rust: debugfs: Add initial support for directories

Adds a `debugfs::Dir` type that can be used to create and remove
DebugFS directories. The `Dir` handle automatically cleans up the
directory on `Drop`.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250904-debugfs-rust-v11-1-7d12a165685a@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 82 insertions, 0 deletions

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
new file mode 100644
index 000000000000..65be71600b8e
--- /dev/null
+++ b/rust/kernel/debugfs.rs
@@ -0,0 +1,82 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2025 Google LLC.
+
+//! DebugFS Abstraction
+//!
+//! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
+
+// When DebugFS is disabled, many parameters are dead. Linting for this isn't helpful.
+#![cfg_attr(not(CONFIG_DEBUG_FS), allow(unused_variables))]
+
+#[cfg(CONFIG_DEBUG_FS)]
+use crate::prelude::*;
+use crate::str::CStr;
+#[cfg(CONFIG_DEBUG_FS)]
+use crate::sync::Arc;
+
+#[cfg(CONFIG_DEBUG_FS)]
+mod entry;
+#[cfg(CONFIG_DEBUG_FS)]
+use entry::Entry;
+
+/// Owning handle to a DebugFS directory.
+///
+/// The directory in the filesystem represented by [`Dir`] will be removed when handle has been
+/// dropped *and* all children have been removed.
+// If we have a parent, we hold a reference to it in the `Entry`. This prevents the `dentry`
+// we point to from being cleaned up if our parent `Dir`/`Entry` is dropped before us.
+//
+// The `None` option indicates that the `Arc` could not be allocated, so our children would not be
+// able to refer to us. In this case, we need to silently fail. All future child directories/files
+// will silently fail as well.
+#[derive(Clone)]
+pub struct Dir(#[cfg(CONFIG_DEBUG_FS)] Option<Arc<Entry>>);
+
+impl Dir {
+    /// Create a new directory in DebugFS. If `parent` is [`None`], it will be created at the root.
+    fn create(name: &CStr, parent: Option<&Dir>) -> Self {
+        #[cfg(CONFIG_DEBUG_FS)]
+        {
+            let parent_entry = match parent {
+                // If the parent couldn't be allocated, just early-return
+                Some(Dir(None)) => return Self(None),
+                Some(Dir(Some(entry))) => Some(entry.clone()),
+                None => None,
+            };
+            Self(
+                // If Arc creation fails, the `Entry` will be dropped, so the directory will be
+                // cleaned up.
+                Arc::new(Entry::dynamic_dir(name, parent_entry), GFP_KERNEL).ok(),
+            )
+        }
+        #[cfg(not(CONFIG_DEBUG_FS))]
+        Self()
+    }
+
+    /// Create a new directory in DebugFS at the root.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// let debugfs = Dir::new(c_str!("parent"));
+    /// ```
+    pub fn new(name: &CStr) -> Self {
+        Dir::create(name, None)
+    }
+
+    /// Creates a subdirectory within this directory.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// let parent = Dir::new(c_str!("parent"));
+    /// let child = parent.subdir(c_str!("child"));
+    /// ```
+    pub fn subdir(&self, name: &CStr) -> Self {
+        Dir::create(name, Some(self))
+    }
+}