gpio: omap: return error if requested debounce time is not possible

omap_gpio_debounce() does not validate that the requested debounce
is within a range it can handle. Instead it lets the register value
wrap silently, and always returns success.

This can lead to all sorts of unexpected behavior, such as gpio_keys
asking for a too-long debounce, but getting a very short debounce in
practice.

Fix this by returning -EINVAL if the requested value does not fit into
the register field. If there is no debounce clock available at all,
return -ENOTSUPP.

Fixes: e85ec6c3047b ("gpio: omap: fix omap2_set_gpio_debounce")
Cc: <[e-mail]> # 4.3+
Signed-off-by: David Rivshin <[e-mail]>
Acked-by: Grygorii Strashko <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpio-omap.c

···
  * OMAP's debounce time is in 31us steps
  *   <debounce time> = (GPIO_DEBOUNCINGTIME[7:0].DEBOUNCETIME + 1) x 31
  * so we need to convert and round up to the closest unit.
+ *
+ * Return: 0 on success, negative error otherwise.
  */
-static void omap2_set_gpio_debounce(struct gpio_bank *bank, unsigned offset,
-				    unsigned debounce)
+static int omap2_set_gpio_debounce(struct gpio_bank *bank, unsigned offset,
+				   unsigned debounce)
 {
 	void __iomem		*reg;
 	u32			val;
···
 	bool			enable = !!debounce;
 
 	if (!bank->dbck_flag)
-		return;
+		return -ENOTSUPP;
 
 	if (enable) {
 		debounce = DIV_ROUND_UP(debounce, 31) - 1;
-		debounce &= OMAP4_GPIO_DEBOUNCINGTIME_MASK;
+		if ((debounce & OMAP4_GPIO_DEBOUNCINGTIME_MASK) != debounce)
+			return -EINVAL;
 	}
 
 	l = BIT(offset);
···
 		bank->context.debounce = debounce;
 		bank->context.debounce_en = val;
 	}
+
+	return 0;
 }
 
 /**
···
 {
 	struct gpio_bank *bank;
 	unsigned long flags;
+	int ret;
 
 	bank = gpiochip_get_data(chip);
 
 	raw_spin_lock_irqsave(&bank->lock, flags);
-	omap2_set_gpio_debounce(bank, offset, debounce);
+	ret = omap2_set_gpio_debounce(bank, offset, debounce);
 	raw_spin_unlock_irqrestore(&bank->lock, flags);
 
-	return 0;
+	if (ret)
+		dev_info(chip->parent,
+			 "Could not set line %u debounce to %u microseconds (%d)",
+			 offset, debounce, ret);
+
+	return ret;
 }
 
 static int omap_gpio_set_config(struct gpio_chip *chip, unsigned offset,