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 13:10:35 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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 Wed, Feb 6, 2019 at 12:24 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
>
> On Wed, 6 Feb 2019 at 10:56, Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > 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.
>
> Well, I would say it sucks in both cases. :-)
>
> >
> > 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.
>
> I get your point, but unfortunate I don't think it's that simple.
>
> For example, someone (like a child) may prevent runtime suspend for
> the consumer. Hence, also the supplier is prevented from being runtime
> suspended.

Well, in that case the supplier should not be suspended until the
consumer can be suspended too.

IOW, if you call device_link_del() in that case, it would be a bug if
it allowed the supplier suspend.

> So, if you want to push this responsibility to the driver, then I
> think we need make __pm_runtime_set_status() to respect device links,
> similar to how it already deals with child/parents.
>
> In that way, the driver could call pm_runtime_set_suspended(), before
> dropping the device link in ->probe(), which would allow the supplier
> to also become runtime suspended.

I guess you mean that runtime PM would be disabled for the consumer at
that point?

> I did a quick research of users of device links, unless I am mistaken,
> this seems like an okay approach.
>
> What do you think?

Well, I think I need to know the exact use case you have in mind. :-)