rust: drm: file: Add File abstraction

A DRM File is the DRM counterpart to a kernel file structure,
representing an open DRM file descriptor.

Add a Rust abstraction to allow drivers to implement their own File types
that implement the DriverFile trait.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250410235546.43736-7-dakr@kernel.org
[ Rework of drm::File
    * switch to the Opaque<T> type
    * fix (mutable) references to struct drm_file (which in this context
      is UB)
    * restructure and rename functions to align with common kernel
      schemes
    * write and fix safety and invariant comments
    * remove necessity for and convert 'as' casts
    * original source archive: https://archive.is/GH8oy

  - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

4 files changed, 106 insertions, 2 deletions

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index d6c6cecbdd51..520f51bb40af 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -63,8 +63,8 @@ pub struct Device<T: drm::Driver> {
 impl<T: drm::Driver> Device<T> {
     const VTABLE: bindings::drm_driver = drm_legacy_fields! {
         load: None,
-        open: None, // TODO: File abstraction
-        postclose: None, // TODO: File abstraction
+        open: Some(drm::File::<T::File>::open_callback),
+        postclose: Some(drm::File::<T::File>::postclose_callback),
         unload: None,
         release: None,
         master_set: None,
diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index 4f80bc448c09..f0c2936c3374 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -105,6 +105,9 @@ pub trait Driver {
     /// The type used to manage memory for this driver.
     type Object: AllocImpl;
 
+    /// The type used to represent a DRM File (client)
+    type File: drm::file::DriverFile;
+
     /// Driver metadata
     const INFO: DriverInfo;
 
diff --git a/rust/kernel/drm/file.rs b/rust/kernel/drm/file.rs
new file mode 100644
index 000000000000..b9527705e551
--- /dev/null
+++ b/rust/kernel/drm/file.rs
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0 OR MIT
+
+//! DRM File objects.
+//!
+//! C header: [`include/linux/drm/drm_file.h`](srctree/include/linux/drm/drm_file.h)
+
+use crate::{bindings, drm, error::Result, prelude::*, types::Opaque};
+use core::marker::PhantomData;
+use core::pin::Pin;
+
+/// Trait that must be implemented by DRM drivers to represent a DRM File (a client instance).
+pub trait DriverFile {
+    /// The parent `Driver` implementation for this `DriverFile`.
+    type Driver: drm::Driver;
+
+    /// Open a new file (called when a client opens the DRM device).
+    fn open(device: &drm::Device<Self::Driver>) -> Result<Pin<KBox<Self>>>;
+}
+
+/// An open DRM File.
+///
+/// # Invariants
+///
+/// `self.0` is a valid instance of a `struct drm_file`.
+#[repr(transparent)]
+pub struct File<T: DriverFile>(Opaque<bindings::drm_file>, PhantomData<T>);
+
+impl<T: DriverFile> File<T> {
+    #[doc(hidden)]
+    /// Not intended to be called externally, except via declare_drm_ioctls!()
+    ///
+    /// # Safety
+    ///
+    /// `raw_file` must be a valid pointer to an open `struct drm_file`, opened through `T::open`.
+    pub unsafe fn as_ref<'a>(ptr: *mut bindings::drm_file) -> &'a File<T> {
+        // SAFETY: `raw_file` is valid by the safety requirements of this function.
+        unsafe { &*ptr.cast() }
+    }
+
+    pub(super) fn as_raw(&self) -> *mut bindings::drm_file {
+        self.0.get()
+    }
+
+    fn driver_priv(&self) -> *mut T {
+        // SAFETY: By the type invariants of `Self`, `self.as_raw()` is always valid.
+        unsafe { (*self.as_raw()).driver_priv }.cast()
+    }
+
+    /// Return a pinned reference to the driver file structure.
+    pub fn inner(&self) -> Pin<&T> {
+        // SAFETY: By the type invariant the pointer `self.as_raw()` points to a valid and opened
+        // `struct drm_file`, hence `driver_priv` has been properly initialized by `open_callback`.
+        unsafe { Pin::new_unchecked(&*(self.driver_priv())) }
+    }
+
+    /// The open callback of a `struct drm_file`.
+    pub(crate) extern "C" fn open_callback(
+        raw_dev: *mut bindings::drm_device,
+        raw_file: *mut bindings::drm_file,
+    ) -> core::ffi::c_int {
+        // SAFETY: A callback from `struct drm_driver::open` guarantees that
+        // - `raw_dev` is valid pointer to a `struct drm_device`,
+        // - the corresponding `struct drm_device` has been registered.
+        let drm = unsafe { drm::Device::as_ref(raw_dev) };
+
+        // SAFETY: `raw_file` is a valid pointer to a `struct drm_file`.
+        let file = unsafe { File::<T>::as_ref(raw_file) };
+
+        let inner = match T::open(drm) {
+            Err(e) => {
+                return e.to_errno();
+            }
+            Ok(i) => i,
+        };
+
+        // SAFETY: This pointer is treated as pinned, and the Drop guarantee is upheld in
+        // `postclose_callback()`.
+        let driver_priv = KBox::into_raw(unsafe { Pin::into_inner_unchecked(inner) });
+
+        // SAFETY: By the type invariants of `Self`, `self.as_raw()` is always valid.
+        unsafe { (*file.as_raw()).driver_priv = driver_priv.cast() };
+
+        0
+    }
+
+    /// The postclose callback of a `struct drm_file`.
+    pub(crate) extern "C" fn postclose_callback(
+        _raw_dev: *mut bindings::drm_device,
+        raw_file: *mut bindings::drm_file,
+    ) {
+        // SAFETY: This reference won't escape this function
+        let file = unsafe { File::<T>::as_ref(raw_file) };
+
+        // SAFETY: `file.driver_priv` has been created in `open_callback` through `KBox::into_raw`.
+        let _ = unsafe { KBox::from_raw(file.driver_priv()) };
+    }
+}
+
+impl<T: DriverFile> super::private::Sealed for File<T> {}
diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
index 2d88e70ba607..b36223e5bd98 100644
--- a/rust/kernel/drm/mod.rs
+++ b/rust/kernel/drm/mod.rs
@@ -4,12 +4,14 @@
 
 pub mod device;
 pub mod driver;
+pub mod file;
 pub mod ioctl;
 
 pub use self::device::Device;
 pub use self::driver::Driver;
 pub use self::driver::DriverInfo;
 pub use self::driver::Registration;
+pub use self::file::File;
 
 pub(crate) mod private {
     pub trait Sealed {}