries,
+	.tx_timestamp_timeout = &sk_tx_timeout,
 
 	.clock_servo = CLOCK_SERVO_PI,
 

sk.c

@@ -28,13 +28,14 @@
 #include <unistd.h>
 #include <ifaddrs.h>
 #include <stdlib.h>
+#include <poll.h>
 
 #include "print.h"
 #include "sk.h"
 
 /* globals */
 
-int sk_tx_retries = 100;
+int sk_tx_timeout = 1;
 
 /* private methods */
 
@@ -200,7 +201,7 @@ int sk_receive(int fd, void *buf, int buflen,
 	       struct hw_timestamp *hwts, int flags)
 {
 	char control[256];
-	int cnt = 0, level, try_again, type;
+	int cnt = 0, res = 0, level, type;
 	struct cmsghdr *cm;
 	struct iovec iov = { buf, buflen };
 	struct msghdr msg;
@@ -213,28 +214,22 @@ int sk_receive(int fd, void *buf, int buflen,
 	msg.msg_control = control;
 	msg.msg_controllen = sizeof(control);
 
-	try_again = flags == MSG_ERRQUEUE ? sk_tx_retries : 1;
+	if (flags == MSG_ERRQUEUE) {
+		struct pollfd pfd = { fd, 0, 0 };
+		res = poll(&pfd, 1, sk_tx_timeout);
+		if (res < 1) {
+			pr_err("poll tx timestamp failed: %m");
+			return res;
+		} else if (!(pfd.revents & POLLERR)) {
+			pr_err("poll tx woke up on non ERR event");
+			return -1;
+		}
+	}
 
-	for ( ; try_again; try_again--) {
 	cnt = recvmsg(fd, &msg, flags);
-		if (cnt >= 0) {
-			break;
-		}
-		if (errno == EINTR) {
-			try_again++;
-		} else if (errno == EAGAIN) {
-			usleep(1);
-		} else {
-			break;
-		}
-	}
-
-	if (cnt < 1) {
-		if (flags == MSG_ERRQUEUE)
-			pr_err("recvmsg tx timestamp failed: %m");
-		else
-			pr_err("recvmsg failed: %m");
-	}
+	if (cnt < 1)
+		pr_err("recvmsg%sfailed: %m",
+		       flags == MSG_ERRQUEUE ? " tx timestamp " : " ");
 
 	for (cm = CMSG_FIRSTHDR(&msg); cm != NULL; cm = CMSG_NXTHDR(&msg, cm)) {
 		level = cm->cmsg_level;

sk.h

@@ -97,9 +97,9 @@ int sk_timestamping_init(int fd, char *device, enum timestamp_type type,
 			 enum transport_type transport);
 
 /**
- * Limits the number of RECVMSG(2) calls when attempting to obtain a
- * transmit time stamp on an event message.
+ * Limits the time that RECVMSG(2) will poll while waiting for the tx timestamp
+ * if MSG_ERRQUEUE is set. Specified in milliseconds.
  */
-extern int sk_tx_retries;
+extern int sk_tx_timeout;
 
 #endif