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

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

On 26.04.25 16:19, Danilo Krummrich wrote:
> On Sat, Apr 26, 2025 at 01:08:39PM +0200, Remo Senekowitsch wrote:
>> On Sat Apr 26, 2025 at 12:15 PM CEST, Danilo Krummrich wrote:
>>> 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.
>>
>> One alternative would be to store a reference count to the device in
>> `FwNode`. At that point we'd be guaranteed to have a valid reference
>> whenever we want to log something.
> 
> Yes, that would work. However, I'm not convinced that it's worth to store an
> ARef<Device> (i.e. take a device reference) in each FwNode structure *only* to
> be able to force an error print if a required device property isn't available.
> 
> Why do you think it is important to force this error print by having it in
> PropertyGuard::required() and even take an additional device reference for this
> purpose, rather than leaving it to the driver when to print a message for an
> error condition that makes it fail to probe()?


To my understanding doing the error print in "core" was proposed by
Rob [1]:

"If the property is missing and required, then we may want to print an
error msg (in the core, not every caller)"

Dirk

[1]
https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/DS90UB954.20driver.20done.2C.20ready.20to.20upstream.3F/near/496884813