[PATCH v2 3/9] rust: debugfs: add Entry::from_raw and ScopedDir::from_dentry

[Alvin Sun <alvin.sun@linux.dev>]

Add methods to construct debugfs abstractions from raw C dentry
pointers. Needed by DRM debugfs_init callback to create ScopedDir
from the root dentry provided by C core.

Signed-off-by: Alvin Sun <alvin.sun@linux.dev>
---
 rust/kernel/debugfs.rs       | 19 +++++++++++++++++++
 rust/kernel/debugfs/entry.rs | 15 ++++++++++++++-
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index dea0e2c953039..0f521762352ad 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -735,4 +735,23 @@ fn new(name: &CStr) -> ScopedDir<'data, 'static> {
             _phantom: PhantomData,
         }
     }
+
+    /// Creates a [`ScopedDir`] wrapping an existing debugfs dentry.
+    ///
+    /// Files created under this directory are not automatically removed on drop;
+    /// their lifetime is tied to the dentry owner.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure the dentry remains valid for the lifetime of the
+    /// returned `ScopedDir`.
+    pub unsafe fn from_dentry(dentry: *mut bindings::dentry) -> Self {
+        let _ = dentry;
+        ScopedDir {
+            #[cfg(CONFIG_DEBUG_FS)]
+            // SAFETY: The caller guarantees the dentry is valid and outlives this `ScopedDir`.
+            entry: ManuallyDrop::new(unsafe { Entry::from_raw(dentry) }),
+            _phantom: PhantomData,
+        }
+    }
 }
diff --git a/rust/kernel/debugfs/entry.rs b/rust/kernel/debugfs/entry.rs
index 46aad64896ecb..7643ff0fa6093 100644
--- a/rust/kernel/debugfs/entry.rs
+++ b/rust/kernel/debugfs/entry.rs
@@ -8,7 +8,7 @@
         CStr,
         CStrExt as _, //
     },
-    sync::Arc,
+    sync::Arc, //
 };
 
 use core::marker::PhantomData;
@@ -87,6 +87,19 @@ pub(crate) unsafe fn dynamic_file<T>(
 }
 
 impl<'a> Entry<'a> {
+    /// Wraps a raw dentry pointer.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure the dentry is valid and outlives this `Entry`.
+    pub(crate) unsafe fn from_raw(entry: *mut bindings::dentry) -> Self {
+        Self {
+            entry,
+            _parent: None,
+            _phantom: PhantomData,
+        }
+    }
+
     pub(crate) fn dir(name: &CStr, parent: Option<&'a Entry<'_>>) -> Self {
         let parent_ptr = match &parent {
             Some(entry) => entry.as_ptr(),

-- 
2.43.0