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From:   Ulf Hansson <ulf.hansson@linaro.org>
To:     "Rafael J . Wysocki" <rjw@rjwysocki.net>, linux-pm@vger.kernel.org
Cc:     Ulf Hansson <ulf.hansson@linaro.org>,
        Saravana Kannan <saravanak@google.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Tony Lindgren <tony@atomide.com>,
        Kevin Hilman <khilman@kernel.org>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: [PATCH v2 0/3] PM: runtime: Update behaviour for no callbacks
Date:   Tue,  8 Jun 2021 11:02:47 +0200
Message-Id: <20210608090250.85256-1-ulf.hansson@linaro.org>
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While reviewing a patch on the mmc-list, I ended up inspecting the behaviour of
how we deal with the no callback case for runtime PM.

A couple of observations:

*) When pm_runtime_no_callbacks() have been called, it allows the PM core to
takes a quicker path, but at the same time, consumer/supplier device links are
being skipped in rpm_resume|suspend().

**) Calling pm_runtime_no_callbacks() to avoid boiler plate code (assigning
empty functions to ->runtime_suspend|resume()), doesn't work if there could be
consumer/supplier device link being used or a platform dependent PM domain that
could get attached to the device.

Therefore, this series suggests to change the behaviour in the PM core, to
allow the ->runtime_suspend|resume() callbacks to be unassigned. This is already
supported for ->runtime_idle() callbacks, so it would also move things into a
more consistent behaviour.

I have looked at various error paths, in the kernel of callers of
pm_runtime_get_sync(). I couldn't find anyone that made sense, that looked for
the special error code, -ENOSYS, which is the error code getting returned when a
callback is missing. Whether that is sufficient proof that these changes are
100% safe, I can't guarantee, but I think it would be worth a try as the
benefits of avoiding boilerplate code and the corresponding additional code
paths are quite nice, if you ask me.

Kind regards
Ulf Hansson


Ulf Hansson (3):
  PM: runtime: Improve path in rpm_idle() when no callback
  PM: runtime: Allow unassigned ->runtime_suspend|resume callbacks
  PM: runtime: Clarify documentation when callbacks are unassigned

 Documentation/power/runtime_pm.rst |  8 ++++++++
 drivers/base/power/runtime.c       | 18 ++++++++----------
 2 files changed, 16 insertions(+), 10 deletions(-)

-- 
2.25.1