rust: debugfs: Add support for callback-based files

Extends the `debugfs` API to support creating files with content
generated and updated by callbacks. This is done via the
`read_callback_file`, `write_callback_file`, and
`read_write_callback_file` methods.

These methods allow for more flexible file definition, either because
the type already has a `Writer` or `Reader` method that doesn't
do what you'd like, or because you cannot implement it (e.g. because
it's a type defined in another crate or a primitive type).

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-v11-4-7d12a165685a@google.com
[ Fix up Result<(), Error> -> Result. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/rust/kernel/debugfs/file_ops.rs b/rust/kernel/debugfs/file_ops.rs
index 2060c8d14d83..50fead17b6f3 100644
--- a/rust/kernel/debugfs/file_ops.rs
+++ b/rust/kernel/debugfs/file_ops.rs
@@ -2,6 +2,7 @@
 // Copyright (C) 2025 Google LLC.
 
 use super::{Reader, Writer};
+use crate::debugfs::callback_adapters::Adapter;
 use crate::prelude::*;
 use crate::seq_file::SeqFile;
 use crate::seq_print;
@@ -46,6 +47,13 @@ impl<T> FileOps<T> {
     }
 }
 
+impl<T: Adapter> FileOps<T> {
+    pub(super) const fn adapt(&self) -> &FileOps<T::Inner> {
+        // SAFETY: `Adapter` asserts that `T` can be legally cast to `T::Inner`.
+        unsafe { core::mem::transmute(self) }
+    }
+}
+
 #[cfg(CONFIG_DEBUG_FS)]
 impl<T> Deref for FileOps<T> {
     type Target = bindings::file_operations;