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

[Guenter Roeck]

On 7/13/26 07:35, Rasmus Villemoes wrote:
> On Wed, Jul 08 2026, "Guenter Roeck" <linux@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >
> > > 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