[PATCH v2 0/3] Introduce DeviceContext

[PATCH v2 0/3] Introduce DeviceContext

[Lyude Paul]

Previous version of this patch series:
  https://lists.freedesktop.org/archives/dri-devel/2025-November/535387.html

One of the unsolved issues we still have with the rust DRM bindings is
the ability to limit certain Device operations to contexts where we can
guarantee that a Device has been fully initialized and registered with
userspace, or vice-versa (e.g. must be unregistered).

While the previous solution for this that I had was simply not exposing
drm::Device at all until the device has been registered with userspace,
unfortunately this isn't enough since:

* As we found out with Tyr, drivers occasionally need to be able to
  create GEM objects before device registration
* We would still need to be able to handle KMS callbacks which could be
  invoked after KMS init but before userspace registration (not handled
  in this series specifically, but DeviceContext will be required for
  handling this).

This patch series provides a pretty nice solution to this, by
implementing a very similar solution to kernel::device::DeviceContext:
introducing our own DeviceContext type state.

                          Series-wide changes

V2:
* s/DeviceCtx/DeviceContext/ for consistency
* Move private driver-data availability to the Registration
  DeviceContext
* s/AnyCtx/Init/

More changes described in each patch description.

Lyude Paul (3):
  rust/drm: Introduce DeviceContext
  rust/drm: Don't setup private driver data until registration
  rust/drm/gem: Use DeviceContext with GEM objects

 drivers/gpu/drm/nova/driver.rs |  10 +-
 drivers/gpu/drm/nova/gem.rs    |  11 +-
 drivers/gpu/drm/tyr/driver.rs  |  12 +-
 drivers/gpu/drm/tyr/gem.rs     |   3 +-
 rust/kernel/drm/device.rs      | 231 +++++++++++++++++++++++++--------
 rust/kernel/drm/driver.rs      |  56 ++++++--
 rust/kernel/drm/gem/mod.rs     |  66 ++++++----
 rust/kernel/drm/mod.rs         |   4 +
 8 files changed, 292 insertions(+), 101 deletions(-)


base-commit: 086714bbb96f63785da251e1f8d1ce3e716a6e42
-- 
2.52.0

[PATCH v2 1/3] rust/drm: Introduce DeviceContext

[Lyude Paul]

One of the tricky things about DRM bindings in Rust is the fact that
initialization of a DRM device is a multi-step process. It's quite normal
for a device driver to start making use of its DRM device for tasks like
creating GEM objects before userspace registration happens. This is an
issue in rust though, since prior to userspace registration the device is
only partly initialized. This means there's a plethora of DRM device
operations we can't yet expose without opening up the door to UB if the DRM
device in question isn't yet registered.

Additionally, this isn't something we can reliably check at runtime. And
even if we could, performing an operation which requires the device be
registered when the device isn't actually registered is a programmer bug,
meaning there's no real way to gracefully handle such a mistake at runtime.
And even if that wasn't the case, it would be horrendously annoying and
noisy to have to check if a device is registered constantly throughout a
driver.

In order to solve this, we first take inspiration from
`kernel::device::DeviceContext` and introduce `kernel::drm::DeviceContext`.
This provides us with a ZST type that we can generalize over to represent
contexts where a device is known to have been registered with userspace at
some point in time (`Registered`), along with contexts where we can't make
such a guarantee (`Uninit`).

It's important to note we intentionally do not provide a `DeviceContext`
which represents an unregistered device. This is because there's no
reasonable way to guarantee that a device with long-living references to
itself will not be registered eventually with userspace. Instead, we
provide a new-type for this: `UnregisteredDevice` which can
provide a guarantee that the `Device` has never been registered with
userspace. To ensure this, we modify `Registration` so that creating a new
`Registration` requires passing ownership of an `UnregisteredDevice`.

Signed-off-by: Lyude Paul <lyude@redhat.com>

---
V2:
* Make sure that `UnregisteredDevice` is not thread-safe (since DRM device
  initialization is also not thread-safe)
* Rename from AnyCtx to Uninit, I think this name actually makes a bit more
  sense.
* Change assume_registered() to assume_ctx()
  Since it looks like in some situations, we'll want to update the
  DeviceContext of a object to the latest DeviceContext we know the Device
  to be in.
* Rename Init to Uninit
  When we eventually add KMS support, we're going to have 3 different
  DeviceContexts - Uninit, Init, Registered. Additionally, aside from not
  being registered there are a number of portions of the rest of the Device
  which also aren't usable before at least the Init context - so the naming
  of Uninit makes this a little clearer.
* s/DeviceContext/DeviceContext/
  For consistency with the rest of the kernel
* Drop as_ref::<Device<T, Uninit>>() for now since I don't actually think
  we need this quite yet

 drivers/gpu/drm/nova/driver.rs |   8 +-
 drivers/gpu/drm/tyr/driver.rs  |  10 +-
 rust/kernel/drm/device.rs      | 172 +++++++++++++++++++++++++++------
 rust/kernel/drm/driver.rs      |  35 +++++--
 rust/kernel/drm/mod.rs         |   4 +
 5 files changed, 182 insertions(+), 47 deletions(-)

diff --git a/drivers/gpu/drm/nova/driver.rs b/drivers/gpu/drm/nova/driver.rs
index b1af0a099551d..99d6841b69cbc 100644
--- a/drivers/gpu/drm/nova/driver.rs
+++ b/drivers/gpu/drm/nova/driver.rs
@@ -21,7 +21,7 @@ pub(crate) struct NovaDriver {
 }
 
 /// Convienence type alias for the DRM device type for this driver
-pub(crate) type NovaDevice = drm::Device<NovaDriver>;
+pub(crate) type NovaDevice<Ctx = drm::Registered> = drm::Device<NovaDriver, Ctx>;
 
 #[pin_data]
 pub(crate) struct NovaData {
@@ -56,10 +56,10 @@ impl auxiliary::Driver for NovaDriver {
     fn probe(adev: &auxiliary::Device<Core>, _info: &Self::IdInfo) -> impl PinInit<Self, Error> {
         let data = try_pin_init!(NovaData { adev: adev.into() });
 
-        let drm = drm::Device::<Self>::new(adev.as_ref(), data)?;
-        drm::Registration::new_foreign_owned(&drm, adev.as_ref(), 0)?;
+        let drm = drm::UnregisteredDevice::<Self>::new(adev.as_ref(), data)?;
+        let drm = drm::Registration::new_foreign_owned(drm, adev.as_ref(), 0)?;
 
-        Ok(Self { drm })
+        Ok(Self { drm: drm.into() })
     }
 }
 
diff --git a/drivers/gpu/drm/tyr/driver.rs b/drivers/gpu/drm/tyr/driver.rs
index f0da589327027..09c3157531072 100644
--- a/drivers/gpu/drm/tyr/driver.rs
+++ b/drivers/gpu/drm/tyr/driver.rs
@@ -29,7 +29,7 @@
 pub(crate) type IoMem = kernel::io::mem::IoMem<SZ_2M>;
 
 /// Convenience type alias for the DRM device type for this driver.
-pub(crate) type TyrDevice = drm::Device<TyrDriver>;
+pub(crate) type TyrDevice<Ctx = drm::Registered> = drm::Device<TyrDriver, Ctx>;
 
