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

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Wed, Jul 08 2026, "Guenter Roeck" <linux@roeck-us.net> 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 <linux@rasmusvillemoes.dk>
>> ---
>>
>> 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).

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?

Rasmus