gpiolib: cdev: add fdinfo output for line request file descriptors

Add fdinfo output for file descriptors created for user-space line
requests in GPIO uAPI v2. The fdinfo file now contains the name of the
GPIO chip that is the "parent" of the request as well as offsets of
the lines requested. This allows user-space to parse the /proc/$PID/fdinfo
entries and deduce the PID of the process that requested a specific line.
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Kent Gibson <warthog618@gmail.com>

drivers/gpio/gpiolib-cdev.c

@@ -1497,6 +1497,21 @@ static int linereq_release(struct inode *inode, struct file *file)
 	return 0;
 }
 
+#ifdef CONFIG_PROC_FS
+static void linereq_show_fdinfo(struct seq_file *out, struct file *file)
+{
+	struct linereq *lr = file->private_data;
+	struct device *dev = &lr->gdev->dev;
+	u16 i;
+
+	seq_printf(out, "gpio-chip:\t%s\n", dev_name(dev));
+
+	for (i = 0; i < lr->num_lines; i++)
+		seq_printf(out, "gpio-line:\t%d\n",
+			   gpio_chip_hwgpio(lr->lines[i].desc));
+}
+#endif
+
 static const struct file_operations line_fileops = {
 	.release = linereq_release,
 	.read = linereq_read,
@@ -1507,6 +1522,9 @@ static const struct file_operations line_fileops = {
 #ifdef CONFIG_COMPAT
 	.compat_ioctl = linereq_ioctl_compat,
 #endif
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo = linereq_show_fdinfo,
+#endif
 };
 
 static int linereq_create(struct gpio_device *gdev, void __user *ip)