gpiolib: Use seq_file's iterator interface

When dumping a collection of items via seq_file, it is recommended to
use the iterator interface. For the gpiolib debugfs interface this can
be done to dump each GPIO chip in turn.

Note that for gpiolib this is a little cumbersome because it does not
provide a list of registered GPIO chips and the only way to iterate is
over each GPIO individually. Once a chip is found, the number of GPIOs
it provides can be skipped as a small optimization.

This patch was requested by Arnd Bergmann here:

	http://article.gmane.org/gmane.linux.ports.tegra/3535

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -1773,56 +1773,102 @@ static void gpiolib_dbg_show(struct seq_file *s, struct gpio_chip *chip)
 	}
 }
 
-static int gpiolib_show(struct seq_file *s, void *unused)
+static void *gpiolib_seq_start(struct seq_file *s, loff_t *pos)
 {
-	struct gpio_chip	*chip = NULL;
-	unsigned		gpio;
-	int			started = 0;
+	struct gpio_chip *chip = NULL;
+	unsigned int gpio;
+	void *ret = NULL;
+	loff_t index = 0;
 
 	/* REVISIT this isn't locked against gpio_chip removal ... */
 
 	for (gpio = 0; gpio_is_valid(gpio); gpio++) {
-		struct device *dev;
-
-		if (chip == gpio_desc[gpio].chip)
+		if (gpio_desc[gpio].chip == chip)
 			continue;
+
 		chip = gpio_desc[gpio].chip;
 		if (!chip)
 			continue;
 
-		seq_printf(s, "%sGPIOs %d-%d",
-				started ? "\n" : "",
-				chip->base, chip->base + chip->ngpio - 1);
-		dev = chip->dev;
-		if (dev)
-			seq_printf(s, ", %s/%s",
-				dev->bus ? dev->bus->name : "no-bus",
-				dev_name(dev));
-		if (chip->label)
-			seq_printf(s, ", %s", chip->label);
-		if (chip->can_sleep)
-			seq_printf(s, ", can sleep");
-		seq_printf(s, ":\n");
-
-		started = 1;
-		if (chip->dbg_show)
-			chip->dbg_show(s, chip);
-		else
-			gpiolib_dbg_show(s, chip);
+		if (index++ >= *pos) {
+			ret = chip;
+			break;
+		}
 	}
+
+	s->private = "";
+
+	return ret;
+}
+
+static void *gpiolib_seq_next(struct seq_file *s, void *v, loff_t *pos)
+{
+	struct gpio_chip *chip = v;
+	unsigned int gpio;
+	void *ret = NULL;
+
+	/* skip GPIOs provided by the current chip */
+	for (gpio = chip->base + chip->ngpio; gpio_is_valid(gpio); gpio++) {
+		chip = gpio_desc[gpio].chip;
+		if (chip) {
+			ret = chip;
+			break;
+		}
+	}
+
+	s->private = "\n";
+	++*pos;
+
+	return ret;
+}
+
+static void gpiolib_seq_stop(struct seq_file *s, void *v)
+{
+}
+
+static int gpiolib_seq_show(struct seq_file *s, void *v)
+{
+	struct gpio_chip *chip = v;
+	struct device *dev;
+
+	seq_printf(s, "%sGPIOs %d-%d", (char *)s->private,
+			chip->base, chip->base + chip->ngpio - 1);
+	dev = chip->dev;
+	if (dev)
+		seq_printf(s, ", %s/%s", dev->bus ? dev->bus->name : "no-bus",
+			dev_name(dev));
+	if (chip->label)
+		seq_printf(s, ", %s", chip->label);
+	if (chip->can_sleep)
+		seq_printf(s, ", can sleep");
+	seq_printf(s, ":\n");
+
+	if (chip->dbg_show)
+		chip->dbg_show(s, chip);
+	else
+		gpiolib_dbg_show(s, chip);
+
 	return 0;
 }
 
+static const struct seq_operations gpiolib_seq_ops = {
+	.start = gpiolib_seq_start,
+	.next = gpiolib_seq_next,
+	.stop = gpiolib_seq_stop,
+	.show = gpiolib_seq_show,
+};
+
 static int gpiolib_open(struct inode *inode, struct file *file)
 {
-	return single_open(file, gpiolib_show, NULL);
+	return seq_open(file, &gpiolib_seq_ops);
 }
 
 static const struct file_operations gpiolib_operations = {
+	.owner		= THIS_MODULE,
 	.open		= gpiolib_open,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
-	.release	= single_release,
+	.release	= seq_release,
 };
 
 static int __init gpiolib_debugfs_init(void)