linux-kernel - Re: [PATCH] net: dsa: mv88e6xxx: add warning for truncated mdio bus id

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Date: Thu, 21 Mar 2024 10:26:54 +0000
From: Josua Mayer <josua@...id-run.com>
To: Andrew Lunn <andrew@...n.ch>
CC: Jiri Pirko <jiri@...nulli.us>, Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...il.com>, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: dsa: mv88e6xxx: add warning for truncated mdio bus
 id

Am 20.03.24 um 19:57 schrieb Andrew Lunn:
>> With separators ('!') we have:
>> cp0!config-space@...00000!mdio@...200!ethernet-switch@...dio
>> cp0!config-space@...00000!mdio@...200!ethernet-switch@...dio-external
>> Truncated to MII_BUS_ID_SIZE:
>> cp0!config-space@...00000!mdio@...200!ethernet-switch@...di
>> cp0!config-space@...00000!mdio@...200!ethernet-switch@...di
> This has been made worse by the DT maintainers wanting
> ethernet-switch@4, not switch@4. And i guess config-space was also
> something shorter in the past.
>
> I think your idea of cropping from the beginning, not the end, is in
> general a good solution. However, is there any danger of
>
> cp0!config-space@...00000!mdio@...200!ethernet-switch@...dio-external
>
> and
>
> cp1!config-space@...00000!mdio@...200!ethernet-switch@...dio-external
Since these will appear as links in /sys/bus/mdio_bus/devices,
this danger exists.
If the prefix is too similar, we can run into duplicates also when
cropping from the front.

So we could crop at the front and reduce likelihood of this situation,
but (imo) should still print a warning since it is not working as intended.

>
> I assume the two instances of cp have the same peripherals, at the
> same address?
In this particular platform the config-space layer uses the actual base address for each cp:
cp0!config-space@...00000
cp1!config-space@...00000
cp2!config-space@...00000
>
> Another option would be if the name needs to be truncated, use the
> fallback as if there was no np:
>
>                 bus->name = "mv88e6xxx SMI";
>                 snprintf(bus->id, MII_BUS_ID_SIZE, "mv88e6xxx-%d", index++);
>
> That at least gives you unique names.
This ensures a unique suffix within a branch of device-tree.
It could still collide with same structure on a cp1 or cp2.


The platform where this is triggered does not (currently) require declaring
external mdio bus (because hardware bug renders that bus useless).
For now my priority is helping future developers running into this.