netdev - Re: [PATCH net-next 06/12] net: dsa: rzn1-a5psw: add Renesas RZ/N1 advanced 5 port switch driver

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Message-ID: <20220415120147.7ya2rwtxcldahh4n@skbuf>
Date:   Fri, 15 Apr 2022 15:01:47 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Clément Léger <clement.leger@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Herve Codina <herve.codina@...tlin.com>,
        Miquèl Raynal <miquel.raynal@...tlin.com>,
        Milan Stevanovic <milan.stevanovic@...com>,
        Jimmy Lalande <jimmy.lalande@...com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org, netdev@...r.kernel.org,
        Jean-Pierre Geslin <jean-pierre.geslin@....se.com>,
        Phil Edworthy <phil.edworthy@...esas.com>
Subject: Re: [PATCH net-next 06/12] net: dsa: rzn1-a5psw: add Renesas RZ/N1
 advanced 5 port switch driver

On Fri, Apr 15, 2022 at 12:23:30PM +0100, Russell King (Oracle) wrote:
> If ->shutdown has been called, the system is going down, and userspace
> is probably already dead.

There isn't anything preventing ->remove from being called after ->shutdown
has been called.

It all starts with the pattern that some driver authors prefer, which is
to redirect their ->shutdown method to ->remove. They argue that it
provides for a well-tested common path, so in turn, this pattern is
quite widespread and I'm not one to argue for removing it.

When such driver (redirecting ->shutdown to ->remove) is a bus driver
(SPI, I2C, lately even the fsl-mc bus), the implication is that the
controller will be unregistered on shutdown. To unregister a bus, you
need to unregister all devices on the bus too.

Due to implicit device ordering on the dpm_list, the ->shutdown() method
of children on said bus has already executed, now we're in the context
of the ->shutdown() procedure of the bus driver itself.

You can argue "hey, that's SPI/I2C and this is a platform driver, there
isn't any bus that unregisters on shutdown here", and you may have a
point there. But platform devices aren't just memory-mapped devices,
they can also be children of mfd devices on SPI/I2C buses. So in theory
you can see this pattern happen on platform devices as well.

This is the reason why I insist for uniformity in the DSA layer in the
way that shutdown is handled. People copy and paste code a lot, and by
leaving them with less variance in the code that they copy, subtle
differences that are not understood but do matter are less likely to
creep in.