netdev - Re: [PATCH net-next v3] net: dsa: mv88e6xxx: control mdio bus-id truncation for long paths

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Message-ID: <A40C71BD-A733-43D2-A563-FEB1322ECB5C@gmail.com>
Date: Sun, 05 May 2024 12:11:57 -0700
From: Florian Fainelli <f.fainelli@...il.com>
To: Josua Mayer <josua@...id-run.com>, Andrew Lunn <andrew@...n.ch>,
 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>
CC: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Mor Nagli <mor.nagli@...id-run.com>
Subject: Re: [PATCH net-next v3] net: dsa: mv88e6xxx: control mdio bus-id truncation for long paths

Le 5 mai 2024 02:52:45 GMT-07:00, Josua Mayer <josua@...id-run.com> a écrit :
>mv88e6xxx supports multiple mdio buses as children, e.g. to model both
>internal and external phys. If the child buses mdio ids are truncated,
>they might collide with each other leading to an obscure error from
>kobject_add.
>
>The maximum length of bus id is currently defined as 61
>(MII_BUS_ID_SIZE). Truncation can occur on platforms with long node
>names and multiple levels before the parent bus on which the dsa switch
>itself sits, e.g. CN9130 [1].
>
>Compare the return value of snprintf against maximum bus-id length to
>detect truncation. In that case write an incrementing marker to the end
>to avoid name collisions.
>This changes the problematic bus-ids mdio and mdio-external from [1]
>to [2].
>
>Truncation at the beginning was considered as a workaround, however that
>is still subject to name collisions in sysfs where only the first
>characters differ.
>
>[1]
>[    8.324631] mv88e6085 f212a200.mdio-mii:04: switch 0x1760 detected: Marvell 88E6176, revision 1
>[    8.389516] mv88e6085 f212a200.mdio-mii:04: Truncated bus-id may collide.
>[    8.592367] mv88e6085 f212a200.mdio-mii:04: Truncated bus-id may collide.
>[    8.623593] sysfs: cannot create duplicate filename '/devices/platform/cp0/cp0:config-space@...00000/f212a200.mdio/mdio_bus/f212a200.mdio-mii/f212a200.mdio-mii:04/mdio_bus/!cp0!config-space@...00000!mdio@...200!ethernet-switch@...di'
>[    8.785480] kobject: kobject_add_internal failed for !cp0!config-space@...00000!mdio@...200!ethernet-switch@...di with -EEXIST, don't try to register things with the same name in the same directory.
>[    8.936514] libphy: mii_bus /cp0/config-space@...00000/mdio@...200/ethernet-switch@...di failed to register
>[    8.946300] mdio_bus !cp0!config-space@...00000!mdio@...200!ethernet-switch@...di: __mdiobus_register: -22
>[    8.956003] mv88e6085 f212a200.mdio-mii:04: Cannot register MDIO bus (-22)
>[    8.965329] mv88e6085: probe of f212a200.mdio-mii:04 failed with error -22
>
>[2]
>/devices/platform/cp0/cp0:config-space@...00000/f212a200.mdio/mdio_bus/f212a200.mdio-mii/f212a200.mdio-mii:04/mdio_bus/!cp0!config-space@...00000!mdio@...200!ethernet-switch...!-0
>/devices/platform/cp0/cp0:config-space@...00000/f212a200.mdio/mdio_bus/f212a200.mdio-mii/f212a200.mdio-mii:04/mdio_bus/!cp0!config-space@...00000!mdio@...200!ethernet-switch...!-1
>
>Signed-off-by: Josua Mayer <josua@...id-run.com>
>---

The idea and implementation is reasonable but this could affect other drivers than mv88e6xxx, why not move that logic to mdiobus_register() and tracking the truncation index globally within the MDIO bus layer?

If we prefer a driver based solution, the mii_bus object could carry a truncation format, at the risk of creating more variation between drivers in case of truncation. We could also wait until we have another driver requiring a similar solution before promoting this to a wider range.

Florian