 #[pin_data(PinnedDrop)]
 pub(crate) struct TyrDriver {
@@ -139,10 +139,12 @@ fn probe(
                 gpu_info,
         });
 
-        let tdev: ARef<TyrDevice> = drm::Device::new(pdev.as_ref(), data)?;
-        drm::driver::Registration::new_foreign_owned(&tdev, pdev.as_ref(), 0)?;
+        let tdev = drm::UnregisteredDevice::<Self>::new(pdev.as_ref(), data)?;
+        let tdev = drm::driver::Registration::new_foreign_owned(tdev, pdev.as_ref(), 0)?;
 
-        let driver = TyrDriver { device: tdev };
+        let driver = TyrDriver {
+            device: tdev.into(),
+        };
 
         // We need this to be dev_info!() because dev_dbg!() does not work at
         // all in Rust for now, and we need to see whether probe succeeded.
diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 3ce8f62a00569..53ca71daf2f86 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -7,14 +7,20 @@
 use crate::{
     alloc::allocator::Kmalloc,
     bindings, device, drm,
-    drm::driver::AllocImpl,
+    drm::{driver::AllocImpl, private::Sealed},
     error::from_err_ptr,
     error::Result,
     prelude::*,
     sync::aref::{ARef, AlwaysRefCounted},
-    types::Opaque,
+    types::{NotThreadSafe, Opaque},
+};
+use core::{
+    alloc::Layout,
+    marker::PhantomData,
+    mem::{self},
+    ops::Deref,
+    ptr::{self, NonNull},
 };
-use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
 
 #[cfg(CONFIG_DRM_LEGACY)]
 macro_rules! drm_legacy_fields {
@@ -47,26 +53,88 @@ macro_rules! drm_legacy_fields {
     }
 }
 
-/// A typed DRM device with a specific `drm::Driver` implementation.
+macro_rules! drm_dev_ctx {
+    (
+        $( #[$attrs:meta] )*
+        $name:ident
+    ) => {
+        $( #[$attrs] )*
+        pub struct $name;
+
+        impl DeviceContext for $name {}
+        impl Sealed for $name {}
+
+        // SAFETY: All registration states are free of side-effects (e.g. no Drop) and are ZSTs,
+        // thus they are always thread-safe.
+        unsafe impl Send for $name {}
+        // SAFETY: All registration states are free of side-effects (e.g. no Drop) and are ZSTs,
+        // thus they are always thread-safe.
+        unsafe impl Sync for $name {}
+    };
+}
+
+/// A trait implemented by all possible contexts a [`Device`] can be used in.
+pub trait DeviceContext: Sealed + Send + Sync {}
+
+drm_dev_ctx! {
+    /// The [`DeviceContext`] of a [`Device`] that was registered with userspace at some point.
+    ///
+    /// This represents a [`Device`] which is guaranteed to have been registered with userspace at
+    /// some point in time. Such a DRM device is guaranteed to have been fully-initialized.
+    ///
+    /// Note: A device in this context is not guaranteed to remain registered with userspace for its
+    /// entire lifetime, as this is impossible to guarantee at compile-time. However, any
+    /// userspace-dependent operations performed with an unregistered device in this [`DeviceContext`]
+    /// are guaranteed to be no-ops.
+    ///
+    /// # Invariants
+    ///
+    /// A [`Device`] in this [`DeviceContext`] is guaranteed to have called `drm_dev_register` once.
+    Registered
+}
+
+drm_dev_ctx! {
+    /// The [`DeviceContext`] of a [`Device`] that may be unregistered and partly uninitialized.
+    ///
+    /// A [`Device`] in this context is only guaranteed to be partly initialized, and may or may not
+    /// be registered with userspace. Thus operations which depend on the [`Device`] being fully
+    /// initialized, or which depend on the [`Device`] being registered with userspace are not
+    /// available through this [`DeviceContext`].
+    ///
+    /// A [`Device`] in this context can be used to create a
+    /// [`Registration`](drm::driver::Registration).
+    Uninit
+}
+
+/// A [`Device`] which is known at compile-time to be unregistered with userspace.
 ///
-/// The device is always reference-counted.
+/// This type allows performing operations which are only safe to do before userspace registration,
+/// and can be used to create a [`Registration`](drm::driver::Registration) once the driver is ready
+/// to register the device with userspace.
+///
+/// Since DRM device initialization must be single-threaded, this object is not thread-safe.
 ///
 /// # Invariants
 ///
-/// `self.dev` is a valid instance of a `struct device`.
-#[repr(C)]
-pub struct Device<T: drm::Driver> {
-    dev: Opaque<bindings::drm_device>,
-    data: T::Data,
+/// The device in `self.0` is guaranteed to be a newly created [`Device`] that has not yet been
+/// registered with userspace until this type is dropped.
+pub struct UnregisteredDevice<T: drm::Driver>(ARef<Device<T, Uninit>>, NotThreadSafe);
+
+impl<T: drm::Driver> Deref for UnregisteredDevice<T> {
+    type Target = Device<T, Uninit>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
 }
 
-impl<T: drm::Driver> Device<T> {
+impl<T: drm::Driver> UnregisteredDevice<T> {
     const VTABLE: bindings::drm_driver = drm_legacy_fields! {
         load: None,
         open: Some(drm::File::<T::File>::open_callback),
         postclose: Some(drm::File::<T::File>::postclose_callback),
         unload: None,
-        release: Some(Self::release),
+        release: Some(Device::<T>::release),
         master_set: None,
         master_drop: None,
         debugfs_init: None,
@@ -94,8 +162,10 @@ impl<T: drm::Driver> Device<T> {
 
     const GEM_FOPS: bindings::file_operations = drm::gem::create_fops();
 
-    /// Create a new `drm::Device` for a `drm::Driver`.
-    pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<ARef<Self>> {
+    /// Create a new `UnregisteredDevice` for a `drm::Driver`.
+    ///
+    /// This can be used to create a [`Registration`](kernel::drm::Registration).
+    pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<Self> {
         // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
         // compatible `Layout`.
         let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
@@ -103,12 +173,12 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
         // SAFETY:
         // - `VTABLE`, as a `const` is pinned to the read-only section of the compilation,
         // - `dev` is valid by its type invarants,
-        let raw_drm: *mut Self = unsafe {
+        let raw_drm: *mut Device<T, Uninit> = unsafe {
             bindings::__drm_dev_alloc(
                 dev.as_raw(),
                 &Self::VTABLE,
                 layout.size(),
-                mem::offset_of!(Self, dev),
+                mem::offset_of!(Device<T, Uninit>, dev),
             )
         }
         .cast();
@@ -123,7 +193,7 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
         unsafe { data.__pinned_init(raw_data) }.inspect_err(|_| {
             // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
             // successful.
-            let drm_dev = unsafe { Self::into_drm_device(raw_drm) };
+            let drm_dev = unsafe { Device::into_drm_device(raw_drm) };
 
             // SAFETY: `__drm_dev_alloc()` was successful, hence `drm_dev` must be valid and the
             // refcount must be non-zero.
@@ -132,9 +202,40 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
 
