gpio: Add comments on single direction chips

A patch from Ricardo got me thinking about some gpio chip
semantics so let's drop in some comments to make things
more clear around that.

Cc: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -2548,20 +2548,31 @@ int gpiod_direction_input(struct gpio_desc *desc)
 	VALIDATE_DESC(desc);
 	chip = desc->gdev->chip;
 
+	/*
+	 * It is legal to have no .get() and .direction_input() specified if
+	 * the chip is output-only, but you can't specify .direction_input()
+	 * and not support the .get() operation, that doesn't make sense.
+	 */
 	if (!chip->get && chip->direction_input) {
 		gpiod_warn(desc,
-			"%s: missing get() and direction_input() operations\n",
-			__func__);
+			   "%s: missing get() but have direction_input()\n",
+			   __func__);
 		return -EIO;
 	}
 
+	/*
+	 * If we have a .direction_input() callback, things are simple,
+	 * just call it. Else we are some input-only chip so try to check the
+	 * direction (if .get_direction() is supported) else we silently
+	 * assume we are in input mode after this.
+	 */
 	if (chip->direction_input) {
 		status = chip->direction_input(chip, gpio_chip_hwgpio(desc));
 	} else if (chip->get_direction &&
 		  (chip->get_direction(chip, gpio_chip_hwgpio(desc)) != 1)) {
 		gpiod_warn(desc,
-			"%s: missing direction_input() operation\n",
-			__func__);
+			   "%s: missing direction_input() operation and line is output\n",
+			   __func__);
 		return -EIO;
 	}
 	if (status == 0)
@@ -2587,16 +2598,22 @@ static int gpiod_direction_output_raw_commit(struct gpio_desc *desc, int value)
 	int val = !!value;
 	int ret = 0;
 
+	/*
+	 * It's OK not to specify .direction_output() if the gpiochip is
+	 * output-only, but if there is then not even a .set() operation it
+	 * is pretty tricky to drive the output line.
+	 */
 	if (!gc->set && !gc->direction_output) {
 		gpiod_warn(desc,
-		       "%s: missing set() and direction_output() operations\n",
-		       __func__);
+			   "%s: missing set() and direction_output() operations\n",
+			   __func__);
 		return -EIO;
 	}
 
 	if (gc->direction_output) {
 		ret = gc->direction_output(gc, gpio_chip_hwgpio(desc), val);
 	} else {
+		/* Check that we are in output mode if we can */
 		if (gc->get_direction &&
 		    gc->get_direction(gc, gpio_chip_hwgpio(desc))) {
 			gpiod_warn(desc,
@@ -2604,6 +2621,10 @@ static int gpiod_direction_output_raw_commit(struct gpio_desc *desc, int value)
 				__func__);
 			return -EIO;
 		}
+		/*
+		 * If we can't actively set the direction, we are some
+		 * output-only chip, so just drive the output as desired.
+		 */
 		gc->set(gc, gpio_chip_hwgpio(desc), val);
 	}
 

include/linux/gpio/driver.h

@@ -178,9 +178,13 @@ static inline struct gpio_irq_chip *to_gpio_irq_chip(struct irq_chip *chip)
  * @free: optional hook for chip-specific deactivation, such as
  *	disabling module power and clock; may sleep
  * @get_direction: returns direction for signal "offset", 0=out, 1=in,
- *	(same as GPIOF_DIR_XXX), or negative error
+ *	(same as GPIOF_DIR_XXX), or negative error.
+ *	It is recommended to always implement this function, even on
+ *	input-only or output-only gpio chips.
  * @direction_input: configures signal "offset" as input, or returns error
+ *	This can be omitted on input-only or output-only gpio chips.
  * @direction_output: configures signal "offset" as output, or returns error
+ *	This can be omitted on input-only or output-only gpio chips.
  * @get: returns value for signal "offset", 0=low, 1=high, or negative error
  * @get_multiple: reads values for multiple signals defined by "mask" and
  *	stores them in "bits", returns 0 on success or negative error