+ * @prepare: Prepare the platform for entering the system sleep state indicated
+ * by @set_target() or represented by the argument if @set_target() is not
+ * implemented.
+ * @prepare() is called right after devices have been suspended (ie. the
+ * appropriate .suspend() method has been executed for each device) and
+ * before the nonboot CPUs are disabled (it is executed with IRQs enabled).
+ * This callback is optional. It returns 0 on success or a negative
+ * error code otherwise, in which case the system cannot enter the desired
+ * sleep state (@enter() and @finish() will not be called in that case).
+ *
+ * @enter: Enter the system sleep state indicated by @set_target() or
+ * represented by the argument if @set_target() is not implemented.
+ * This callback is mandatory. It returns 0 on success or a negative
+ * error code otherwise, in which case the system cannot enter the desired
+ * sleep state.
+ *
+ * @finish: Called when the system has just left a sleep state, right after
+ * the nonboot CPUs have been enabled and before devices are resumed (it is
+ * executed with IRQs enabled). If @set_target() is not implemented, the
+ * argument represents the sleep state being left.
+ * This callback is optional, but should be implemented by the platforms
+ * that implement @prepare(). If implemented, it is always called after
+ * @enter() (even if @enter() fails).