         // SAFETY: The reference count is one, and now we take ownership of that reference as a
         // `drm::Device`.
-        Ok(unsafe { ARef::from_raw(raw_drm) })
+        // INVARIANT: We just created the device above, but have yet to call `drm_dev_register`.
+        // `Self` cannot be copied or sent to another thread - ensuring that `drm_dev_register`
+        // won't be called during its lifetime and that the device is unregistered
+        Ok(Self(unsafe { ARef::from_raw(raw_drm) }, NotThreadSafe))
     }
+}
 
+/// A typed DRM device with a specific [`drm::Driver`] implementation and [`DeviceContext`].
+///
+/// Since DRM devices can be used before being fully initialized and registered with userspace, `C`
+/// represents the furthest [`DeviceContext`] we can guarantee that this [`Device`] has reached.
+///
+/// Keep in mind: this means that an unregistered device can still have the registration state
+/// [`Registered`] as long as it was registered with userspace once in the past, and that the
+/// behavior of such a device is still well-defined. In such a situation, the behavior of any
+/// functions which interact with userspace will simply be no-ops. Additionally, a device with the
+/// registration state [`Uninit`] simply does not have a guaranteed registration state at compile
+/// time, and could be either registered or unregistered. Since there is no way to guarantee a
+/// long-lived reference to an unregistered device would remain unregistered, we do not provide a
+/// [`DeviceContext`] for this.
+///
+/// # Invariants
+///
+/// * `self.dev` is a valid instance of a `struct device`.
+/// * The data layout of `Self` remains the same across all implementations of `C`.
+/// * Any invariants for `C` also apply.
+#[repr(C)]
+pub struct Device<T: drm::Driver, C: DeviceContext = Registered> {
+    dev: Opaque<bindings::drm_device>,
+    data: T::Data,
+    _ctx: PhantomData<C>,
+}
+
+impl<T: drm::Driver, C: DeviceContext> Device<T, C> {
     pub(crate) fn as_raw(&self) -> *mut bindings::drm_device {
         self.dev.get()
     }
@@ -160,13 +261,13 @@ unsafe fn into_drm_device(ptr: NonNull<Self>) -> *mut bindings::drm_device {
     ///
     /// # Safety
     ///
-    /// Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count,
-    /// i.e. it must be ensured that the reference count of the C `struct drm_device` `ptr` points
-    /// to can't drop to zero, for the duration of this function call and the entire duration when
-    /// the returned reference exists.
-    ///
-    /// Additionally, callers must ensure that the `struct device`, `ptr` is pointing to, is
-    /// embedded in `Self`.
+    /// * Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count,
+    ///   i.e. it must be ensured that the reference count of the C `struct drm_device` `ptr` points
+    ///   to can't drop to zero, for the duration of this function call and the entire duration when
+    ///   the returned reference exists.
+    /// * Additionally, callers must ensure that the `struct device`, `ptr` is pointing to, is
+    ///   embedded in `Self`.
+    /// * Callers promise that any type invariants of `C` will be upheld.
     #[doc(hidden)]
     pub unsafe fn from_raw<'a>(ptr: *const bindings::drm_device) -> &'a Self {
         // SAFETY: By the safety requirements of this function `ptr` is a valid pointer to a
@@ -186,9 +287,20 @@ extern "C" fn release(ptr: *mut bindings::drm_device) {
         // - `this` is valid for dropping.
         unsafe { core::ptr::drop_in_place(this) };
     }
+
+    /// Change the [`DeviceContext`] for a [`Device`].
+    ///
+    /// # Safety
+    ///
+    /// The caller promises that `self` fulfills all of the guarantees provided by the given
+    /// [`DeviceContext`].
+    pub(crate) unsafe fn assume_ctx<NewCtx: DeviceContext>(&self) -> &Device<T, NewCtx> {
+        // SAFETY: The data layout is identical via our type invariants.
+        unsafe { mem::transmute(self) }
+    }
 }
 
-impl<T: drm::Driver> Deref for Device<T> {
+impl<T: drm::Driver, C: DeviceContext> Deref for Device<T, C> {
     type Target = T::Data;
 
     fn deref(&self) -> &Self::Target {
@@ -198,7 +310,7 @@ fn deref(&self) -> &Self::Target {
 
 // 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> {
+unsafe impl<T: drm::Driver, C: DeviceContext> AlwaysRefCounted for Device<T, C> {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::drm_dev_get(self.as_raw()) };
@@ -213,7 +325,7 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
-impl<T: drm::Driver> AsRef<device::Device> for Device<T> {
+impl<T: drm::Driver, C: DeviceContext> AsRef<device::Device> for Device<T, C> {
     fn as_ref(&self) -> &device::Device {
         // SAFETY: `bindings::drm_device::dev` is valid as long as the DRM device itself is valid,
         // which is guaranteed by the type invariant.
@@ -222,8 +334,8 @@ fn as_ref(&self) -> &device::Device {
 }
 
 // SAFETY: A `drm::Device` can be released from any thread.
-unsafe impl<T: drm::Driver> Send for Device<T> {}
+unsafe impl<T: drm::Driver, C: DeviceContext> Send for Device<T, C> {}
 
 // SAFETY: A `drm::Device` can be shared among threads because all immutable methods are protected
 // by the synchronization in `struct drm_device`.
-unsafe impl<T: drm::Driver> Sync for Device<T> {}
+unsafe impl<T: drm::Driver, C: DeviceContext> Sync for Device<T, C> {}
diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index f30ee4c6245cd..fb4f215320bd9 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -10,6 +10,7 @@
     prelude::*,
     sync::aref::ARef,
 };
+use core::{mem, ptr::NonNull};
 use macros::vtable;
 
 /// Driver use the GEM memory manager. This should be set for all modern drivers.
@@ -122,30 +123,46 @@ pub trait Driver {
 
 impl<T: Driver> Registration<T> {
     /// Creates a new [`Registration`] and registers it.
-    fn new(drm: &drm::Device<T>, flags: usize) -> Result<Self> {
+    fn new(drm: drm::UnregisteredDevice<T>, flags: usize) -> Result<Self> {
         // SAFETY: `drm.as_raw()` is valid by the invariants of `drm::Device`.
         to_result(unsafe { bindings::drm_dev_register(drm.as_raw(), flags) })?;
 
-        Ok(Self(drm.into()))
+        // SAFETY: We just called `drm_dev_register` above
+        let new = NonNull::from(unsafe { drm.assume_ctx() });
+
+        // Leak the ARef from UnregisteredDevice in preparation for transferring its ownership.
+        mem::forget(drm);
+
+        // SAFETY: `drm`'s `Drop` constructor was never called, ensuring that there remains at least
+        // one reference to the device - which we take ownership over here.
+        let new = unsafe { ARef::from_raw(new) };
+
+        Ok(Self(new))
     }
 
