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

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Date: Thu, 21 Mar 2024 16:10:34 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Josua Mayer <josua@...id-run.com>
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

On Thu, Mar 21, 2024 at 10:26:54AM +0000, Josua Mayer wrote:
> 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.

The problem with a warning is, what do you tell people who ask how to
fix the warning? Hack your device tree to short the node names?

A warning is best done when there is something which can be done to
fix the problem. If it is not fixable, it is just noise.

> > 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.

static int mv88e6xxx_mdio_register(struct mv88e6xxx_chip *chip,
                                   struct device_node *np,
                                   bool external)
{
        static int index;
        struct mv88e6xxx_mdio_bus *mdio_bus;
        struct mii_bus *bus;
        int err;

index is static, so it is simply a counter. So you should get the
names mv88e6xxx-0, mv88e6xxx-1, mv88e6xxx-2, mv88e6xxx-3...

	Andrew