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

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Message-ID: <CAJZ5v0hRMcsdGVOHCfeK4_k81L+PJSNq80kx1BpwDBs+WtgqpA@mail.gmail.com>
Date:   Fri, 30 Oct 2020 17:38:29 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
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: Re: [PATCH 0/3] PM: runtime: Fixes related to device links management

Hi Greg,

On Wed, Oct 21, 2020 at 9:14 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> 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.

Any concerns regarding this series?

If not, I'd like to queue it up for -rc3, because the current behavior
in there is quite confusing (or worse).

Cheers!