Re: [PATCH v7 5/9] rust: device: Introduce PropertyGuard

["Remo Senekowitsch" <remo@buenzli.dev>]

On Thu Jun 5, 2025 at 5:08 PM CEST, Rob Herring wrote:
> On Fri, May 30, 2025 at 09:28:52PM +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 | 59 ++++++++++++++++++++++++++++++++++
>>  1 file changed, 59 insertions(+)
>> 
>> diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
>> index 8e0414b0517e4..b789fbbd0e6cc 100644
>> --- a/rust/kernel/device/property.rs
>> +++ b/rust/kernel/device/property.rs
>> @@ -155,3 +155,62 @@ 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(),
>
> Is it possible to make "{self.fwnode}: property..." work? Just need to 
> implement Display trait on FwNode, right?
>
> Doesn't look to me like we can alter what we print like in C, but for 
> dmesg it's usually the full path we want anyways.

Well, field access is not allowed directly in the format string. But if
there is a variable `fwnode`, then yes, one can do "{fwnode}" directly.
Implementing Display directly on FwNode seems like a good idea if the
full path is the intuitive default.

I tried it with an `ARef<FwNode>`, but that didn't work. I had to force
the dereference with `&*fwnode`. Maybe an `impl<T: Display> for ARef<T>`
could be added, the standard library does that for its smart pointers.

Remo