Re: [PATCH v4 3/3] PM: runtime: Clarify documentation when callbacks are unassigned

Recent changes to the PM core allows ->runtime_suspend|resume callbacks to
be unassigned.

In the earlier behaviour the PM core would return -ENOSYS, when trying to
runtime resume a device, for example. Let's update the documentation to
clarify this.

Signed-off-by: Ulf Hansson <redacted>
---

Changes in v4:
        - This time, really, fix spelling and further clarified the behaviour,
	according to comments from Alan.

---
 Documentation/power/runtime_pm.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
index 18ae21bf7f92..8a0a43811e3a 100644
--- a/Documentation/power/runtime_pm.rst
+++ b/Documentation/power/runtime_pm.rst

@@ -827,6 +827,15 @@ or driver about runtime power changes.  Instead, the driver for the device's
 parent must take responsibility for telling the device's driver when the
 parent's power state changes.
 
+Note that, in some cases it may not be desirable for subsystems/drivers to call
+pm_runtime_no_callbacks() for their devices. This could be because a subset of
+the runtime PM callbacks needs to be implemented, a platform dependent PM
+domain could get attached to the device or that the device is power managed
+through a supplier device link. For these reasons and to avoid boilerplate code
+in subsystems/drivers, the PM core allows runtime PM callbacks to be
+unassigned. More precisely, if a callback pointer is NULL, the PM core will act
+as though there was a callback and it returned 0.
+
 9. Autosuspend, or automatically-delayed suspends
 =================================================