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

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

Am 20.03.24 um 17:03 schrieb Jiri Pirko:
> Wed, Mar 20, 2024 at 03:33:24PM CET, josua@solid-run.com wrote:
>> 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,
> Read again, my question is about the else branch.
Oh!
The else case occurs when the switch node does
not have a child node named "mdio":

    /* Always register one mdio bus for the internal/default mdio

     * bus. This maybe represented in the device tree, but is
     * optional.
     */
    child = of_get_child_by_name(np, "mdio");
    err = mv88e6xxx_mdio_register(chip, child, false);
    of_node_put(child);

In this case child is passed to np as NULL.

For example if we have no "mdio" child node, but do have "mdio-external",
the else case creates an mdio bus named "mv88e6xxx-%d".

Then we would end up - in my example - with these two:
cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mv88e6xxx-0
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!mv8
cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi

They are not duplicates, but too close for comfort.
Different device may exceed maximum size again.

>> 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>
>>>>
>>>>