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

[Josua Mayer <josua@solid-run.com>]

Am 20.03.24 um 15:09 schrieb Jiri Pirko:
> Wed, Mar 20, 2024 at 02:48:55PM CET, josua@solid-run.com wrote:
>> 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 which 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 whiich the dsa switch
> s/whiich/which/
>
>
>> sits such as on CN9130 [1].
>>
>> Test whether the return value of snprintf exceeds the maximum bus id
>> length and print a warning.
>>
>> [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@f2000000/f212a200.mdio/mdio_bus/f212a200.mdio-mii/f212a200.mdio-mii:04/mdio_bus/!cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi'
>> [    8.785480] kobject: kobject_add_internal failed for !cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi with -EEXIST, don't try to register things with the same name in the same directory.
>> [    8.936514] libphy: mii_bus /cp0/config-space@f2000000/mdio@12a200/ethernet-switch@4/mdi failed to register
>> [    8.946300] mdio_bus !cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi: __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
>>
>> Signed-off-by: Josua Mayer <josua@solid-run.com>
> This is not bug fix, assume you target net-next. Please:
> 1) Next time, indicate that in the patch subject like this:
>    [patch net-next] xxx
> 2) net-next is currently closed, repost next week.
Correct, thanks - will do.
Just for future reference for those occasional contributors -
is there such a thing as an lkml calendar?
>
>> ---
>> drivers/net/dsa/mv88e6xxx/chip.c | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
>> index 614cabb5c1b0..1c40f7631ab1 100644
>> --- a/drivers/net/dsa/mv88e6xxx/chip.c
>> +++ b/drivers/net/dsa/mv88e6xxx/chip.c
>> @@ -3731,10 +3731,12 @@ static int mv88e6xxx_mdio_register(struct mv88e6xxx_chip *chip,
>>
>> 	if (np) {
>> 		bus->name = np->full_name;
>> -		snprintf(bus->id, MII_BUS_ID_SIZE, "%pOF", np);
>> +		if (snprintf(bus->id, MII_BUS_ID_SIZE, "%pOF", np) >= MII_BUS_ID_SIZE)
>> +			dev_warn(chip->dev, "Truncated bus-id may collide.\n");
> How about instead of warn&fail fallback to some different name in this
> case?
Duplicate could be avoided by truncating from the start,
however I don't know if that is a good idea.
It affects naming of paths in sysfs, and the root cause is
difficult to spot.
>> 	} else {
>> 		bus->name = "mv88e6xxx SMI";
>> -		snprintf(bus->id, MII_BUS_ID_SIZE, "mv88e6xxx-%d", index++);
>> +		if (snprintf(bus->id, MII_BUS_ID_SIZE, "mv88e6xxx-%d", index++) >= MII_BUS_ID_SIZE)
> How exactly this may happen?
It can happen on switch nodes at deep levels in the device-tree,
while describing both internal and external mdio buses of a switch.
E.g. Documentation/devicetree/bindings/net/dsa/marvell,mv88e6xxx.yaml

On CN9130 platform device-tree looks like this:

/ {
    cp0 {
        config-space@f2000000 {
            mdio@12a200 {
                ethernet-switch@4 {
                    mdio { ... };
                    mdio-external { ... };
                };
            };
        };
    };
};

For mdio-external child all the names alone, without separators,
make up 66 characters, exceeding: MII_BUS_ID_SIZE:
cp0config-space@f2000000mdio@12a200ethernet-switch@4mdio-external

With separators ('!') we have:
cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdio
cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdio-external
Truncated to MII_BUS_ID_SIZE:
cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi
cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi
They become duplicates.

>> +			dev_warn(chip->dev, "Truncated bus-id may collide.\n");
Another option (imo) is to force the issue and return error code.
Then the only way out would be increase of MII_BUS_ID_SIZE.
>> 	}
>>
>> 	bus->read = mv88e6xxx_mdio_read;
>>
>> ---
>> base-commit: e8f897f4afef0031fe618a8e94127a0934896aba
>> change-id: 20240320-mv88e6xxx-truncate-busid-34a1d2769bbf
>>
>> Sincerely,
>> -- 
>> Josua Mayer <josua@solid-run.com>
>>
>>