[PATCH 04/21] gpiolib: provide gpio_device_get_desc()

[Bartosz Golaszewski <brgl@xxxxxxxx>]

From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>

Getting the GPIO descriptor directly from the gpio_chip struct is
dangerous as we don't take the reference to the underlying GPIO device.
In order to start working towards removing gpiochip_get_desc(), let's
provide a safer variant that works with an existing reference to struct
gpio_device.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
---
 drivers/gpio/gpiolib.c      | 45 +++++++++++++++++++++++++++----------
 include/linux/gpio/driver.h |  2 ++
 2 files changed, 35 insertions(+), 12 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 74b837671d30..4a9af6bfc6d4 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -147,27 +147,48 @@ struct gpio_desc *gpio_to_desc(unsigned gpio)
 }
 EXPORT_SYMBOL_GPL(gpio_to_desc);
 
-/**
- * gpiochip_get_desc - get the GPIO descriptor corresponding to the given
- *                     hardware number for this chip
- * @gc: GPIO chip
- * @hwnum: hardware number of the GPIO for this chip
- *
- * Returns:
- * A pointer to the GPIO descriptor or ``ERR_PTR(-EINVAL)`` if no GPIO exists
- * in the given chip for the specified hardware number.
- */
+/* This function is deprecated and will be removed soon, don't use. */
 struct gpio_desc *gpiochip_get_desc(struct gpio_chip *gc,
 				    unsigned int hwnum)
 {
-	struct gpio_device *gdev = gc->gpiodev;
+	return gpio_device_get_desc(gc->gpiodev, hwnum);
+}
+EXPORT_SYMBOL_GPL(gpiochip_get_desc);
+
+/**
+ * gpio_device_get_desc() - get the GPIO descriptor corresponding to the given
+ *                          hardware number for this GPIO device
+ * @gdev: GPIO device to get the descriptor from
+ * @hwnum: hardware number of the GPIO for this chip
+ *
+ * Returns:
+ * A pointer to the GPIO descriptor or ``ERR_PTR(-EINVAL)`` if no GPIO exists
+ * in the given chip for the specified hardware number or ``ERR_PTR(-ENODEV)``
+ * if the underlying chip already vanished.
+ *
+ * The reference count of struct gpio_device is *NOT* increased like when the
+ * GPIO is being requested for exclusive usage. It's up to the caller to make
+ * sure the GPIO device will stay alive together with the descriptor returned
+ * by this function.
+ */
+struct gpio_desc *
+gpio_device_get_desc(struct gpio_device *gdev, unsigned int hwnum)
+{
+	struct gpio_chip *gc = gdev->chip;
+
+	/*
+	 * FIXME: This will be locked once we protect gdev->chip everywhere
+	 * with SRCU.
+	 */
+	if (!gc)
+		return ERR_PTR(-ENODEV);
 
 	if (hwnum >= gdev->ngpio)
 		return ERR_PTR(-EINVAL);
 
 	return &gdev->descs[hwnum];
 }
-EXPORT_SYMBOL_GPL(gpiochip_get_desc);
+EXPORT_SYMBOL_GPL(gpio_device_get_desc);
 
 /**
  * desc_to_gpio - convert a GPIO descriptor to the integer namespace
diff --git a/include/linux/gpio/driver.h b/include/linux/gpio/driver.h
index 92f7143bad3e..a769baf3d731 100644
--- a/include/linux/gpio/driver.h
+++ b/include/linux/gpio/driver.h
@@ -766,6 +766,8 @@ struct gpio_desc *gpiochip_request_own_desc(struct gpio_chip *gc,
 void gpiochip_free_own_desc(struct gpio_desc *desc);
 
 struct gpio_desc *gpiochip_get_desc(struct gpio_chip *gc, unsigned int hwnum);
+struct gpio_desc *
+gpio_device_get_desc(struct gpio_device *gdev, unsigned int hwnum);
 
 #ifdef CONFIG_GPIOLIB
 
-- 
2.39.2