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Date:   Thu, 27 Oct 2022 19:47:05 -0700
From:   Florian Fainelli <>
To:     Vladimir Oltean <>,
Cc:     Andrew Lunn <>,
        Vivien Didelot <>,,
        "David S. Miller" <>,
        Eric Dumazet <>,
        Jakub Kicinski <>,
        Paolo Abeni <>,
        Colin Foster <>,
        Michael Walle <>,
        Heiko Thiery <>,
        Tobias Waldekranz <>
Subject: Re: [PATCH net] net: dsa: fall back to default tagger if we can't
 load the one from DT

On 10/27/2022 7:54 AM, Vladimir Oltean wrote:
> DSA tagging protocol drivers can be changed at runtime through sysfs and
> at probe time through the device tree (support for the latter was added
> later).
> When changing through sysfs, it is assumed that the module for the new
> tagging protocol was already loaded into the kernel (in fact this is
> only a concern for Ocelot/Felix switches, where we have tag_ocelot.ko
> and tag_ocelot_8021q.ko; for every other switch, the default and
> alternative protocols are compiled within the same .ko, so there is
> nothing for the user to load).
> The kernel cannot currently call request_module(), because it has no way
> of constructing the modalias name of the tagging protocol driver
> ("dsa_tag-%d", where the number is one of DSA_TAG_PROTO_*_VALUE).
> The device tree only contains the string name of the tagging protocol
> ("ocelot-8021q"), and the only mapping between the string and the
> DSA_TAG_PROTO_OCELOT_8021Q_VALUE is present in tag_ocelot_8021q.ko.
> So this is a chicken-and-egg situation and dsa_core.ko has nothing based
> on which it can automatically request the insertion of the module.
> As a consequence, if CONFIG_NET_DSA_TAG_OCELOT_8021Q is built as module,
> the switch will forever defer probing.
> The long-term solution is to make DSA call request_module() somehow,
> but that probably needs some refactoring.
> What we can do to keep operating with existing device tree blobs is to
> cancel the attempt to change the tagging protocol with the one specified
> there, and to remain operating with the default one. Depending on the
> situation, the default protocol might still allow some functionality
> (in the case of ocelot, it does), and it's better to have that than to
> fail to probe.
> Fixes: deff710703d8 ("net: dsa: Allow default tag protocol to be overridden from DT")
> Link:
> Reported-by: Heiko Thiery <>
> Reported-by: Michael Walle <>
> Signed-off-by: Vladimir Oltean <>

Reviewed-by: Florian Fainelli <>

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