gpio: set up initial state from .get_direction()

If the gpiochip supports the .get_direction() callback, then
the initial state of the descriptor flags should be set up
as output accordingly. Also put in comments explaining what is
going on.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -565,14 +565,31 @@ int gpiochip_add_data(struct gpio_chip *chip, void *data)
 		struct gpio_desc *desc = &gdev->descs[i];
 
 		desc->gdev = gdev;
+		/*
+		 * REVISIT: most hardware initializes GPIOs as inputs
+		 * (often with pullups enabled) so power usage is
+		 * minimized. Linux code should set the gpio direction
+		 * first thing; but until it does, and in case
+		 * chip->get_direction is not set, we may expose the
+		 * wrong direction in sysfs.
+		 */
+
+		if (chip->get_direction) {
+			/*
+			 * If we have .get_direction, set up the initial
+			 * direction flag from the hardware.
+			 */
+			int dir = chip->get_direction(chip, i);
 
-		/* REVISIT: most hardware initializes GPIOs as inputs (often
-		 * with pullups enabled) so power usage is minimized. Linux
-		 * code should set the gpio direction first thing; but until
-		 * it does, and in case chip->get_direction is not set, we may
-		 * expose the wrong direction in sysfs.
+			if (!dir)
+				set_bit(FLAG_IS_OUT, &desc->flags);
+		} else if (!chip->direction_input) {
+			/*
+			 * If the chip lacks the .direction_input callback
+			 * we logically assume all lines are outputs.
 			 */
-		desc->flags = !chip->direction_input ? (1 << FLAG_IS_OUT) : 0;
+			set_bit(FLAG_IS_OUT, &desc->flags);
+		}
 	}
 
 	spin_unlock_irqrestore(&gpio_lock, flags);