gpio: improve the API contract for setting direction

From: Bartosz Golaszewski <[email protected]>

If a GPIO driver returns a positive integer from one of the direction setter callbacks, we'll end up propagating it to user-space. Whether we should sanitize the values returned by callbacks is a different question but let's first improve the documentation and fortify the contract with GPIO providers.

Reported-by José Guilherme de Castro Rodrigues <[email protected]>

Signed-off-by Bartosz Golaszewski <[email protected]>

---
v1 -> v2:
- add a mention of 0 being return on success

 include/linux/gpio/driver.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

include/linux/gpio/driver.h +6 -5

index 9a5c6c76e653..90cd57796b36 100644
@@ -335,10 +335,12 @@ struct gpio_irq_chip {
  *	(same as GPIO_LINE_DIRECTION_OUT / GPIO_LINE_DIRECTION_IN),
  *	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.
+ * @direction_input: configures signal "offset" as input, returns 0 on success
+ *	or a negative error number. This can be omitted on input-only or
+ *	output-only gpio chips.
+ * @direction_output: configures signal "offset" as output, returns 0 on
+ *	success or a negative error number. 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
--

2.40.1