rust: device: Add child accessor and iterator

Allow Rust drivers to access children of a fwnode either by name or by
iterating over all of them.

In C, there is the function `fwnode_get_next_child_node` for iteration
and the macro `fwnode_for_each_child_node` that helps with handling the
pointers. Instead of a macro, a native iterator is used in Rust such
that regular for-loops can be used.

Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>

Author: senekor

rust/kernel/device/property.rs

@@ -190,6 +190,62 @@ impl FwNode {
             name,
         }
     }
+
+    /// Returns first matching named child node handle.
+    pub fn get_child_by_name(&self, name: &CStr) -> Option<ARef<Self>> {
+        // SAFETY: `self` and `name` are valid by their type invariants.
+        let child =
+            unsafe { bindings::fwnode_get_named_child_node(self.as_raw(), name.as_char_ptr()) };
+        if child.is_null() {
+            return None;
+        }
+        // SAFETY:
+        // - `fwnode_get_named_child_node` returns a pointer with its refcount
+        //   incremented.
+        // - That increment is relinquished, i.e. the underlying object is not
+        //   used anymore except via the newly created `ARef`.
+        Some(unsafe { Self::from_raw(child) })
+    }
+
+    /// Returns an iterator over a node's children.
+    pub fn children<'a>(&'a self) -> impl Iterator<Item = ARef<FwNode>> + 'a {
+        let mut prev: Option<ARef<FwNode>> = None;
+
+        core::iter::from_fn(move || {
+            let prev_ptr = match prev.take() {
+                None => ptr::null_mut(),
+                Some(prev) => {
+                    // We will pass `prev` to `fwnode_get_next_child_node`,
+                    // which decrements its refcount, so we use
+                    // `ARef::into_raw` to avoid decrementing the refcount
+                    // twice.
+                    let prev = ARef::into_raw(prev);
+                    prev.as_ptr().cast()
+                }
+            };
+            // SAFETY:
+            // - `self.as_raw()` is valid by its type invariant.
+            // - `prev_ptr` may be null, which is allowed and corresponds to
+            //   getting the first child. Otherwise, `prev_ptr` is valid, as it
+            //   is the stored return value from the previous invocation.
+            // - `prev_ptr` has its refount incremented.
+            // - The increment of `prev_ptr` is relinquished, i.e. the
+            //   underlying object won't be used anymore.
+            let next = unsafe { bindings::fwnode_get_next_child_node(self.as_raw(), prev_ptr) };
+            if next.is_null() {
+                return None;
+            }
+            // SAFETY:
+            // - `next` is valid because `fwnode_get_next_child_node` returns a
+            //   pointer with its refcount incremented.
+            // - That increment is relinquished, i.e. the underlying object
+            //   won't be used anymore, except via the newly created
+            //   `ARef<Self>`.
+            let next = unsafe { FwNode::from_raw(next) };
+            prev = Some(next.clone());
+            Some(next)
+        })
+    }
 }
 
 // SAFETY: Instances of `FwNode` are always reference-counted.