linux-kernel - [PATCH 0/3] PM: runtime: Fixes related to device links management

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Date:   Wed, 21 Oct 2020 21:10:08 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>,
        Saravana Kannan <saravanak@...gle.com>,
        Xiang Chen <chenxiang66@...ilicon.com>
Subject: [PATCH 0/3] PM: runtime: Fixes related to device links management

Hi Greg & all,

Commit d12544fb2aa9 ("PM: runtime: Remove link state checks in
rpm_get/put_supplier()") merged recently introduced a weakness
in the handling of device links in the runtime PM framework that
may be confusing and even harmful.

Namely, the checks removed by that commit prevented PM-runtime from
getting or dropping references to the supplier device whose driver
was going away via its links to consumers, which specifically allowed
the pm_runtime_clean_up_links() called from __device_release_driver()
to run without interfering with runtime suspend/resume of consumer
devices (which still might happen even though the drivers had been
unbound from them by that time).

After the above commit, calling pm_runtime_clean_up_links() from
__device_release_driver() makes a little sense and it may be interfering
destructively with regular PM-runtime suspend/resume control flows, so
it needs to be either fixed or dropped altogether.  I prefer the latter,
because among other things this removes an arbitrary difference in the
handling of managed device links with respect to the stateless ones,
so patch [2/3] is doing just that.

However, in some rare cases pm_runtime_clean_up_links() may help to clean
up leftover PM-runtime references, so if that function goes away, they
need to be cleaned up elsewhere.  That's why patch [1/3] modifies
__device_link_del() to drop them upon device link removal (which also
needs to be done for stateless device links and that's why I'm regarding
this patch as a fix).

Finally, to avoid pointless overhead related to suspending and resuming
the target device for multiple times in a row in __device_release_driver(),
it is better to resume it upfront before checking its links to consumers,
which is done by patch [3/3].

While this series touches the driver core, it really is mostly related to
runtime PM, so I can apply it if that's OK.

Thanks!