reset: ACPI reset support

Some of the IO devices like I2C or SPI require reset at runtime to
recover from an error condition without changing the power state of
the system. Added check for ACPI handle and a call to method '__RST'
if supported. Devices using device tree method are unaffected by this.

Signed-off-by: Krishna Yarlagadda <[e-mail]>
[[e-mail]: wrap in #ifdef CONFIG_ACPI due to missing stubs]
Signed-off-by: Philipp Zabel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/reset/core.c

···
 #include <linux/kref.h>
 #include <linux/module.h>
 #include <linux/of.h>
+#include <linux/acpi.h>
 #include <linux/reset.h>
 #include <linux/reset-controller.h>
 #include <linux/slab.h>
···
  *
  * Convenience wrapper for __reset_control_get() and reset_control_reset().
  * This is useful for the common case of devices with single, dedicated reset
- * lines.
+ * lines. _RST firmware method will be called for devices with ACPI.
  */
 int __device_reset(struct device *dev, bool optional)
 {
 	struct reset_control *rstc;
 	int ret;
+
+#ifdef CONFIG_ACPI
+	acpi_handle handle = ACPI_HANDLE(dev);
+
+	if (handle) {
+		if (!acpi_has_method(handle, "_RST"))
+			return optional ? 0 : -ENOENT;
+		if (ACPI_FAILURE(acpi_evaluate_object(handle, "_RST", NULL,
+						      NULL)))
+			return -EIO;
+	}
+#endif
 
 	rstc = __reset_control_get(dev, NULL, 0, 0, optional, true);
 	if (IS_ERR(rstc))