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

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Date:   Thu, 27 Oct 2022 18:04:46 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Michael Walle <michael@...le.cc>
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" 
        <linux-arm-kernel@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...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"

On Thu, Oct 27, 2022 at 06:00:04PM +0200, Michael Walle wrote:
> > - 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.

Yeah, that's worth exploring, but it means that we have 2 code paths for
request_module() with different string formats. To me this is slightly
undesirable, we should try to consolidate the mechanisms in the core.

> > - 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).

Well, to be fair, this is not exactly true. As far as I could find
(grep for "ops->name" in net/dsa), there are only 3 instances of reading
the "name" field of struct dsa_device_ops, and none of them are from a
fast path.

I can imagine a table along the lines of:

static const char * const dsa_tag_proto_names[] = {
	[DSA_TAG_PROTO_NONE] = "none",
	[DSA_TAG_PROTO_BRCM] = "brcm",
	....
};

which is then used to directly replace ops->name
(becomes dsa_tag_proto_names[ops->proto]).

Then, we could add a new function "dsa_tag_protocol_name_to_id()" or
something along those lines, and construct the modalias string based on
that.

No duplication necessary, since we would remove dsa_device_ops :: name.