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.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

4 files changed, 134 insertions, 2 deletions

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 7651246d38dc..c90ce2d71095 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -57,8 +57,8 @@ pub struct Device<T: drm::drv::Driver>(Opaque<bindings::drm_device>, PhantomData
 impl<T: drm::drv::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::open_callback::<T::File>),
+        postclose: Some(drm::file::postclose_callback::<T::File>),
         unload: None,
         release: Some(Self::release),
         master_set: None,
diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs
index ebb79a8c90ee..895409f04664 100644
--- a/rust/kernel/drm/drv.rs
+++ b/rust/kernel/drm/drv.rs
@@ -139,6 +139,9 @@ pub trait Driver {
     /// Should be either `drm::gem::Object<T>` or `drm::gem::shmem::Object<T>`.
     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..4214d042a46e
--- /dev/null
+++ b/rust/kernel/drm/file.rs
@@ -0,0 +1,128 @@
+// 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::*};
+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::drv::Driver;
+
+    /// Open a new file (called when a client opens the DRM device).
+    fn open(device: &drm::device::Device<Self::Driver>) -> Result<Pin<KBox<Self>>>;
+}
+
+/// An open DRM File.
+///
+/// # Invariants
+/// `raw` is a valid pointer to an open `drm_file` struct.
+#[repr(transparent)]
+pub struct File<T: DriverFile> {
+    raw: *mut bindings::drm_file,
+    _p: PhantomData<T>,
+}
+
+#[allow(clippy::missing_safety_doc)]
+/// The open callback of a `struct drm_file`.
+pub(super) unsafe extern "C" fn open_callback<T: DriverFile>(
+    raw_dev: *mut bindings::drm_device,
+    raw_file: *mut bindings::drm_file,
+) -> core::ffi::c_int {
+    // SAFETY: A callback from `struct drm_driver::open` gurantees that `raw_dev` is valid
+    // pointer to a `sturct drm_device`.
+    let drm = unsafe { drm::device::Device::borrow(raw_dev) };
+    // SAFETY: This reference won't escape this function
+    let file = unsafe { &mut *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 below.
+    file.driver_priv = KBox::into_raw(unsafe { Pin::into_inner_unchecked(inner) }) as *mut _;
+
+    0
+}
+
+#[allow(clippy::missing_safety_doc)]
+/// The postclose callback of a `struct drm_file`.
+pub(super) unsafe extern "C" fn postclose_callback<T: DriverFile>(
+    _raw_dev: *mut bindings::drm_device,
+    raw_file: *mut bindings::drm_file,
+) {
+    // SAFETY: This reference won't escape this function
+    let file = unsafe { &*raw_file };
+
+    // SAFETY: `file.driver_priv` has been created in `open_callback` through `KBox::into_raw`.
+    unsafe {
+        let _ = KBox::from_raw(file.driver_priv as *mut 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 from_raw(raw_file: *mut bindings::drm_file) -> File<T> {
+        File {
+            raw: raw_file,
+            _p: PhantomData,
+        }
+    }
+
+    #[allow(dead_code)]
+    /// Return the raw pointer to the underlying `drm_file`.
+    pub(super) fn raw(&self) -> *const bindings::drm_file {
+        self.raw
+    }
+
+    /// Return an immutable reference to the raw `drm_file` structure.
+    pub(super) fn file(&self) -> &bindings::drm_file {
+        // SAFETY: `self.raw` is a valid pointer to a `struct drm_file`.
+        unsafe { &*self.raw }
+    }
+
+    /// Return a pinned reference to the driver file structure.
+    pub fn inner(&self) -> Pin<&T> {
+        // SAFETY: By the type invariant the pointer `self.raw` points to a valid and opened
+        // `struct drm_file`, hence `self.raw.driver_priv` has been properly initialized by
+        // `open_callback`.
+        unsafe { Pin::new_unchecked(&*(self.file().driver_priv as *const T)) }
+    }
+}
+
+impl<T: DriverFile> crate::private::Sealed for File<T> {}
+
+/// Generic trait to allow users that don't care about driver specifics to accept any `File<T>`.
+///
+/// # Safety
+///
+/// Must only be implemented for `File<T>` and return the pointer, following the normal invariants
+/// of that type.
+pub unsafe trait GenericFile: crate::private::Sealed {
+    /// Returns the raw const pointer to the `struct drm_file`
+    fn raw(&self) -> *const bindings::drm_file;
+    /// Returns the raw mut pointer to the `struct drm_file`
+    fn raw_mut(&mut self) -> *mut bindings::drm_file;
+}
+
+// SAFETY: Implementation for `File<T>`, holding up its type invariants.
+unsafe impl<T: DriverFile> GenericFile for File<T> {
+    fn raw(&self) -> *const bindings::drm_file {
+        self.raw
+    }
+    fn raw_mut(&mut self) -> *mut bindings::drm_file {
+        self.raw
+    }
+}
diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
index 69376b3c6db9..a767942d0b52 100644
--- a/rust/kernel/drm/mod.rs
+++ b/rust/kernel/drm/mod.rs
@@ -4,4 +4,5 @@
 
 pub mod device;
 pub mod drv;
+pub mod file;
 pub mod ioctl;