Rather than always using Display, allow hooking arbitrary functions to
arbitrary files. Display technically has the expressiveness to do this,
but requires a new type be declared for every different way to render
things, which can be very clumsy.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 rust/kernel/debugfs.rs              | 41 ++++++++++++++++++++++++++
 rust/kernel/debugfs/display_file.rs | 57 ++++++++++++++++++++++++++++++++++++-
 2 files changed, 97 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index cd83f21cf2818f406575941ebbc6c426575643e4..78544653b46d959c1520b1210170d22a0e973989 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -10,6 +10,7 @@
 use crate::str::CStr;
 #[cfg(CONFIG_DEBUG_FS)]
 use crate::sync::Arc;
+use core::fmt;
 use core::fmt::Display;
 use core::ops::Deref;
 
@@ -153,6 +154,46 @@ pub fn display_file<D: Deref<Target = T> + 'static + Send + Sync, T: Display>(
         self.create_file(name, data)
     }
 
+    /// Create a file in a DebugFS directory with the provided name, and contents from invoking `f`
+    /// on the provided reference.
+    ///
+    /// `f` must be a function item or a non-capturing closure, or this will fail to compile.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use core::sync::atomic::{AtomicU32, Ordering};
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// let dir = Dir::new(c_str!("foo"));
+    /// static MY_ATOMIC: AtomicU32 = AtomicU32::new(3);
+    /// let file = dir.fmt_file(c_str!("bar"), &MY_ATOMIC, &|val, f| {
+    ///   let out = val.load(Ordering::Relaxed);
+    ///   writeln!(f, "{out:#010x}")
+    /// });
+    /// MY_ATOMIC.store(10, Ordering::Relaxed);
+    /// ```
+    pub fn fmt_file<
+        D: Deref<Target = T> + 'static + Send + Sync,
+        T: Sync,
+        F: Fn(&T, &mut fmt::Formatter<'_>) -> fmt::Result + Send + Sync,
+    >(
+        &self,
+        name: &CStr,
+        data: D,
+        f: &'static F,
+    ) -> File {
+        #[cfg(CONFIG_DEBUG_FS)]
+        let data_adapted = display_file::FormatAdapter::new(data, f);
+        #[cfg(not(CONFIG_DEBUG_FS))]
+        let data_adapted = {
+            // Mark used
+            let (_, _) = (data, f);
+            &0
+        };
+        self.display_file(name, data_adapted)
+    }
+
     /// Create a new directory in DebugFS at the root.
     ///
     /// # Examples
diff --git a/rust/kernel/debugfs/display_file.rs b/rust/kernel/debugfs/display_file.rs
index 65e37e34d7b587482492dc296637a3bc517b9fe3..75f2e1c560840324117bc1bc73aa89e3823aaf0e 100644
--- a/rust/kernel/debugfs/display_file.rs
+++ b/rust/kernel/debugfs/display_file.rs
@@ -3,7 +3,9 @@
 
 use crate::seq_file::SeqFile;
 use crate::seq_print;
-use core::fmt::Display;
+use core::fmt::{Display, Formatter, Result};
+use core::marker::PhantomData;
+use core::ops::Deref;
 
 /// Implements `open` for `file_operations` via `single_open` to fill out a `seq_file`.
 ///
@@ -58,3 +60,56 @@ pub(crate) trait DisplayFile: Display + Sized {
 }
 
 impl<T: Display + Sized> DisplayFile for T {}
+
+/// Adapter to implement `Display` via a callback with the same representation as `T`.
+///
+/// # Invariants
+///
+/// If an instance for `FormatAdapter<_, F>` is constructed, `F` is inhabited.
+#[repr(transparent)]
+pub(crate) struct FormatAdapter<D, F> {
+    inner: D,
+    _formatter: PhantomData<F>,
+}
+
+impl<D, F> FormatAdapter<D, F> {
+    pub(crate) fn new(inner: D, _f: &'static F) -> Self {
+        // INVARIANT: We were passed a reference to F, so it is inhabited.
+        FormatAdapter {
+            inner,
+            _formatter: PhantomData,
+        }
+    }
+}
+
+impl<D: Deref<Target = T>, T, F> Display for FormatAdapter<D, F>
+where
+    F: Fn(&T, &mut Formatter<'_>) -> Result + 'static,
+{
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
+        // SAFETY: FormatAdapter<_, F> can only be constructed if F is inhabited
+        let f: &F = unsafe { materialize_zst_fmt() };
+        f(&self.inner, fmt)
+    }
+}
+
+impl<D, F> Deref for FormatAdapter<D, F> {
+    type Target = Self;
+    fn deref(&self) -> &Self {
+        self
+    }
+}
+
+/// For types with a unique value, produce a static reference to it.
+///
+/// # Safety
+///
+/// The caller asserts that F is inhabited
+unsafe fn materialize_zst_fmt<F>() -> &'static F {
+    const { assert!(core::mem::size_of::<F>() == 0) };
+    let zst_dangle: core::ptr::NonNull<F> = core::ptr::NonNull::dangling();
+    // SAFETY: While the pointer is dangling, it is a dangling pointer to a ZST, based on the
+    // assertion above. The type is also inhabited, by the caller's assertion. This means
+    // we can materialize it.
+    unsafe { zst_dangle.as_ref() }
+}

-- 
2.50.0.rc2.696.g1fc2a0284f-goog