Re: [PATCH v3 3/7] rust: property: Introduce PropertyGuard

[Danilo Krummrich <dakr@kernel.org>]

On Sun, Apr 27, 2025 at 08:11:58AM +0200, Dirk Behme wrote:
> On 26.04.25 12:15, Danilo Krummrich wrote:
> > On Sat, Apr 26, 2025 at 08:19:09AM +0200, Dirk Behme wrote:
> >> On 25.04.25 17:35, Danilo Krummrich wrote:
> >>> On Fri, Apr 25, 2025 at 05:01:26PM +0200, Remo Senekowitsch wrote:
> >>>> +impl<T> PropertyGuard<'_, '_, T> {
> >>>> +    /// Access the property, indicating it is required.
> >>>> +    ///
> >>>> +    /// If the property is not present, the error is automatically logged. If a
> >>>> +    /// missing property is not an error, use [`Self::optional`] instead.
> >>>> +    pub fn required(self) -> Result<T> {
> >>>> +        if self.inner.is_err() {
> >>>> +            pr_err!(
> >>>> +                "{}: property '{}' is missing\n",
> >>>> +                self.fwnode.display_path(),
> >>>> +                self.name
> >>>> +            );
> >>>
> >>> Hm, we can't use the device pointer of the fwnode_handle, since it is not
> >>> guaranteed to be valid, hence the pr_*() print...
> >>>
> >>> Anyways, I'm not sure we need to print here at all. If a driver wants to print
> >>> that it is unhappy about a missing required property it can do so by itself, I
> >>> think.
> >>
> >> Hmm, the driver said by using 'required' that it *is* required. So a
> >> missing property is definitely an error here. Else it would have used
> >> 'optional'. Which doesn't print in case the property is missing.
> >>
> >> If I remember correctly having 'required' and 'optional' is the result
> >> of some discussion on Zulip. And one conclusion of that discussion was
> >> to move checking & printing the error out of the individual drivers
> >> into a central place to avoid this error checking & printing in each
> >> and every driver. I think the idea is that the drivers just have to do
> >> ...required()?; and that's it, then.
> > 
> > Yes, I get the idea.
> > 
> > If it'd be possible to use dev_err!() instead I wouldn't object in this specific
> > case. But this code is used by drivers from probe(), hence printing the error
> > without saying for which device it did occur is a bit pointless.
> 
> Thinking a little about this, yes, we don't know the device here. But:
> Does the device matter here?

If the above fails it means that for a (specific) device a driver expects that
a specific property of some firmware node is present. So, yes, I think it does
matter.

> There is nothing wrong with the (unknown)
> device, no? What is wrong here is the firmware (node). It misses
> something.

How do we know the firmware node is wrong? Maybe the driver has wrong
expectations for this device?

> And this is exactly what the message tells: "There is an
> error due to the missing node 'name' in 'path', please fix it". That
> should be sufficient to identify the firmware/device tree description
> and fix it.

I think we can't always fix them, even if they're wrong. How do we fix ACPI
firmware nodes for instance?

(Software nodes provide a solution for that, see also commit 59abd83672f7
("drivers: base: Introducing software nodes to the firmware node framework").)