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

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Message-ID: <CAGETcx9rEHG4xj1X6ERtPbL2GjzuNU+9OZPb6zY2hxvHFXnq-g@mail.gmail.com>
Date:   Tue, 14 Sep 2021 09:23:14 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vladimir Oltean <olteanv@...il.com>,
        Android Kernel Team <kernel-team@...roid.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/5] driver core: fw_devlink: Improve handling of
 cyclic dependencies

On Tue, Sep 14, 2021 at 5:35 AM Rob Herring <robh+dt@...nel.org> wrote:
>
> On Mon, Sep 13, 2021 at 11:39 PM Saravana Kannan <saravanak@...gle.com> 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>
> > Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> > ---
> >  drivers/base/core.c | 17 ++++++++++++-----
> >  1 file changed, 12 insertions(+), 5 deletions(-)
>
> Am I supposed to apply this? What about the revert and mdio-parent-bus
> support you mentioned? Those are needed too? Please send me a series
> with what I should apply for 5.15, not fixes and new features
> combined.

You don't need to pick up any of this. I just added you for 5/5 and
you've seen how a lot of the debugging goes.

-Saravana