selftests: gpio: add a helper for reading GPIO line names

Add a simple program that allows to read GPIO line names from the
character device. This will be used in gpio-sim selftests.
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>

tools/testing/selftests/gpio/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 gpio-mockup-cdev
 gpio-chip-info
+gpio-line-name

tools/testing/selftests/gpio/Makefile

@@ -2,6 +2,6 @@
 
 TEST_PROGS := gpio-mockup.sh
 TEST_FILES := gpio-mockup-sysfs.sh
-TEST_GEN_PROGS_EXTENDED := gpio-mockup-cdev gpio-chip-info
+TEST_GEN_PROGS_EXTENDED := gpio-mockup-cdev gpio-chip-info gpio-line-name
 
 include ../lib.mk

tools/testing/selftests/gpio/gpio-line-name.c

+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * GPIO character device helper for reading line names.
+ *
+ * Copyright (C) 2021 Bartosz Golaszewski <brgl@bgdev.pl>
+ */
+
+#include <fcntl.h>
+#include <linux/gpio.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/types.h>
+
+static void print_usage(void)
+{
+	printf("usage:\n");
+	printf("  gpio-line-name <chip path> <line offset>\n");
+}
+
+int main(int argc, char **argv)
+{
+	struct gpio_v2_line_info info;
+	int fd, ret;
+	char *endp;
+
+	if (argc != 3) {
+		print_usage();
+		return EXIT_FAILURE;
+	}
+
+	fd = open(argv[1], O_RDWR);
+	if (fd < 0) {
+		perror("unable to open the GPIO chip");
+		return EXIT_FAILURE;
+	}
+
+	memset(&info, 0, sizeof(info));
+	info.offset = strtoul(argv[2], &endp, 10);
+	if (*endp != '\0') {
+		print_usage();
+		return EXIT_FAILURE;
+	}
+
+	ret = ioctl(fd, GPIO_V2_GET_LINEINFO_IOCTL, &info);
+	if (ret) {
+		perror("line info ioctl failed");
+		return EXIT_FAILURE;
+	}
+
+	printf("%s\n", info.name);
+
+	return EXIT_SUCCESS;
+}