gpiolib: dynamic gpio number allocation

If gpio_chip->base is negative during registration, gpiolib performs dynamic
base allocation.  This is useful for devices that aren't always present, such
as GPIOs on hotplugged devices rather than mainboards.  (This behavior was
previously specified but not implemented.)

To avoid using any numbers that may have been explicitly assigned but not yet
registered, this dynamic allocation assigns GPIO numbers from the biggest
number on down, instead of from the smallest on up.
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpio/gpiolib.c

@@ -80,6 +80,33 @@ static inline struct gpio_chip *gpio_to_chip(unsigned gpio)
 	return gpio_desc[gpio].chip;
 }
 
+/* dynamic allocation of GPIOs, e.g. on a hotplugged device */
+static int gpiochip_find_base(int ngpio)
+{
+	int i;
+	int spare = 0;
+	int base = -ENOSPC;
+
+	for (i = ARCH_NR_GPIOS - 1; i >= 0 ; i--) {
+		struct gpio_chip *chip = gpio_desc[i].chip;
+
+		if (!chip) {
+			spare++;
+			if (spare == ngpio) {
+				base = i;
+				break;
+			}
+		} else {
+			spare = 0;
+			i -= chip->ngpio - 1;
+		}
+	}
+
+	if (gpio_is_valid(base))
+		pr_debug("%s: found new base at %d\n", __func__, base);
+	return base;
+}
+
 /**
  * gpiochip_add() - register a gpio_chip
  * @chip: the chip to register, with chip->base initialized
@@ -88,38 +115,49 @@ static inline struct gpio_chip *gpio_to_chip(unsigned gpio)
  * Returns a negative errno if the chip can't be registered, such as
  * because the chip->base is invalid or already associated with a
  * different chip.  Otherwise it returns zero as a success code.
+ *
+ * If chip->base is negative, this requests dynamic assignment of
+ * a range of valid GPIOs.
  */
 int gpiochip_add(struct gpio_chip *chip)
 {
 	unsigned long	flags;
 	int		status = 0;
 	unsigned	id;
+	int		base = chip->base;
 
-	/* NOTE chip->base negative is reserved to mean a request for
-	 * dynamic allocation.  We don't currently support that.
-	 */
-
-	if (chip->base < 0 || !gpio_is_valid(chip->base  + chip->ngpio)) {
+	if ((!gpio_is_valid(base) || !gpio_is_valid(base + chip->ngpio))
+			&& base >= 0) {
 		status = -EINVAL;
 		goto fail;
 	}
 
 	spin_lock_irqsave(&gpio_lock, flags);
 
+	if (base < 0) {
+		base = gpiochip_find_base(chip->ngpio);
+		if (base < 0) {
+			status = base;
+			goto fail_unlock;
+		}
+		chip->base = base;
+	}
+
 	/* these GPIO numbers must not be managed by another gpio_chip */
-	for (id = chip->base; id < chip->base + chip->ngpio; id++) {
+	for (id = base; id < base + chip->ngpio; id++) {
 		if (gpio_desc[id].chip != NULL) {
 			status = -EBUSY;
 			break;
 		}
 	}
 	if (status == 0) {
-		for (id = chip->base; id < chip->base + chip->ngpio; id++) {
+		for (id = base; id < base + chip->ngpio; id++) {
 			gpio_desc[id].chip = chip;
 			gpio_desc[id].flags = 0;
 		}
 	}
 
+fail_unlock:
 	spin_unlock_irqrestore(&gpio_lock, flags);
 fail:
 	/* failures here can mean systems won't boot... */