-    /// Same as [`Registration::new`}, but transfers ownership of the [`Registration`] to
+    /// Same as [`Registration::new`], but transfers ownership of the [`Registration`] to
     /// [`devres::register`].
-    pub fn new_foreign_owned(
-        drm: &drm::Device<T>,
-        dev: &device::Device<device::Bound>,
+    pub fn new_foreign_owned<'a>(
+        drm: drm::UnregisteredDevice<T>,
+        dev: &'a device::Device<device::Bound>,
         flags: usize,
-    ) -> Result
+    ) -> Result<&'a drm::Device<T>>
     where
         T: 'static,
     {
-        if drm.as_ref().as_raw() != dev.as_raw() {
+        let this_dev: &device::Device = drm.as_ref();
+        if this_dev.as_raw() != dev.as_raw() {
             return Err(EINVAL);
         }
 
         let reg = Registration::<T>::new(drm, flags)?;
+        let drm = NonNull::from(reg.device());
+
+        devres::register(dev, reg, GFP_KERNEL)?;
 
-        devres::register(dev, reg, GFP_KERNEL)
+        // SAFETY: Since `reg` was passed to devres::register(), the device now owns the lifetime
+        // of the DRM registration - ensuring that this references lives for at least as long as 'a.
+        Ok(unsafe { drm.as_ref() })
     }
 
     /// Returns a reference to the `Device` instance for this registration.
diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
index 1b82b6945edf2..64a43cb0fe57c 100644
--- a/rust/kernel/drm/mod.rs
+++ b/rust/kernel/drm/mod.rs
@@ -9,6 +9,10 @@
 pub mod ioctl;
 
 pub use self::device::Device;
+pub use self::device::DeviceContext;
+pub use self::device::Registered;
+pub use self::device::Uninit;
+pub use self::device::UnregisteredDevice;
 pub use self::driver::Driver;
 pub use self::driver::DriverInfo;
 pub use self::driver::Registration;
-- 
2.52.0

[PATCH v2 2/3] rust/drm: Don't setup private driver data until registration

[Lyude Paul]

Now that we have a DeviceContext that we can use to represent whether a
Device is known to have been registered, we can make it so that drivers can
create their Devices but wait until the registration phase to assign their
private data to the Device. This is desirable as some drivers need to make
use of the DRM device early on before finalizing their private driver data.

As such, this change makes it so that the driver's private data can
currently only be accessed through Device<T, Registered> types and not
Device<T, Uninit>.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 drivers/gpu/drm/nova/driver.rs |  4 +--
 drivers/gpu/drm/tyr/driver.rs  |  4 +--
 rust/kernel/drm/device.rs      | 59 +++++++++++++++++++++-------------
 rust/kernel/drm/driver.rs      | 23 +++++++++++--
 4 files changed, 60 insertions(+), 30 deletions(-)

diff --git a/drivers/gpu/drm/nova/driver.rs b/drivers/gpu/drm/nova/driver.rs
index 99d6841b69cbc..8cea5f68c3b04 100644
--- a/drivers/gpu/drm/nova/driver.rs
+++ b/drivers/gpu/drm/nova/driver.rs
@@ -56,8 +56,8 @@ impl auxiliary::Driver for NovaDriver {
     fn probe(adev: &auxiliary::Device<Core>, _info: &Self::IdInfo) -> impl PinInit<Self, Error> {
         let data = try_pin_init!(NovaData { adev: adev.into() });
 
-        let drm = drm::UnregisteredDevice::<Self>::new(adev.as_ref(), data)?;
-        let drm = drm::Registration::new_foreign_owned(drm, adev.as_ref(), 0)?;
+        let drm = drm::UnregisteredDevice::<Self>::new(adev.as_ref())?;
+        let drm = drm::Registration::new_foreign_owned(drm, adev.as_ref(), data, 0)?;
 
         Ok(Self { drm: drm.into() })
     }
diff --git a/drivers/gpu/drm/tyr/driver.rs b/drivers/gpu/drm/tyr/driver.rs
index 09c3157531072..768d50bde929e 100644
--- a/drivers/gpu/drm/tyr/driver.rs
+++ b/drivers/gpu/drm/tyr/driver.rs
@@ -139,8 +139,8 @@ fn probe(
                 gpu_info,
         });
 
-        let tdev = drm::UnregisteredDevice::<Self>::new(pdev.as_ref(), data)?;
-        let tdev = drm::driver::Registration::new_foreign_owned(tdev, pdev.as_ref(), 0)?;
+        let tdev = drm::UnregisteredDevice::<Self>::new(pdev.as_ref())?;
+        let tdev = drm::driver::Registration::new_foreign_owned(tdev, pdev.as_ref(), data, 0)?;
 
         let driver = TyrDriver {
             device: tdev.into(),
diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 53ca71daf2f86..441ed7c94fcf5 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -16,10 +16,12 @@
 };
 use core::{
     alloc::Layout,
+    cell::UnsafeCell,
     marker::PhantomData,
-    mem::{self},
+    mem::{self, MaybeUninit},
     ops::Deref,
-    ptr::{self, NonNull},
+    ptr::NonNull,
+    sync::atomic::*,
 };
 
 #[cfg(CONFIG_DRM_LEGACY)]
@@ -118,7 +120,7 @@ pub trait DeviceContext: Sealed + Send + Sync {}
 ///
 /// The device in `self.0` is guaranteed to be a newly created [`Device`] that has not yet been
 /// registered with userspace until this type is dropped.
-pub struct UnregisteredDevice<T: drm::Driver>(ARef<Device<T, Uninit>>, NotThreadSafe);
+pub struct UnregisteredDevice<T: drm::Driver>(pub(crate) ARef<Device<T, Uninit>>, NotThreadSafe);
 
 impl<T: drm::Driver> Deref for UnregisteredDevice<T> {
     type Target = Device<T, Uninit>;
@@ -165,7 +167,7 @@ impl<T: drm::Driver> UnregisteredDevice<T> {
     /// Create a new `UnregisteredDevice` for a `drm::Driver`.
     ///
     /// This can be used to create a [`Registration`](kernel::drm::Registration).
-    pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<Self> {
+    pub fn new(dev: &device::Device) -> Result<Self> {
         // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
         // compatible `Layout`.
         let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
@@ -184,22 +186,6 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<S
         .cast();
         let raw_drm = NonNull::new(from_err_ptr(raw_drm)?).ok_or(ENOMEM)?;
 
-        // SAFETY: `raw_drm` is a valid pointer to `Self`.
-        let raw_data = unsafe { ptr::addr_of_mut!((*raw_drm.as_ptr()).data) };
-
-        // SAFETY:
-        // - `raw_data` is a valid pointer to uninitialized memory.
-        // - `raw_data` will not move until it is dropped.
-        unsafe { data.__pinned_init(raw_data) }.inspect_err(|_| {
-            // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
-            // successful.
-            let drm_dev = unsafe { Device::into_drm_device(raw_drm) };
-
-            // SAFETY: `__drm_dev_alloc()` was successful, hence `drm_dev` must be valid and the
-            // refcount must be non-zero.
-            unsafe { bindings::drm_dev_put(drm_dev) };
-        })?;
-
         // SAFETY: The reference count is one, and now we take ownership of that reference as a
         // `drm::Device`.
         // INVARIANT: We just created the device above, but have yet to call `drm_dev_register`.
@@ -231,7 +217,15 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<S
 #[repr(C)]
 pub struct Device<T: drm::Driver, C: DeviceContext = Registered> {
     dev: Opaque<bindings::drm_device>,
-    data: T::Data,
+
+    /// Keeps track of whether we've initialized the device data yet.
+    pub(crate) data_is_init: AtomicBool,
+
+    /// The Driver's private data.
+    ///
+    /// This must only be written to from [`drm::Registration::new`].
+    pub(crate) data: UnsafeCell<MaybeUninit<T::Data>>,
+
     _ctx: PhantomData<C>,
 }
 
