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

[Danilo Krummrich <dakr@kernel.org>]

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:
> >> This abstraction is a way to force users to specify whether a property
> >> is supposed to be required or not. This allows us to move error
> >> logging of missing required properties into core, preventing a lot of
> >> boilerplate in drivers.
> >>
> >> It will be used by upcoming methods for reading device properties.
> >>
> >> Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>
> >> ---
> >>  rust/kernel/device/property.rs | 57 ++++++++++++++++++++++++++++++++++
> >>  1 file changed, 57 insertions(+)
> >>
> >> diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
> >> index 28850aa3b..de31a1f56 100644
> >> --- a/rust/kernel/device/property.rs
> >> +++ b/rust/kernel/device/property.rs
> >> @@ -146,3 +146,60 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
> >>          unsafe { bindings::fwnode_handle_put(obj.cast().as_ptr()) }
> >>      }
> >>  }
> >> +
> >> +/// A helper for reading device properties.
> >> +///
> >> +/// Use [`Self::required`] if a missing property is considered a bug and
> >> +/// [`Self::optional`] otherwise.
> >> +///
> >> +/// For convenience, [`Self::or`] and [`Self::or_default`] are provided.
> >> +pub struct PropertyGuard<'fwnode, 'name, T> {
> >> +    /// The result of reading the property.
> >> +    inner: Result<T>,
> >> +    /// The fwnode of the property, used for logging in the "required" case.
> >> +    fwnode: &'fwnode FwNode,
> >> +    /// The name of the property, used for logging in the "required" case.
> >> +    name: &'name CStr,
> >> +}
> >> +
> >> +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.

Drivers can still decide to properly print the error if the returned Result
indicates one.