linux-kernel - Re: [PATCH] Revert "arm64: dts: ls1028a: sl28: use ocelot-8021q tagging by default"

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Message-ID: <84c5e0a041909615a1ba8a4508131206@walle.cc>
Date:   Thu, 27 Oct 2022 18:00:04 +0200
From:   Michael Walle <michael@...le.cc>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     Shawn Guo <shawnguo@...nel.org>, Leo Li <leoyang.li@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Heiko Thiery <heiko.thiery@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH] Revert "arm64: dts: ls1028a: sl28: use ocelot-8021q
 tagging by default"

Am 2022-10-27 14:27, schrieb Vladimir Oltean:

>> Pretty nasty. Of all the switch drivers that support tagging protocol
>> change, Ocelot/Felix is the only one with this bug, because in all 
>> other
>> cases, the default and the alternative tagging protocol are part of 
>> the
>> same .ko. Only here we have tag_ocelot.ko and tag_ocelot_8021q.ko.
>> 
>> The problem preventing us from calling request_module() is that 
>> currently,
>> the string identifying the tagging protocol (to which we match device
>> tree information) is part of the tag_*.ko. I think we'd need the
>> translation table between string and enum dsa_tag_protocol to be part 
>> of
>> dsa_core.ko.
> 
> I think we should treat what we committed to in terms of dt-bindings
> with utmost respect, so I would consider your proposed revert as the
> absolute last option. Reverting a device tree change doesn't mean that
> the device trees without the revert will disappear from circulation.
> 
> So far we have 3 options for fixing this within the kernel
> 
> - make tag_ocelot.o and tag_ocelot_8021q.o link into the same
>   tag_ocelot.ko
> 
> - change the MODULE_ALIAS() of all tagging protocol driver modules from
>   "dsa_tag-<number" to something containing their string name - what 
> you
>   proposed. I don't know why the current MODULE_ALIAS() is formatted 
> the
>   way it is. Maybe Andrew can comment on whether this is feasible.
>   I think there isn't any backwards compatibility concern, since only
>   modules compiled for a certain kernel version are expected to be
>   loaded.

FWIW, you can have multiple aliases if we somehow need to keep the IDs,
too.

> - put a translation table between string and MODULE_ALIAS() inside
>   dsa_core.ko, which potentially duplicates code. Maybe if we
>   auto-generate it somehow?

Yeah, I also thought of a table with of name to module alias mapping.
But then you'd have two places to keep in sync (of not autogenerated).

-michael