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Date: Sun, 05 May 2024 12:11:57 -0700
From: Florian Fainelli <f.fainelli@gmail.com>
To: Josua Mayer <josua@solid-run.com>, Andrew Lunn <andrew@lunn.ch>,
 Vladimir Oltean <olteanv@gmail.com>, "David S. Miller" <davem@davemloft.net>,
 Eric Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>,
 Paolo Abeni <pabeni@redhat.com>
CC: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
 Mor Nagli <mor.nagli@solid-run.com>
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_net-next_v3=5D_net=3A_dsa=3A_mv88e6xxx=3A_?=
 =?US-ASCII?Q?control_mdio_bus-id_truncation_for_long_paths?=
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Le 5 mai 2024 02:52:45 GMT-07:00, Josua Mayer <josua@solid-run=2Ecom> a =C3=
=A9crit=C2=A0:
>mv88e6xxx supports multiple mdio buses as children, e=2Eg=2E to model bot=
h
>internal and external phys=2E If the child buses mdio ids are truncated,
>they might collide with each other leading to an obscure error from
>kobject_add=2E
>
>The maximum length of bus id is currently defined as 61
>(MII_BUS_ID_SIZE)=2E Truncation can occur on platforms with long node
>names and multiple levels before the parent bus on which the dsa switch
>itself sits, e=2Eg=2E CN9130 [1]=2E
>
>Compare the return value of snprintf against maximum bus-id length to
>detect truncation=2E In that case write an incrementing marker to the end
>to avoid name collisions=2E
>This changes the problematic bus-ids mdio and mdio-external from [1]
>to [2]=2E
>
>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=2E
>
>[1]
>[    8=2E324631] mv88e6085 f212a200=2Emdio-mii:04: switch 0x1760 detected=
: Marvell 88E6176, revision 1
>[    8=2E389516] mv88e6085 f212a200=2Emdio-mii:04: Truncated bus-id may c=
ollide=2E
>[    8=2E592367] mv88e6085 f212a200=2Emdio-mii:04: Truncated bus-id may c=
ollide=2E
>[    8=2E623593] sysfs: cannot create duplicate filename '/devices/platfo=
rm/cp0/cp0:config-space@f2000000/f212a200=2Emdio/mdio_bus/f212a200=2Emdio-m=
ii/f212a200=2Emdio-mii:04/mdio_bus/!cp0!config-space@f2000000!mdio@12a200!e=
thernet-switch@4!mdi'
>[    8=2E785480] kobject: kobject_add_internal failed for !cp0!config-spa=
ce@f2000000!mdio@12a200!ethernet-switch@4!mdi with -EEXIST, don't try to re=
gister things with the same name in the same directory=2E
>[    8=2E936514] libphy: mii_bus /cp0/config-space@f2000000/mdio@12a200/e=
thernet-switch@4/mdi failed to register
>[    8=2E946300] mdio_bus !cp0!config-space@f2000000!mdio@12a200!ethernet=
-switch@4!mdi: __mdiobus_register: -22
>[    8=2E956003] mv88e6085 f212a200=2Emdio-mii:04: Cannot register MDIO b=
us (-22)
>[    8=2E965329] mv88e6085: probe of f212a200=2Emdio-mii:04 failed with e=
rror -22
>
>[2]
>/devices/platform/cp0/cp0:config-space@f2000000/f212a200=2Emdio/mdio_bus/=
f212a200=2Emdio-mii/f212a200=2Emdio-mii:04/mdio_bus/!cp0!config-space@f2000=
000!mdio@12a200!ethernet-switch=2E=2E=2E!-0
>/devices/platform/cp0/cp0:config-space@f2000000/f212a200=2Emdio/mdio_bus/=
f212a200=2Emdio-mii/f212a200=2Emdio-mii:04/mdio_bus/!cp0!config-space@f2000=
000!mdio@12a200!ethernet-switch=2E=2E=2E!-1
>
>Signed-off-by: Josua Mayer <josua@solid-run=2Ecom>
>---

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

If we prefer a driver based solution, the mii_bus object could carry a tru=
ncation format, at the risk of creating more variation between drivers in c=
ase of truncation=2E We could also wait until we have another driver requir=
ing a similar solution before promoting this to a wider range=2E


Florian