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

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Date:   Mon, 4 Feb 2019 12:40:25 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "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 Fri, Feb 1, 2019 at 4:18 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
>
> On Fri, 1 Feb 2019 at 02:04, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >
> > Hi Greg at al,
> >
> > This is a combination of the two device links series I have posted
> > recently (https://lore.kernel.org/lkml/2493187.oiOpCWJBV7@aspire.rjw.lan/
> > and https://lore.kernel.org/lkml/2405639.4es7pRLqn0@aspire.rjw.lan/) rebased
> > on top of your driver-core-next branch.
> >
> > Recently I have been looking at the device links code because of the
> > recent discussion on possibly using them in the DRM subsystem (see for
> > example https://marc.info/?l=linux-pm&m=154832771905309&w=2) and I have
> > found a few issues in that code which should be addressed by this patch
> > series.  Please refer to the patch changelogs for details.
> >
> > None of the problems addressed here should be manifesting themselves in
> > mainline kernel today, but if there are more device links users in the
> > future, they most likely will be encountered sooner or later.  Also they
> > need to be fixed for the DRM use case to be supported IMO.
> >
> > On top of this the series makes device links support the "composite device"
> > use case in the DRM subsystem mentioned above (essentially, the last patch
> > in the series is for that purpose).
> >
>
> Rafael, Greg, I have reviewed patch 1 -> 7, they all look good to me.
>
> If not too late, feel free to add for the first 7 patches:
>
> Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>

Thanks!

> Although, I want to point out one problem that I have found when using
> device links. I believe it's already there, even before this series,
> but just wanted to described it for your consideration.
>
> This is what happens:
> I have a platform driver is being probed. During ->probe() the driver
> adds a device link like this:
>
> link = device_link_add(consumer-dev, supplier-dev, DL_FLAG_STATELESS |
> DL_FLAG_PM_RUNTIME | DL_FLAG_RPM_ACTIVE);
>
> At some point later in ->probe(), the driver realizes that it must
> remove the device link, either because it encountered an error or
> simply because it doesn't need the device link to be there anymore.
> Thus it calls:
>
> device_link_del(link);
>
> When probe finished of the driver, the runtime PM usage count for the
> supplier-dev remains increased to 1 and thus it never becomes runtime
> suspended.

OK, so this is a tricky one.

With this series applied, if the link actually goes away after the
cleanup device_link_del(), device_link_free() should take care of
dropping the PM-runtime count of the supplier.  If it doesn't do that,
there is a mistake in the code that needs to be fixed.

However, if the link doesn't go away after the cleanup
device_link_del(), the supplier's PM-runtime count will not be
dropped, because the core doesn't know whether or not the
device_link_del() has been called by the same entity that caused the
supplier's PM-runtime count to be incremented.  For example, if the
consumer device is suspended after the device_link_add() that
incremented the supplier's PM-runtime count and then suspended again,
the link's rpm_active refcount is one already and so the supplier's
PM-runtime count should not be dropped.

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.