Re: [PATCH v6 4/9] rust: device: Enable printing fwnode name and path

[Danilo Krummrich <dakr@kernel.org>]

On Sat, May 24, 2025 at 09:22:27PM +0200, Remo Senekowitsch wrote:
> +    /// Returns an object that implements [`Display`](core::fmt::Display) for
> +    /// printing the full path of a node.
> +    pub fn display_path(&self) -> impl core::fmt::Display + '_ {

While testing the code I found the following buggy print statement:

	"test-device@2/test-device@2/test-device@2/test-device@2: property 'test,u32-required-prop' is missing"

and I think the following changes fix this:

> +        struct FwNodeDisplayPath<'a>(&'a FwNode);
> +
> +        impl core::fmt::Display for FwNodeDisplayPath<'_> {
> +            fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
> +                // The logic here is the same as the one in lib/vsprintf.c
> +                // (fwnode_full_name_string).
> +
> +                // SAFETY: `self.0.as_raw()` is valid by its type invariant.
> +                let num_parents = unsafe { bindings::fwnode_count_parents(self.0.as_raw()) };
> +
> +                for depth in (0..=num_parents).rev() {
> +                    let fwnode = if depth == 0 {
> +                        self.0.as_raw()

	self.0

> +                    } else {
> +                        // SAFETY: `self.0.as_raw()` is valid.
> +                        unsafe { bindings::fwnode_get_nth_parent(self.0.as_raw(), depth) }

	let ptr = unsafe { bindings::fwnode_get_nth_parent(self.0.as_raw(), depth) };
	FwNode::as_ref(ptr)

where FwNode::as_ref() is

	unsafe fn as_ref<'a>(ptr: *mut bindings::fwnode_handle) -> &'a Self

> +                    };
> +
> +                    // SAFETY: `fwnode` is valid, it is either `self.0.as_raw()`
> +                    // or the return value of `bindings::fwnode_get_nth_parent`
> +                    // which returns a valid pointer to a `fwnode_handle` if the
> +                    // provided depth is within the valid range, which we know
> +                    // to be true.
> +                    let prefix = unsafe { bindings::fwnode_get_name_prefix(fwnode) };
> +                    if !prefix.is_null() {
> +                        // SAFETY: `fwnode_get_name_prefix` returns null or a
> +                        // valid C string.
> +                        let prefix = unsafe { CStr::from_char_ptr(prefix) };
> +                        write!(f, "{prefix}")?;
> +                    }
> +                    write!(f, "{}", self.0.display_name())?;

	write!(f, "{}", fwnode.0.display_name())?;

Otherwise we always write the name of self, no matter the actual parent depth.

> +
> +                    if depth != 0 {
> +                        // SAFETY:
> +                        // - `fwnode` is valid, because `depth` is
> +                        //   a valid depth of a parent of `self.0.as_raw()`.
> +                        // - `fwnode_get_nth_parent` increments the refcount and
> +                        //   we are responsible to decrement it.
> +                        unsafe { bindings::fwnode_handle_put(fwnode) }
> +                    }
> +                }
> +
> +                Ok(())
> +            }
> +        }
> +
> +        FwNodeDisplayPath(self)
> +    }