@@ -282,6 +276,21 @@ extern "C" fn release(ptr: *mut bindings::drm_device) {
         // SAFETY: `ptr` is a valid pointer to a `struct drm_device` and embedded in `Self`.
         let this = unsafe { Self::from_drm_device(ptr) };
 
+        {
+            // SAFETY:
+            // - Since we are in release(), we are guaranteed that no one else has access to `this`.
+            // - We confirmed above that `this` is a valid pointer to an initialized `Self`.
+            let this = unsafe { &mut *this };
+            if this.data_is_init.load(Ordering::Relaxed) {
+                // SAFETY:
+                // - Since we are in release(), we are guaranteed that no one else has access to
+                //   `this`.
+                // - We checked that the data is initialized above.
+                // - We do not use `data` any point after calling this function.
+                unsafe { (&mut *this.data.get()).assume_init_drop() };
+            }
+        }
+
         // SAFETY:
         // - When `release` runs it is guaranteed that there is no further access to `this`.
         // - `this` is valid for dropping.
@@ -300,11 +309,15 @@ pub(crate) unsafe fn assume_ctx<NewCtx: DeviceContext>(&self) -> &Device<T, NewC
     }
 }
 
-impl<T: drm::Driver, C: DeviceContext> Deref for Device<T, C> {
+impl<T: drm::Driver> Deref for Device<T, Registered> {
     type Target = T::Data;
 
     fn deref(&self) -> &Self::Target {
-        &self.data
+        // SAFETY:
+        // - `data` is initialized before any `Device`s with the `Registered` context are available
+        //   to the user.
+        // - `data` is only written to once in `Registration::new()`, so this read will never race.
+        unsafe { (&*self.data.get()).assume_init_ref() }
     }
 }
 
diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index fb4f215320bd9..108d789e43759 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -10,7 +10,12 @@
     prelude::*,
     sync::aref::ARef,
 };
-use core::{mem, ptr::NonNull};
+use core::{
+    mem,
+    ptr::NonNull,
+    sync::atomic::*,
+    //
+};
 use macros::vtable;
 
 /// Driver use the GEM memory manager. This should be set for all modern drivers.
@@ -123,7 +128,18 @@ pub trait Driver {
 
 impl<T: Driver> Registration<T> {
     /// Creates a new [`Registration`] and registers it.
-    fn new(drm: drm::UnregisteredDevice<T>, flags: usize) -> Result<Self> {
+    fn new(
+        drm: drm::UnregisteredDevice<T>,
+        data: impl PinInit<T::Data, Error>,
+        flags: usize,
+    ) -> Result<Self> {
+        // SAFETY:
+        // - `raw_data` is a valid pointer to uninitialized memory.
+        // - `raw_data` will not move until it is dropped.
+        unsafe { data.__pinned_init(drm.0.data.get().cast()) }?;
+
+        drm.data_is_init.store(true, Ordering::Relaxed);
+
         // SAFETY: `drm.as_raw()` is valid by the invariants of `drm::Device`.
         to_result(unsafe { bindings::drm_dev_register(drm.as_raw(), flags) })?;
 
@@ -145,6 +161,7 @@ fn new(drm: drm::UnregisteredDevice<T>, flags: usize) -> Result<Self> {
     pub fn new_foreign_owned<'a>(
         drm: drm::UnregisteredDevice<T>,
         dev: &'a device::Device<device::Bound>,
+        data: impl PinInit<T::Data, Error>,
         flags: usize,
     ) -> Result<&'a drm::Device<T>>
     where
@@ -155,7 +172,7 @@ pub fn new_foreign_owned<'a>(
             return Err(EINVAL);
         }
 
-        let reg = Registration::<T>::new(drm, flags)?;
+        let reg = Registration::<T>::new(drm, data, flags)?;
         let drm = NonNull::from(reg.device());
 
         devres::register(dev, reg, GFP_KERNEL)?;
-- 
2.52.0

[PATCH v2 3/3] rust/drm/gem: Use DeviceContext with GEM objects

[Lyude Paul]

Now that we have the ability to represent the context in which a DRM device
is in at compile-time, we can start carrying around this context with GEM
object types in order to allow a driver to safely create GEM objects before
a DRM device has registered with userspace.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 drivers/gpu/drm/nova/driver.rs |  2 +-
 drivers/gpu/drm/nova/gem.rs    | 11 +++---
 drivers/gpu/drm/tyr/driver.rs  |  2 +-
 drivers/gpu/drm/tyr/gem.rs     |  3 +-
 rust/kernel/drm/device.rs      | 14 ++++----
 rust/kernel/drm/driver.rs      |  2 +-
 rust/kernel/drm/gem/mod.rs     | 66 ++++++++++++++++++++++------------
 7 files changed, 63 insertions(+), 37 deletions(-)

diff --git a/drivers/gpu/drm/nova/driver.rs b/drivers/gpu/drm/nova/driver.rs
index 8cea5f68c3b04..2c13261450406 100644
--- a/drivers/gpu/drm/nova/driver.rs
+++ b/drivers/gpu/drm/nova/driver.rs
@@ -67,7 +67,7 @@ fn probe(adev: &auxiliary::Device<Core>, _info: &Self::IdInfo) -> impl PinInit<S
 impl drm::Driver for NovaDriver {
     type Data = NovaData;
     type File = File;
-    type Object = gem::Object<NovaObject>;
+    type Object<Ctx: drm::DeviceContext> = gem::Object<NovaObject, Ctx>;
 
     const INFO: drm::DriverInfo = INFO;
 
diff --git a/drivers/gpu/drm/nova/gem.rs b/drivers/gpu/drm/nova/gem.rs
index 6ccfa5da57617..f6e98b9db58d8 100644
--- a/drivers/gpu/drm/nova/gem.rs
+++ b/drivers/gpu/drm/nova/gem.rs
@@ -2,7 +2,7 @@
 
 use kernel::{
     drm,
-    drm::{gem, gem::BaseObject},
+    drm::{gem, gem::BaseObject, DeviceContext},
     page,
     prelude::*,
     sync::aref::ARef,
@@ -20,20 +20,23 @@ pub(crate) struct NovaObject {}
 impl gem::DriverObject for NovaObject {
     type Driver = NovaDriver;
 
-    fn new(_dev: &NovaDevice, _size: usize) -> impl PinInit<Self, Error> {
+    fn new<Ctx: DeviceContext>(_dev: &NovaDevice<Ctx>, _size: usize) -> impl PinInit<Self, Error> {
         try_pin_init!(NovaObject {})
     }
 }
 
 impl NovaObject {
     /// Create a new DRM GEM object.
-    pub(crate) fn new(dev: &NovaDevice, size: usize) -> Result<ARef<gem::Object<Self>>> {
+    pub(crate) fn new<Ctx: DeviceContext>(
+        dev: &NovaDevice<Ctx>,
+        size: usize,
+    ) -> Result<ARef<gem::Object<Self, Ctx>>> {
         if size == 0 {
             return Err(EINVAL);
         }
         let aligned_size = page::page_align(size).ok_or(EINVAL)?;
 
