netdev - Re: [PATCH net-next] dsa: sja1105: fix reverse dependency

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Date:   Thu, 5 Aug 2021 14:05:11 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     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>,
        Arnd Bergmann <arnd@...db.de>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Yangbo Lu <yangbo.lu@....com>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] dsa: sja1105: fix reverse dependency

On Thu, Aug 5, 2021 at 1:49 PM Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Thu, Aug 05, 2021 at 01:39:34PM +0200, Arnd Bergmann wrote:
> > Do you have any opinion on whether that 'select' going the other way is still
> > relevant?
>
> Yes, of course it is. It also has nothing to do with build dependencies.
> With the original DSA design from 2008, an Ethernet switch has separate
> drivers for
> (a) accessing its registers
> (b) manipulating the packets that the switch sends towards a host
>     Ethernet controller ("DSA master")
>
> The register access drivers are in drivers/net/dsa/*, the packet
> manipulation ("tagging protocol") drivers are in net/dsa/tag_*.c.
>
> [ This is because it was originally thought that a "tagging protocol" is
>   completely stateless and you should never need to access a hardware
>   register when manipulating a packet. ]
>
> When you enable a driver for a switch, you absolutely want to ping
> through it too, so all register access drivers enable the tagging
> protocol driver specific to their hardware as well, using 'select'.
> This works just fine because tagging protocol drivers generally have no
> dependencies, or if they do, the register access driver inherits them too.
> So a user does not need to manually enable the tagging protocol driver.

Got it, thanks for the explanation.

      Arnd