gpio: timberdale: Fix potential deadlock on &tgpio->lock

As timbgpio_irq_enable()/timbgpio_irq_disable() callback could be
executed under irq context, it could introduce double locks on
&tgpio->lock if it preempts other execution units requiring
the same locks.

timbgpio_gpio_set()
--> timbgpio_update_bit()
--> spin_lock(&tgpio->lock)
<interrupt>
   --> timbgpio_irq_disable()
   --> spin_lock_irqsave(&tgpio->lock)

This flaw was found by an experimental static analysis tool I am
developing for irq-related deadlock.

To prevent the potential deadlock, the patch uses spin_lock_irqsave()
on &tgpio->lock inside timbgpio_gpio_set() to prevent the possible
deadlock scenario.

Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpio-timberdale.c

@@ -43,9 +43,10 @@ static int timbgpio_update_bit(struct gpio_chip *gpio, unsigned index,
 	unsigned offset, bool enabled)
 {
 	struct timbgpio *tgpio = gpiochip_get_data(gpio);
+	unsigned long flags;
 	u32 reg;
 
-	spin_lock(&tgpio->lock);
+	spin_lock_irqsave(&tgpio->lock, flags);
 	reg = ioread32(tgpio->membase + offset);
 
 	if (enabled)
@@ -54,7 +55,7 @@ static int timbgpio_update_bit(struct gpio_chip *gpio, unsigned index,
 		reg &= ~(1 << index);
 
 	iowrite32(reg, tgpio->membase + offset);
-	spin_unlock(&tgpio->lock);
+	spin_unlock_irqrestore(&tgpio->lock, flags);
 
 	return 0;
 }