[PATCH v4 09/16] rust: drm: add Ioctl device context typestate

[Danilo Krummrich <dakr@kernel.org>]

Add the Ioctl DeviceContext for DRM devices that have been registered
with userspace previously.

A Device<T, Ioctl> has been registered at some point, but may be
concurrently unregistering or already unregistered. drm_dev_enter() can
guard against this, ensuring the device remains registered for the
duration of the critical section.

This typestate will be used in ioctl dispatch context where registration
is guaranteed by the DRM core, and RegistrationGuard can safely be
acquired.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/drm/device.rs | 34 +++++++++++++++++++++++++++++++---
 rust/kernel/drm/mod.rs    |  1 +
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 86a7fca1d33f..d429b4655449 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -74,14 +74,16 @@ macro_rules! drm_legacy_fields {
 
 /// A trait implemented by all possible contexts a [`Device`] can be used in.
 ///
-/// A [`Device`] can be in one of two contexts:
+/// A [`Device`] can be in one of the following contexts:
 ///
 /// - [`Normal`]: The general-purpose, reference-counted context. A [`Device`] in this context may
 ///   or may not be registered with userspace.
+/// - [`Ioctl`]: The device has been registered with userspace at some point; used in ioctl
+///   dispatch context.
 /// - [`Registered`]: The device has been registered with userspace at some point.
 ///
-/// `Device<T, Registered>` dereferences to `Device<T>` ([`Normal`]), so any method available on a
-/// [`Normal`] device is also available on a [`Registered`] one.
+/// Both `Device<T, Ioctl>` and `Device<T, Registered>` dereference to `Device<T>` ([`Normal`]),
+/// so any method available on a [`Normal`] device is also available in the other contexts.
 pub trait DeviceContext: Sealed + Send + Sync {}
 
 /// The general-purpose, reference-counted [`DeviceContext`].
@@ -113,6 +115,21 @@ impl DeviceContext for Normal {}
 impl Sealed for Registered {}
 impl DeviceContext for Registered {}
 
+/// The [`DeviceContext`] of a [`Device`] that has been registered with userspace previously.
+///
+/// A [`Device`] in this context has been registered at some point, but may be concurrently
+/// unregistering or already unregistered. `drm_dev_enter()` can guard against this, ensuring the
+/// device remains registered for the duration of the critical section.
+///
+/// # Invariants
+///
+/// A [`Device`] in this context has been registered with userspace via `drm_dev_register()` at
+/// some point.
+pub struct Ioctl;
+
+impl Sealed for Ioctl {}
+impl DeviceContext for Ioctl {}
+
 /// A [`Device`] which is known at compile-time to be unregistered with userspace.
 ///
 /// This type allows performing operations which are only safe to do before userspace registration,
@@ -342,6 +359,17 @@ fn deref(&self) -> &Self::Target {
     }
 }
 
+impl<T: drm::Driver> Deref for Device<T, Ioctl> {
+    type Target = Device<T>;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The caller holds a `Device<T, Ioctl>`, which guarantees all invariants
+        // of the weaker `Normal` context.
+        unsafe { self.assume_ctx() }
+    }
+}
+
 // SAFETY: DRM device objects are always reference counted and the get/put functions
 // satisfy the requirements.
 unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {
diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
index e5bfaf130342..a6693d2b84b8 100644
--- a/rust/kernel/drm/mod.rs
+++ b/rust/kernel/drm/mod.rs
@@ -11,6 +11,7 @@
 
 pub use self::device::Device;
 pub use self::device::DeviceContext;
+pub use self::device::Ioctl;
 pub use self::device::Normal;
 pub use self::device::Registered;
 pub use self::device::UnregisteredDevice;
-- 
2.54.0