linux-kernel - Re: [PATCH v1 1/5] driver core: fw_devlink: Improve handling of cyclic dependencies

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Date:   Tue, 14 Sep 2021 01:01:52 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vladimir Oltean <olteanv@...il.com>, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/5] driver core: fw_devlink: Improve handling of
 cyclic dependencies

On Mon, Sep 13, 2021 at 11:16 PM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> On 14.09.2021 06:39, Saravana Kannan wrote:
> > When we have a dependency of the form:
> >
> > Device-A -> Device-C
> >       Device-B
> >
> > Device-C -> Device-B
> >
> > Where,
> > * Indentation denotes "child of" parent in previous line.
> > * X -> Y denotes X is consumer of Y based on firmware (Eg: DT).
> >
> > We have cyclic dependency: device-A -> device-C -> device-B -> device-A
> >
> > fw_devlink current treats device-C -> device-B dependency as an invalid
> > dependency and doesn't enforce it but leaves the rest of the
> > dependencies as is.
> >
> > While the current behavior is necessary, it is not sufficient if the
> > false dependency in this example is actually device-A -> device-C. When
> > this is the case, device-C will correctly probe defer waiting for
> > device-B to be added, but device-A will be incorrectly probe deferred by
> > fw_devlink waiting on device-C to probe successfully. Due to this, none
> > of the devices in the cycle will end up probing.
> >
> > To fix this, we need to go relax all the dependencies in the cycle like
> > we already do in the other instances where fw_devlink detects cycles.
> > A real world example of this was reported[1] and analyzed[2].
> >
> > [1] - https://lore.kernel.org/lkml/0a2c4106-7f48-2bb5-048e-8c001a7c3fda@samsung.com/
> > [2] - https://lore.kernel.org/lkml/CAGETcx8peaew90SWiux=TyvuGgvTQOmO4BFALz7aj0Za5QdNFQ@mail.gmail.com/
> > Fixes: f9aa460672c9 ("driver core: Refactor fw_devlink feature")
> > Reported-by: Marek Szyprowski <m.szyprowski@...sung.com>
> Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>

Thanks!

-Saravana

> > Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> > ---
> >   drivers/base/core.c | 17 ++++++++++++-----
> >   1 file changed, 12 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index e65dd803a453..316df6027093 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -1772,14 +1772,21 @@ static int fw_devlink_create_devlink(struct device *con,
> >        * be broken by applying logic. Check for these types of cycles and
> >        * break them so that devices in the cycle probe properly.
> >        *
> > -      * If the supplier's parent is dependent on the consumer, then
> > -      * the consumer-supplier dependency is a false dependency. So,
> > -      * treat it as an invalid link.
> > +      * If the supplier's parent is dependent on the consumer, then the
> > +      * consumer and supplier have a cyclic dependency. Since fw_devlink
> > +      * can't tell which of the inferred dependencies are incorrect, don't
> > +      * enforce probe ordering between any of the devices in this cyclic
> > +      * dependency. Do this by relaxing all the fw_devlink device links in
> > +      * this cycle and by treating the fwnode link between the consumer and
> > +      * the supplier as an invalid dependency.
> >        */
> >       sup_dev = fwnode_get_next_parent_dev(sup_handle);
> >       if (sup_dev && device_is_dependent(con, sup_dev)) {
> > -             dev_dbg(con, "Not linking to %pfwP - False link\n",
> > -                     sup_handle);
> > +             dev_info(con, "Fixing up cyclic dependency with %pfwP (%s)\n",
> > +                      sup_handle, dev_name(sup_dev));
> > +             device_links_write_lock();
> > +             fw_devlink_relax_cycle(con, sup_dev);
> > +             device_links_write_unlock();
> >               ret = -EINVAL;
> >       } else {
> >               /*
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>