-        gem::Object::new(dev, aligned_size)
+        gem::Object::<Self, Ctx>::new(dev, aligned_size)
     }
 
     /// Look up a GEM object handle for a `File` and return an `ObjectRef` for it.
diff --git a/drivers/gpu/drm/tyr/driver.rs b/drivers/gpu/drm/tyr/driver.rs
index 768d50bde929e..c94aa1f12ca61 100644
--- a/drivers/gpu/drm/tyr/driver.rs
+++ b/drivers/gpu/drm/tyr/driver.rs
@@ -183,7 +183,7 @@ fn drop(self: Pin<&mut Self>) {
 impl drm::Driver for TyrDriver {
     type Data = TyrData;
     type File = File;
-    type Object = drm::gem::Object<TyrObject>;
+    type Object<R: drm::DeviceContext> = drm::gem::Object<TyrObject, R>;
 
     const INFO: drm::DriverInfo = INFO;
 
diff --git a/drivers/gpu/drm/tyr/gem.rs b/drivers/gpu/drm/tyr/gem.rs
index 1273bf89dbd5d..00804f8c14bd4 100644
--- a/drivers/gpu/drm/tyr/gem.rs
+++ b/drivers/gpu/drm/tyr/gem.rs
@@ -3,6 +3,7 @@
 use crate::driver::TyrDevice;
 use crate::driver::TyrDriver;
 use kernel::drm::gem;
+use kernel::drm::DeviceContext;
 use kernel::prelude::*;
 
 /// GEM Object inner driver data
@@ -12,7 +13,7 @@ pub(crate) struct TyrObject {}
 impl gem::DriverObject for TyrObject {
     type Driver = TyrDriver;
 
-    fn new(_dev: &TyrDevice, _size: usize) -> impl PinInit<Self, Error> {
+    fn new<Ctx: DeviceContext>(_dev: &TyrDevice<Ctx>, _size: usize) -> impl PinInit<Self, Error> {
         try_pin_init!(TyrObject {})
     }
 }
diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 441ed7c94fcf5..0864bd10af339 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -140,13 +140,13 @@ impl<T: drm::Driver> UnregisteredDevice<T> {
         master_set: None,
         master_drop: None,
         debugfs_init: None,
-        gem_create_object: T::Object::ALLOC_OPS.gem_create_object,
-        prime_handle_to_fd: T::Object::ALLOC_OPS.prime_handle_to_fd,
-        prime_fd_to_handle: T::Object::ALLOC_OPS.prime_fd_to_handle,
-        gem_prime_import: T::Object::ALLOC_OPS.gem_prime_import,
-        gem_prime_import_sg_table: T::Object::ALLOC_OPS.gem_prime_import_sg_table,
-        dumb_create: T::Object::ALLOC_OPS.dumb_create,
-        dumb_map_offset: T::Object::ALLOC_OPS.dumb_map_offset,
+        gem_create_object: T::Object::<Uninit>::ALLOC_OPS.gem_create_object,
+        prime_handle_to_fd: T::Object::<Uninit>::ALLOC_OPS.prime_handle_to_fd,
+        prime_fd_to_handle: T::Object::<Uninit>::ALLOC_OPS.prime_fd_to_handle,
+        gem_prime_import: T::Object::<Uninit>::ALLOC_OPS.gem_prime_import,
+        gem_prime_import_sg_table: T::Object::<Uninit>::ALLOC_OPS.gem_prime_import_sg_table,
+        dumb_create: T::Object::<Uninit>::ALLOC_OPS.dumb_create,
+        dumb_map_offset: T::Object::<Uninit>::ALLOC_OPS.dumb_map_offset,
         show_fdinfo: None,
         fbdev_probe: None,
 
diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index 108d789e43759..b37c5685e8ad8 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -109,7 +109,7 @@ pub trait Driver {
     type Data: Sync + Send;
 
     /// The type used to manage memory for this driver.
-    type Object: AllocImpl;
+    type Object<Ctx: drm::DeviceContext>: AllocImpl;
 
     /// The type used to represent a DRM File (client)
     type File: drm::file::DriverFile;
diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index d49a9ba026356..1dd8681a5738b 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -7,13 +7,16 @@
 use crate::{
     alloc::flags::*,
     bindings, drm,
-    drm::driver::{AllocImpl, AllocOps},
+    drm::{
+        device::{DeviceContext, Registered},
+        driver::{AllocImpl, AllocOps},
+    },
     error::{to_result, Result},
     prelude::*,
     sync::aref::{ARef, AlwaysRefCounted},
     types::Opaque,
 };
-use core::{ops::Deref, ptr::NonNull};
+use core::{marker::PhantomData, ops::Deref, ptr::NonNull};
 
 /// A type alias for retrieving a [`Driver`]s [`DriverFile`] implementation from its
 /// [`DriverObject`] implementation.
@@ -22,21 +25,30 @@
 /// [`DriverFile`]: drm::file::DriverFile
 pub type DriverFile<T> = drm::File<<<T as DriverObject>::Driver as drm::Driver>::File>;
 
+/// A type alias for retrieving the current [`AllocImpl`] for a given [`DriverObject`].
+///
+/// [`Driver`]: drm::Driver
+pub type DriverAllocImpl<T, Ctx = Registered> =
+    <<T as DriverObject>::Driver as drm::Driver>::Object<Ctx>;
+
 /// GEM object functions, which must be implemented by drivers.
 pub trait DriverObject: Sync + Send + Sized {
     /// Parent `Driver` for this object.
     type Driver: drm::Driver;
 
     /// Create a new driver data object for a GEM object of a given size.
-    fn new(dev: &drm::Device<Self::Driver>, size: usize) -> impl PinInit<Self, Error>;
+    fn new<Ctx: DeviceContext>(
+        dev: &drm::Device<Self::Driver, Ctx>,
+        size: usize,
+    ) -> impl PinInit<Self, Error>;
 
     /// Open a new handle to an existing object, associated with a File.
-    fn open(_obj: &<Self::Driver as drm::Driver>::Object, _file: &DriverFile<Self>) -> Result {
+    fn open(_obj: &DriverAllocImpl<Self>, _file: &DriverFile<Self>) -> Result {
         Ok(())
     }
 
     /// Close a handle to an existing object, associated with a File.
-    fn close(_obj: &<Self::Driver as drm::Driver>::Object, _file: &DriverFile<Self>) {}
+    fn close(_obj: &DriverAllocImpl<Self>, _file: &DriverFile<Self>) {}
 }
 
 /// Trait that represents a GEM object subtype
@@ -62,9 +74,12 @@ extern "C" fn open_callback<T: DriverObject>(
     // SAFETY: `open_callback` is only ever called with a valid pointer to a `struct drm_file`.
     let file = unsafe { DriverFile::<T>::from_raw(raw_file) };
 
