[PATCH v2 5/7] rust: drm: wrap fops open with RegistrationGuard

[Danilo Krummrich <dakr@kernel.org>]

Add a Minor abstraction with RAII release and a fops_open() wrapper that
holds a RegistrationGuard (drm_dev_enter / drm_dev_exit) across the
entire drm_open() call.

This guarantees that drm_dev_unplug() in Registration::drop() waits for
the full open sequence to complete, so all files are visible in the
filelist when iterating for cleanup.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/drm/device.rs | 63 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 62 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 3340fa0dc46b..b8444499b39c 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -42,6 +42,42 @@
     },
 };
 
+/// A reference to a `struct drm_minor` with RAII release.
+struct Minor(NonNull<bindings::drm_minor>);
+
+// Methods use `#[inline(never)]` to prevent the unexported `drm_minor_acquire()` /
+// `drm_minor_release()` symbols from being inlined into driver modules.
+impl Minor {
+    /// Acquire a minor by ID. Increments the underlying device's refcount.
+    #[inline(never)]
+    fn acquire(minor_id: u32) -> Result<Self> {
+        // SAFETY: `drm_minors_xa` is a valid global xarray; any `minor_id` is safe to
+        // look up (returns ERR_PTR on failure).
+        let ptr =
+            unsafe { bindings::drm_minor_acquire(&raw mut bindings::drm_minors_xa, minor_id) };
+        Ok(Self(NonNull::new(from_err_ptr(ptr)?).ok_or(ENODEV)?))
+    }
+
+    /// Returns a reference to the DRM device for this minor.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the minor belongs to a `Device<T>`.
+    unsafe fn device<T: drm::Driver>(&self) -> &Device<T, Userspace> {
+        // SAFETY: The minor is valid (from `drm_minor_acquire()`) and `minor->dev`
+        // is a valid `drm_device`. The caller guarantees it is a `Device<T>`.
+        unsafe { Device::from_raw((*self.0.as_ptr()).dev) }
+    }
+}
+
+impl Drop for Minor {
+    #[inline(never)]
+    fn drop(&mut self) {
+        // SAFETY: `self.0` came from `drm_minor_acquire()` and has not been released yet.
+        unsafe { bindings::drm_minor_release(self.0.as_ptr()) }
+    }
+}
+
 #[cfg(CONFIG_DRM_LEGACY)]
 macro_rules! drm_legacy_fields {
     ( $($field:ident: $val:expr),* $(,)? ) => {
@@ -198,11 +234,36 @@ const fn compute_features() -> u32 {
         fops: &Self::FOPS,
     };
 
+    /// Wrapper for `fops.open` that holds a [`RegistrationGuard`] across the entire `drm_open()`
+    /// call. This guarantees that `drm_dev_unplug()` in `Registration::drop()` waits for the full
+    /// open sequence.
+    extern "C" fn fops_open(inode: *mut bindings::inode, filp: *mut bindings::file) -> c_int {
+        let f = || -> Result<c_int> {
+            // SAFETY: `inode` is valid.
+            let minor_id = unsafe { bindings::iminor(inode) };
+            let minor = Minor::acquire(minor_id)?;
+
+            // SAFETY: `fops_open` is only installed for devices of type `T` (via `FOPS`).
+            let _guard = (unsafe { minor.device::<T>() })
+                .registration_guard()
+                .ok_or(ENODEV)?;
+
+            // SAFETY: `inode` and `filp` are valid. The RegistrationGuard ensures the entire
+            // `drm_open()` runs within the SRCU critical section.
+            Ok(unsafe { bindings::drm_open(inode, filp) })
+        };
+
+        match f() {
+            Ok(ret) => ret,
+            Err(e) => e.to_errno(),
+        }
+    }
+
     const FOPS: bindings::file_operations = {
         let mut fops: bindings::file_operations = pin_init::zeroed();
 
         fops.owner = crate::module::this_module::<T::OwnerModule>().as_ptr();
-        fops.open = Some(bindings::drm_open);
+        fops.open = Some(Self::fops_open);
         fops.release = Some(bindings::drm_release);
         fops.unlocked_ioctl = Some(bindings::drm_ioctl);
         #[cfg(CONFIG_COMPAT)]
-- 
2.56.0