linux-kernel - [PATCH v2 3/3] PM: runtime: Clarify documentation when callbacks are unassigned

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Date:   Tue,  8 Jun 2021 11:02:50 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J . Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Tony Lindgren <tony@...mide.com>,
        Kevin Hilman <khilman@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 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 <ulf.hansson@...aro.org>
---

Changes in v2:
	- Added a new patch for the updating the docs, as pointed out by Alan.

---
 Documentation/power/runtime_pm.rst | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
index 18ae21bf7f92..3d09c9fd450d 100644
--- a/Documentation/power/runtime_pm.rst
+++ b/Documentation/power/runtime_pm.rst
@@ -827,6 +827,14 @@ 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 manged
+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.
+
 9. Autosuspend, or automatically-delayed suspends
 =================================================
 
-- 
2.25.1