netdev - Re: [PATCH] dpaa_eth: fix usage as DSA master, try 3

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Date:   Tue, 16 Jun 2020 16:00:55 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "Madalin Bucur (OSS)" <madalin.bucur@....nxp.com>,
        netdev <netdev@...r.kernel.org>, joakim.tjernlund@...inera.com
Subject: Re: [PATCH] dpaa_eth: fix usage as DSA master, try 3

On Tue, 26 May 2020 at 03:58, David Miller <davem@...emloft.net> wrote:
>
> From: Vladimir Oltean <olteanv@...il.com>
> Date: Mon, 25 May 2020 00:22:51 +0300
>
> > From: Vladimir Oltean <vladimir.oltean@....com>
> >
> > The dpaa-eth driver probes on compatible string for the MAC node, and
> > the fman/mac.c driver allocates a dpaa-ethernet platform device that
> > triggers the probing of the dpaa-eth net device driver.
> >
> > All of this is fine, but the problem is that the struct device of the
> > dpaa_eth net_device is 2 parents away from the MAC which can be
> > referenced via of_node. So of_find_net_device_by_node can't find it, and
> > DSA switches won't be able to probe on top of FMan ports.
> >
> > It would be a bit silly to modify a core function
> > (of_find_net_device_by_node) to look for dev->parent->parent->of_node
> > just for one driver. We're just 1 step away from implementing full
> > recursion.
> >
> > Actually there have already been at least 2 previous attempts to make
> > this work:
> > - Commit a1a50c8e4c24 ("fsl/man: Inherit parent device and of_node")
> > - One or more of the patches in "[v3,0/6] adapt DPAA drivers for DSA":
> >   https://patchwork.ozlabs.org/project/netdev/cover/1508178970-28945-1-git-send-email-madalin.bucur@nxp.com/
> >   (I couldn't really figure out which one was supposed to solve the
> >   problem and how).
> >
> > Point being, it looks like this is still pretty much a problem today.
> > On T1040, the /sys/class/net/eth0 symlink currently points to
> >
> > ../../devices/platform/ffe000000.soc/ffe400000.fman/ffe4e6000.ethernet/dpaa-ethernet.0/net/eth0
> >
> > which pretty much illustrates the problem. The closest of_node we've got
> > is the "fsl,fman-memac" at /soc@...000000/fman@...000/ethernet@...00,
> > which is what we'd like to be able to reference from DSA as host port.
> >
> > For of_find_net_device_by_node to find the eth0 port, we would need the
> > parent of the eth0 net_device to not be the "dpaa-ethernet" platform
> > device, but to point 1 level higher, aka the "fsl,fman-memac" node
> > directly. The new sysfs path would look like this:
> >
> > ../../devices/platform/ffe000000.soc/ffe400000.fman/ffe4e6000.ethernet/net/eth0
> >
> > And this is exactly what SET_NETDEV_DEV does. It sets the parent of the
> > net_device. The new parent has an of_node associated with it, and
> > of_dev_node_match already checks for the of_node of the device or of its
> > parent.
> >
> > Fixes: a1a50c8e4c24 ("fsl/man: Inherit parent device and of_node")
> > Fixes: c6e26ea8c893 ("dpaa_eth: change device used")
> > Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
>
> Applied and queued up for -stable, thanks.

Joakim notified me that this breaks stable trees.
It turns out that my assessment about who-broke-who was wrong.
The real Fixes: tag should have been:

Fixes: 060ad66f9795 ("dpaa_eth: change DMA device")

which changes the device on which SET_NETDEV_DEV is made.

git describe --tags 060ad66f97954
v5.4-rc3-783-g060ad66f9795

Which means that it shouldn't have been backported to 4.19 and below.

What is the procedure to revert it from those stable trees? Would I
need to revert this patch in "net" and apply another one with the
correct Fixes: tag?

Thanks,
-Vladimir