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

[Dirk Behme <dirk.behme@gmail.com>]

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.

Best regards

Dirk