eal: add option usage string

Add a usage string field in rte_option, allowing to display
help to the user and describe which options are currently available.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>

lib/librte_eal/common/eal_common_options.c

@@ -1515,4 +1515,5 @@ eal_common_usage(void)
 	       "  --"OPT_NO_HPET"           Disable HPET\n"
 	       "  --"OPT_NO_SHCONF"         No shared config (mmap'd files)\n"
 	       "\n", RTE_MAX_LCORE);
+	rte_option_usage();
 }

lib/librte_eal/common/eal_private.h

@@ -374,4 +374,11 @@ rte_option_parse(const char *opt);
 void
 rte_option_init(void);
 
+/**
+ * Iterate through the registered options and show the associated
+ * usage string.
+ */
+void
+rte_option_usage(void);
+
 #endif /* _EAL_PRIVATE_H_ */

lib/librte_eal/common/include/rte_option.h

@@ -35,6 +35,7 @@ typedef int (*rte_option_cb)(void);
 struct rte_option {
 	TAILQ_ENTRY(rte_option) next; /**< Next entry in the list. */
 	char *opt_str;             /**< The option name. */
+	const char *usage; /**< Option summary string. */
 	rte_option_cb cb;          /**< Function called when option is used. */
 	int enabled;               /**< Set when the option is used. */
 };

lib/librte_eal/common/rte_option.c

@@ -61,3 +61,20 @@ rte_option_init(void)
 			option->cb();
 	}
 }
+
+void
+rte_option_usage(void)
+{
+	struct rte_option *option;
+	int opt_count = 0;
+
+	TAILQ_FOREACH(option, &rte_option_list, next)
+		opt_count += 1;
+	if (opt_count == 0)
+		return;
+
+	printf("EAL dynamic options:\n");
+	TAILQ_FOREACH(option, &rte_option_list, next)
+		printf("  --%-*s %s\n", 17, option->opt_str, option->usage);
+	printf("\n");
+}

lib/librte_telemetry/rte_telemetry.c

@@ -1821,6 +1821,7 @@ int telemetry_log_level;
 
 static struct rte_option option = {
 	.opt_str = "telemetry",
+	.usage = "Enable telemetry backend",
 	.cb = &rte_telemetry_init,
 	.enabled = 0
 };