Re: [PATCH v4 5/9] rust: device: Introduce PropertyGuard

[Dirk Behme]

On 04/05/2025 19:31, 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@xxxxxxxxxxx>
> ---
>  rust/kernel/device/property.rs | 59 ++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
> index 6ccc7947f9c31..59c61e2493831 100644
> --- a/rust/kernel/device/property.rs
> +++ b/rust/kernel/device/property.rs
> @@ -123,3 +123,62 @@ 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_by`] 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. The
> +    /// device is required to associate the log with it.
> +    pub fn required_by(self, dev: &super::Device) -> Result<T> {
> +        if self.inner.is_err() {
> +            dev_err!(
> +                dev,
> +                "{}: property '{}' is missing\n",
> +                self.fwnode.display_path(),
> +                self.name
> +            );
> +        }
> +        self.inner
> +    }

Thinking about the .required_by(dev) I wonder if there will be cases
where we do *not* have a device? I.e. where we really have a fwnode,
only. And therefore can't pass a device. If we have such cases do we
need to be able to pass e.g. Option(dev) and switch back to pr_err() in
case of None?

>From the beginning of our discussion I think to remember that the C API
has both the fwnode_property_*() and device_property_*() because there
are use cases for the fwnode_property_*() API where is no device?

Thanks

Dirk