Re: [PATCH] watchdog: take all OF aliases into account when assigning id

On 7/13/26 07:35, Rasmus Villemoes wrote:
> On Wed, Jul 08 2026, "Guenter Roeck" <[email protected]> wrote:
> 
>> On Mon, Jun 15, 2026 at 04:57:59PM +0200, Rasmus Villemoes wrote:
>>> If some, but not all, watchdog devices have device tree aliases, those
>>> without aliases might (depending on probe order) be assigned an id
>>> which would otherwise be assigned to one of those with an alias.
>>>
>>> This is problematic when for example watchdog0 is an alias for an
>>> always-running gpio watchdog that userspace must handle, but the SOC's
>>> watchdog device(s) get probed first and thus one of those become
>>> /dev/watchdog0.
>>>
>>> Ensure that ids for devices without a device tree alias are allocated
>>> from above the highest numbered alias, if any.
>>>
>>> Signed-off-by: Rasmus Villemoes <[email protected]>
>>> ---
>>>
>>> This is similar to how the mmc, i2c, i3c and spi subsystems handle
>>> device tree aliases and avoid using an id that might be assigned to a
>>> device/bus that is probed later.
>>
>> The patch makes sense. Unfortunately, there are systems with aliased
>> watchdogs which do not enable "watchdog0" (e.g., several Nuvoton based
>> boards). On such systems, if they do have an unaliased / auto-generated
>> watchdog, /dev/watchdog0 and with it /dev/watchdog would no longer be
>> created. This would result in a ABI break.
>>
>> On top of that, the patch only affects systems with both aliased and
>> un-aliased watchdogs, which makes me even more concerned.
> 
> Well, yes, the problem only occurs on exactly such systems.
> 
> - If all enabled watchdog devices have DT aliases, they all get their
> assigned id.
> 
> - If no wathcdog device has a DT alias, they'll just get sequentially
> assigned ids in probe order, and none of them will "accidentally" get an
> id that should be assigned to a device with a DT alias.
> 
>> To apply this or a similar patch, we would have to ensure that there
>> is no enabled watchdog with ID == 0.
> 
> I'm not sure I completely understand your concern(s), but I can see that
> if there is any watchdog DT alias, we'll never use id 0 except if there
> is a watchdog0 DT alias (and that device is actually enabled).
> 
... and if ID 0 is not used in that situaton, /dev/watchdog will not be
created since it is tied to /dev/watchdog0.

> What if instead of assigning dynamic ids from above the highest existing
> alias, we assign a dynamic id as usual, but skip existing aliases? So if
> there's a watchdog1 alias, but no watchdog0 alias, the first unaliased
> watchdog device being probed would become /dev/watchdog0 and hence
> /dev/watchdog. Would that work?
> 

Yes, since that would not change existing behavior.

Is this an problem that is actually observed on some system, or a theoretic
one ?

Thanks,
Guenter