perf tools: Add perf_config_scan()

To simplify getting a single config value, add a function to scan a config
variable.
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220905073424.3971-2-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/config.c

@@ -908,3 +908,34 @@ void set_buildid_dir(const char *dir)
 	/* for communicating with external commands */
 	setenv("PERF_BUILDID_DIR", buildid_dir, 1);
 }
+
+struct perf_config_scan_data {
+	const char *name;
+	const char *fmt;
+	va_list args;
+	int ret;
+};
+
+static int perf_config_scan_cb(const char *var, const char *value, void *data)
+{
+	struct perf_config_scan_data *d = data;
+
+	if (!strcmp(var, d->name))
+		d->ret = vsscanf(value, d->fmt, d->args);
+
+	return 0;
+}
+
+int perf_config_scan(const char *name, const char *fmt, ...)
+{
+	struct perf_config_scan_data d = {
+		.name = name,
+		.fmt = fmt,
+	};
+
+	va_start(d.args, fmt);
+	perf_config(perf_config_scan_cb, &d);
+	va_end(d.args);
+
+	return d.ret;
+}

tools/perf/util/config.h

@@ -29,6 +29,7 @@ typedef int (*config_fn_t)(const char *, const char *, void *);
 
 int perf_default_config(const char *, const char *, void *);
 int perf_config(config_fn_t fn, void *);
+int perf_config_scan(const char *name, const char *fmt, ...) __scanf(2, 3);
 int perf_config_set(struct perf_config_set *set,
 		    config_fn_t fn, void *data);
 int perf_config_int(int *dest, const char *, const char *);