netdev - Re: [PATCH net-next] net: build all switchdev drivers as modules when the bridge is a module

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Date:   Tue, 3 Aug 2021 14:33:04 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     Grygorii Strashko <grygorii.strashko@...com>,
        Networking <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Lars Povlsen <lars.povlsen@...rochip.com>,
        Steen Hegelund <Steen.Hegelund@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Ido Schimmel <idosch@...dia.com>, Jiri Pirko <jiri@...dia.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <nikolay@...dia.com>,
        Linux Kernel Functional Testing <lkft@...aro.org>,
        Vignesh Raghavendra <vigneshr@...com>
Subject: Re: [PATCH net-next] net: build all switchdev drivers as modules when
 the bridge is a module

On Tue, Aug 3, 2021 at 1:58 PM Vladimir Oltean <vladimir.oltean@....com> wrote:
>
> On Tue, Aug 03, 2021 at 02:18:38PM +0300, Grygorii Strashko wrote:
> > In my opinion, the problem is a bit bigger here than just fixing the build :(
> >
> > In case, of ^cpsw the switchdev mode is kinda optional and in many cases
> > (especially for testing purposes, NFS) the multi-mac mode is still preferable mode.
> >
> > There were no such tight dependency between switchdev drivers and bridge core before and switchdev serviced as
> > independent, notification based layer between them, so ^cpsw still can be "Y" and bridge can be "M".
> > Now for mostly every kernel build configuration the CONFIG_BRIDGE will need to be set as "Y", or we will have
> > to update drivers to support build with BRIDGE=n and maintain separate builds for networking vs non-networking testing.
> > But is this enough? Wouldn't it cause 'chain reaction' required to add more and more "Y" options (like CONFIG_VLAN_8021Q)?
> >
> > PS. Just to be sure we on the same page - ARM builds will be forced (with this patch) to have CONFIG_TI_CPSW_SWITCHDEV=m
> > and so all our automation testing will just fail with omap2plus_defconfig.
>
> I didn't realize it is such a big use case to have the bridge built as
> module and cpsw as built-in.

I don't think anybody realistically cares about doing, I was only interested in
correctly expressing what the dependency is.

> I will send a patch that converts the
> switchdev_bridge_port_{,un}offload functions to simply emit a blocking
> switchdev notifier which the bridge processes (a la SWITCHDEV_FDB_ADD_TO_BRIDGE),
> therefore making switchdev and the bridge loosely coupled in order to
> keep your setup the way it was before.

That does sounds like it can avoid future build regressions, and simplify the
Kconfig dependencies, so that would probably be a good solution.

       arnd