-    // SAFETY: `open_callback` is specified in the AllocOps structure for `DriverObject<T>`,
-    // ensuring that `raw_obj` is contained within a `DriverObject<T>`
-    let obj = unsafe { <<T::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj) };
+    // SAFETY:
+    // * `open_callback` is specified in the AllocOps structure for `DriverObject`, ensuring that
+    //   `raw_obj` is contained within a `DriverAllocImpl<T>`
+    // * It is only possible for `open_callback` to be called after device registration, ensuring
+    //   that the object's device is in the `Registered` state.
+    let obj: &DriverAllocImpl<T> = unsafe { IntoGEMObject::from_raw(raw_obj) };
 
     match T::open(obj, file) {
         Err(e) => e.to_errno(),
@@ -81,12 +96,12 @@ extern "C" fn close_callback<T: DriverObject>(
 
     // SAFETY: `close_callback` is specified in the AllocOps structure for `Object<T>`, ensuring
     // that `raw_obj` is indeed contained within a `Object<T>`.
-    let obj = unsafe { <<T::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj) };
+    let obj: &DriverAllocImpl<T> = unsafe { IntoGEMObject::from_raw(raw_obj) };
 
     T::close(obj, file);
 }
 
-impl<T: DriverObject> IntoGEMObject for Object<T> {
+impl<T: DriverObject, Ctx: DeviceContext> IntoGEMObject for Object<T, Ctx> {
     fn as_raw(&self) -> *mut bindings::drm_gem_object {
         self.obj.get()
     }
@@ -94,7 +109,7 @@ fn as_raw(&self) -> *mut bindings::drm_gem_object {
     unsafe fn from_raw<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self {
         // SAFETY: `obj` is guaranteed to be in an `Object<T>` via the safety contract of this
         // function
-        unsafe { &*crate::container_of!(Opaque::cast_from(self_ptr), Object<T>, obj) }
+        unsafe { &*crate::container_of!(Opaque::cast_from(self_ptr), Object<T, Ctx>, obj) }
     }
 }
 
@@ -111,7 +126,7 @@ fn size(&self) -> usize {
     fn create_handle<D, F>(&self, file: &drm::File<F>) -> Result<u32>
     where
         Self: AllocImpl<Driver = D>,
-        D: drm::Driver<Object = Self, File = F>,
+        D: drm::Driver<Object<Registered> = Self, File = F>,
         F: drm::file::DriverFile<Driver = D>,
     {
         let mut handle: u32 = 0;
@@ -126,7 +141,7 @@ fn create_handle<D, F>(&self, file: &drm::File<F>) -> Result<u32>
     fn lookup_handle<D, F>(file: &drm::File<F>, handle: u32) -> Result<ARef<Self>>
     where
         Self: AllocImpl<Driver = D>,
-        D: drm::Driver<Object = Self, File = F>,
+        D: drm::Driver<Object<Registered> = Self, File = F>,
         F: drm::file::DriverFile<Driver = D>,
     {
         // SAFETY: The arguments are all valid per the type invariants.
@@ -166,16 +181,18 @@ impl<T: IntoGEMObject> BaseObject for T {}
 ///
 /// # Invariants
 ///
-/// - `self.obj` is a valid instance of a `struct drm_gem_object`.
+/// * `self.obj` is a valid instance of a `struct drm_gem_object`.
+/// * Any type invariants of `Ctx` apply to the parent DRM device for this GEM object.
 #[repr(C)]
 #[pin_data]
-pub struct Object<T: DriverObject + Send + Sync> {
+pub struct Object<T: DriverObject + Send + Sync, Ctx: DeviceContext = Registered> {
     obj: Opaque<bindings::drm_gem_object>,
     #[pin]
     data: T,
+    _ctx: PhantomData<Ctx>,
 }
 
-impl<T: DriverObject> Object<T> {
+impl<T: DriverObject, Ctx: DeviceContext> Object<T, Ctx> {
     const OBJECT_FUNCS: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
         free: Some(Self::free_callback),
         open: Some(open_callback::<T>),
@@ -195,11 +212,12 @@ impl<T: DriverObject> Object<T> {
     };
 
     /// Create a new GEM object.
-    pub fn new(dev: &drm::Device<T::Driver>, size: usize) -> Result<ARef<Self>> {
+    pub fn new(dev: &drm::Device<T::Driver, Ctx>, size: usize) -> Result<ARef<Self>> {
         let obj: Pin<KBox<Self>> = KBox::pin_init(
             try_pin_init!(Self {
                 obj: Opaque::new(bindings::drm_gem_object::default()),
                 data <- T::new(dev, size),
+                _ctx: PhantomData,
             }),
             GFP_KERNEL,
         )?;
@@ -208,6 +226,8 @@ pub fn new(dev: &drm::Device<T::Driver>, size: usize) -> Result<ARef<Self>> {
         unsafe { (*obj.as_raw()).funcs = &Self::OBJECT_FUNCS };
 
         // SAFETY: The arguments are all valid per the type invariants.
+        // INVARIANT: We use `dev` for creating the GEM object, which is known to be in state `Ctx` -
+        // ensuring that the GEM object's pointer to the DRM device is always in the same state.
         to_result(unsafe { bindings::drm_gem_object_init(dev.as_raw(), obj.obj.get(), size) })?;
 
         // SAFETY: We will never move out of `Self` as `ARef<Self>` is always treated as pinned.
@@ -221,13 +241,15 @@ pub fn new(dev: &drm::Device<T::Driver>, size: usize) -> Result<ARef<Self>> {
     }
 
     /// Returns the `Device` that owns this GEM object.
-    pub fn dev(&self) -> &drm::Device<T::Driver> {
+    pub fn dev(&self) -> &drm::Device<T::Driver, Ctx> {
         // SAFETY:
         // - `struct drm_gem_object.dev` is initialized and valid for as long as the GEM
         //   object lives.
         // - The device we used for creating the gem object is passed as &drm::Device<T::Driver> to
         //   Object::<T>::new(), so we know that `T::Driver` is the right generic parameter to use
         //   here.
+        // - Any type invariants of `Ctx` are upheld by using the same `Ctx` for the `Device` we
+        //   return.
         unsafe { drm::Device::from_raw((*self.as_raw()).dev) }
     }
 
@@ -253,7 +275,7 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
 }
 
 // SAFETY: Instances of `Object<T>` are always reference-counted.
-unsafe impl<T: DriverObject> crate::sync::aref::AlwaysRefCounted for Object<T> {
+unsafe impl<T: DriverObject, Ctx: DeviceContext> AlwaysRefCounted for Object<T, Ctx> {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::drm_gem_object_get(self.as_raw()) };
@@ -268,9 +290,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
-impl<T: DriverObject> super::private::Sealed for Object<T> {}
+impl<T: DriverObject, Ctx: DeviceContext> super::private::Sealed for Object<T, Ctx> {}
 
-impl<T: DriverObject> Deref for Object<T> {
+impl<T: DriverObject, Ctx: DeviceContext> Deref for Object<T, Ctx> {
     type Target = T;
 
     fn deref(&self) -> &Self::Target {
@@ -278,7 +300,7 @@ fn deref(&self) -> &Self::Target {
     }
 }
 
-impl<T: DriverObject> AllocImpl for Object<T> {
+impl<T: DriverObject, Ctx: DeviceContext> AllocImpl for Object<T, Ctx> {
     type Driver = T::Driver;
 
     const ALLOC_OPS: AllocOps = AllocOps {
-- 
2.52.0

Re: [PATCH v2 1/3] rust/drm: Introduce DeviceContext

[Danilo Krummrich]

On Fri Jan 16, 2026 at 9:41 PM CET, Lyude Paul wrote:
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index 3ce8f62a00569..53ca71daf2f86 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -7,14 +7,20 @@
>  use crate::{
>      alloc::allocator::Kmalloc,
>      bindings, device, drm,
> -    drm::driver::AllocImpl,
> +    drm::{driver::AllocImpl, private::Sealed},

Please use kernel vertical style [1] when making changes.

