linux-kernel - Re: [PATCH v2 0/9] driver core: Fix some device links issues and add "consumer autoprobe" flag

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Date:   Wed, 6 Feb 2019 10:56:11 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Lukas Wunner <lukas@...ner.de>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Lucas Stach <l.stach@...gutronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH v2 0/9] driver core: Fix some device links issues and add
 "consumer autoprobe" flag

On Tue, Feb 5, 2019 at 12:27 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> On Tuesday, February 5, 2019 9:15:49 AM CET Ulf Hansson wrote:
> > On Mon, 4 Feb 2019 at 12:45, Rafael J. Wysocki <rafael@...nel.org> wrote:
> > >
> > > On Mon, Feb 4, 2019 at 12:40 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > > >
> > > > On Fri, Feb 1, 2019 at 4:18 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:

[cut]

> > >
> > > For example, if the consumer device is suspended after the
> > > device_link_add() that incremented the supplier's PM-runtime count and
> > > then resumed again, the rpm_active refcount will be greater than one
> > > because of the last resume and not because of the initial link
> > > creation.  In that case, dropping the supplier's PM-runtime count on
> > > link deletion may not work as expected.
> >
> > I see what your are saying and I must admit, by looking at the code,
> > that it has turned into being rather complicated. Assuming of good
> > reasons, of course.
> >
> > Anyway, I will play a little bit more with my tests to see what I can find out.
> >
> > >
> > > > Arguably, device_link_del() could be made automatically drop the
> > > > supplier's PM-runtime count by one if the link's rpm_active refcount
> > > > is not one, but there will be failing scenarios in that case too
> > > > AFAICS.
> >
> > Let's see.
>
> So for the record, below is the (untested) patch I'm thinking about.
>
> Having considered this for some time, I think that it would be better to
> try to drop the supplier's PM-runtime usage counter on link removal even if
> the link doesn't go away then.  That would be more consistent at least IMO.

So I can't convince myself that this is the case.

Either way, if there are two callers of device_link_add() for one
consumer-supplier pair trying to add a stateless link between them and
one of these callers passes DL_FLAG_RPM_ACTIVE set in the flags to it,
there may be issues regardless of what device_link_del() and
device_link_remove() do.  However, if they decrement the link's
rpm_active refcount (and possibly the supplier's PM-runtime usage
counter too), the supplier may be suspended prematurely, whereas in
the other case (no decrementation of rpm_active, which how the code
works after this series) it may just be prevented from suspending.  To
me, the former is worse than the latter.

Moreover, there is a workaround for the latter issue that seems to be
easy enough: it is sufficient to let the consumer runtime suspend
after calling device_link_add() to create the link (with
DL_FLAG_RPM_ACTIVE set) and before trying to remove it.

Because of the above, I'm just going to post a patch to document the
current behavior of the code as a known limitation.