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

[Danilo Krummrich <dakr@kernel.org>]

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.