[PATCH v7 6/6] rust: samples: Add debugfs sample

[Matthew Maurer <mmaurer@google.com>]

Provides an example of using the Rust DebugFS bindings.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 MAINTAINERS                  |  1 +
 samples/rust/Kconfig         | 11 +++++++
 samples/rust/Makefile        |  1 +
 samples/rust/rust_debugfs.rs | 76 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 89 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index 551e3a6a16d9051a2d58a77614c458287da2bdcc..f55c99cb2907655d109cb1fa248cdec803b5ce9a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7374,6 +7374,7 @@ F:	rust/kernel/devres.rs
 F:	rust/kernel/driver.rs
 F:	rust/kernel/faux.rs
 F:	rust/kernel/platform.rs
+F:	samples/rust/rust_debugfs.rs
 F:	samples/rust/rust_driver_platform.rs
 F:	samples/rust/rust_driver_faux.rs
 
diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
index 7f7371a004ee0a8f67dca99c836596709a70c4fa..01101db41ae31b08a86d048cdd27da8ef9bb23a2 100644
--- a/samples/rust/Kconfig
+++ b/samples/rust/Kconfig
@@ -62,6 +62,17 @@ config SAMPLE_RUST_DMA
 
 	  If unsure, say N.
 
+config SAMPLE_RUST_DEBUGFS
+	tristate "DebugFS Test Module"
+	depends on DEBUG_FS
+	help
+	  This option builds the Rust DebugFS Test module sample.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_debugfs.
+
+	  If unsure, say N.
+
 config SAMPLE_RUST_DRIVER_PCI
 	tristate "PCI Driver"
 	depends on PCI
diff --git a/samples/rust/Makefile b/samples/rust/Makefile
index bd2faad63b4f3befe7d1ed5139fe25c7a8b6d7f6..61276222a99f8cc6d7f84c26d0533b30815ebadd 100644
--- a/samples/rust/Makefile
+++ b/samples/rust/Makefile
@@ -4,6 +4,7 @@ ccflags-y += -I$(src)				# needed for trace events
 obj-$(CONFIG_SAMPLE_RUST_MINIMAL)		+= rust_minimal.o
 obj-$(CONFIG_SAMPLE_RUST_MISC_DEVICE)		+= rust_misc_device.o
 obj-$(CONFIG_SAMPLE_RUST_PRINT)			+= rust_print.o
+obj-$(CONFIG_SAMPLE_RUST_DEBUGFS)		+= rust_debugfs.o
 obj-$(CONFIG_SAMPLE_RUST_DMA)			+= rust_dma.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_PCI)		+= rust_driver_pci.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_PLATFORM)	+= rust_driver_platform.o
diff --git a/samples/rust/rust_debugfs.rs b/samples/rust/rust_debugfs.rs
new file mode 100644
index 0000000000000000000000000000000000000000..6af9177c711a07764f0323e03a72d115287f1205
--- /dev/null
+++ b/samples/rust/rust_debugfs.rs
@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2025 Google LLC.
+
+//! Sample DebugFS exporting module
+
+use core::sync::atomic::{AtomicU32, Ordering};
+use kernel::c_str;
+use kernel::debugfs::{Dir, File};
+use kernel::prelude::*;
+use kernel::sync::{new_mutex, Arc};
+
+module! {
+    type: RustDebugFs,
+    name: "rust_debugfs",
+    authors: ["Matthew Maurer"],
+    description: "Rust DebugFS usage sample",
+    license: "GPL",
+}
+
+struct RustDebugFs {
+    // As we only hold these for drop effect (to remove the directory/files) we have a leading
+    // underscore to indicate to the compiler that we don't expect to use this field directly.
+    _debugfs: Dir,
+    _subdir: Dir,
+    _file: File,
+    _file_2: File,
+}
+
+static EXAMPLE: AtomicU32 = AtomicU32::new(8);
+
+impl kernel::Module for RustDebugFs {
+    fn init(_this: &'static ThisModule) -> Result<Self> {
+        // Create a debugfs directory in the root of the filesystem called "sample_debugfs".
+        let debugfs = Dir::new(c_str!("sample_debugfs"));
+
+        // Create a subdirectory, so "sample_debugfs/subdir" now exists.
+        let sub = debugfs.subdir(c_str!("subdir"));
+
+        // Create a single file in the subdirectory called "example" that will read from the
+        // `EXAMPLE` atomic variable.
+        // We `forget` the result to avoid deleting the file at the end of the scope.
+        let file = sub.fmt_file(c_str!("example"), &EXAMPLE, &|example, f| {
+            writeln!(f, "Reading atomic: {}", example.load(Ordering::Relaxed))
+        });
+        // Now, "sample_debugfs/subdir/example" will print "Reading atomic: 8\n" when read.
+
+        // Change the value in the variable displayed by the file. This is intended to demonstrate
+        // that the module can continue to change the value used by the file.
+        EXAMPLE.store(10, Ordering::Relaxed);
+        // Now, "sample_debugfs/subdir/example" will print "Reading atomic: 10\n" when read.
+
+        // In addition to globals, we can also attach any kind of owned data. Most commonly, this
+        // will look like an `Arc<MyObject>` as those can be shared with the rest of the module.
+        let my_arc = Arc::pin_init(new_mutex!(10), GFP_KERNEL)?;
+        // An `Arc<Mutex<usize>>` doesn't implement display, so let's give explicit instructions on
+        // how to print it
+        let file_2 = sub.fmt_file(c_str!("arc_backed"), my_arc.clone(), &|val, f| {
+            writeln!(f, "locked value: {:#010x}", *val.lock())
+        });
+
+        // Since it's an `Arc` and we cloned it, we continue to have access to `my_arc`. If this
+        // were real, we'd probably stash it in our module struct and do something with it when
+        // handling real calls.
+        *my_arc.lock() = 99;
+
+        // Save the handles we want to preserve to our module object. They will be automatically
+        // cleaned up when our module is unloaded.
+        Ok(Self {
+            _debugfs: debugfs,
+            _subdir: sub,
+            _file: file,
+            _file_2: file_2,
+        })
+    }
+}

-- 
2.50.0.714.g196bf9f422-goog