(If you want you can also add an additional patch converting the whole DRM
module first for consistency.)

[1] https://docs.kernel.org/rust/coding-guidelines.html#imports

>      error::from_err_ptr,
>      error::Result,
>      prelude::*,
>      sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    types::{NotThreadSafe, Opaque},
> +};
> +use core::{
> +    alloc::Layout,
> +    marker::PhantomData,
> +    mem::{self},
> +    ops::Deref,
> +    ptr::{self, NonNull},
>  };
> -use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
>  
>  #[cfg(CONFIG_DRM_LEGACY)]
>  macro_rules! drm_legacy_fields {
> @@ -47,26 +53,88 @@ macro_rules! drm_legacy_fields {
>      }
>  }
>  
> -/// A typed DRM device with a specific `drm::Driver` implementation.
> +macro_rules! drm_dev_ctx {
> +    (
> +        $( #[$attrs:meta] )*
> +        $name:ident
> +    ) => {
> +        $( #[$attrs] )*
> +        pub struct $name;
> +
> +        impl DeviceContext for $name {}
> +        impl Sealed for $name {}
> +
> +        // SAFETY: All registration states are free of side-effects (e.g. no Drop) and are ZSTs,
> +        // thus they are always thread-safe.
> +        unsafe impl Send for $name {}
> +        // SAFETY: All registration states are free of side-effects (e.g. no Drop) and are ZSTs,
> +        // thus they are always thread-safe.
> +        unsafe impl Sync for $name {}

They are all ZSTs and should be Send + Sync by default, hence the impls are not
needed.

Also, I don't think this macro adds a lot of value when the Send and Sync impls
are removed. In the driver core it looks like this:

	mod private {
	    pub trait Sealed {}
	
	    impl Sealed for super::Bound {}
	    impl Sealed for super::Core {}
	    impl Sealed for super::CoreInternal {}
	    impl Sealed for super::Normal {}
	}
	
	impl DeviceContext for Bound {}
	impl DeviceContext for Core {}
	impl DeviceContext for CoreInternal {}
	impl DeviceContext for Normal {}

I think that's compact enough.

> +    };
> +}
> +
> +/// A trait implemented by all possible contexts a [`Device`] can be used in.

I think this documentation should provide a rough overview of the different
states a DRM device can have.

> +pub trait DeviceContext: Sealed + Send + Sync {}
> +
> +drm_dev_ctx! {
> +    /// The [`DeviceContext`] of a [`Device`] that was registered with userspace at some point.
> +    ///
> +    /// This represents a [`Device`] which is guaranteed to have been registered with userspace at
> +    /// some point in time. Such a DRM device is guaranteed to have been fully-initialized.
> +    ///
> +    /// Note: A device in this context is not guaranteed to remain registered with userspace for its
> +    /// entire lifetime, as this is impossible to guarantee at compile-time. However, any
> +    /// userspace-dependent operations performed with an unregistered device in this [`DeviceContext`]
> +    /// are guaranteed to be no-ops.
> +    ///
> +    /// # Invariants
> +    ///
> +    /// A [`Device`] in this [`DeviceContext`] is guaranteed to have called `drm_dev_register` once.

I'm not sure it makes sense for user facing documentation to menthion the
corresponding C function here. I'd just say that it has been registered with the
DRM core.

> +    Registered
> +}
> +
> @@ -132,9 +202,40 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
>  
>          // SAFETY: The reference count is one, and now we take ownership of that reference as a
>          // `drm::Device`.
> -        Ok(unsafe { ARef::from_raw(raw_drm) })
> +        // INVARIANT: We just created the device above, but have yet to call `drm_dev_register`.
> +        // `Self` cannot be copied or sent to another thread - ensuring that `drm_dev_register`
> +        // won't be called during its lifetime and that the device is unregistered

Please end with a period.

> +        Ok(Self(unsafe { ARef::from_raw(raw_drm) }, NotThreadSafe))
>      }
> +}
>  
> +/// A typed DRM device with a specific [`drm::Driver`] implementation and [`DeviceContext`].
> +///
> +/// Since DRM devices can be used before being fully initialized and registered with userspace, `C`
> +/// represents the furthest [`DeviceContext`] we can guarantee that this [`Device`] has reached.
> +///
> +/// Keep in mind: this means that an unregistered device can still have the registration state
> +/// [`Registered`] as long as it was registered with userspace once in the past, and that the
> +/// behavior of such a device is still well-defined. In such a situation, the behavior of any
> +/// functions which interact with userspace will simply be no-ops. Additionally, a device with the

I don't think those are necessarily no-ops, in hot-unplug cases drivers might
still receive DRM callbacks from userspace. But they won't be able to access bus
device resources anymore. In contrast to the DRM C implementation the
corresponding synchronization happens at driver core level in Rust, whereas on
the C side there is drm_dev_enter() / drm_dev_exit() for this.

> +/// registration state [`Uninit`] simply does not have a guaranteed registration state at compile
> +/// time, and could be either registered or unregistered. Since there is no way to guarantee a
> +/// long-lived reference to an unregistered device would remain unregistered, we do not provide a
> +/// [`DeviceContext`] for this.
> +///
> +/// # Invariants
> +///
> +/// * `self.dev` is a valid instance of a `struct device`.
> +/// * The data layout of `Self` remains the same across all implementations of `C`.
> +/// * Any invariants for `C` also apply.
> +#[repr(C)]
> +pub struct Device<T: drm::Driver, C: DeviceContext = Registered> {
> +    dev: Opaque<bindings::drm_device>,
> +    data: T::Data,
> +    _ctx: PhantomData<C>,
> +}
> +
> +impl<T: drm::Driver, C: DeviceContext> Device<T, C> {
>      pub(crate) fn as_raw(&self) -> *mut bindings::drm_device {
>          self.dev.get()
>      }
> @@ -160,13 +261,13 @@ unsafe fn into_drm_device(ptr: NonNull<Self>) -> *mut bindings::drm_device {
>      ///
>      /// # Safety
>      ///
> -    /// Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count,
> -    /// i.e. it must be ensured that the reference count of the C `struct drm_device` `ptr` points
> -    /// to can't drop to zero, for the duration of this function call and the entire duration when
> -    /// the returned reference exists.
> -    ///
> -    /// Additionally, callers must ensure that the `struct device`, `ptr` is pointing to, is
> -    /// embedded in `Self`.
> +    /// * Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count,
> +    ///   i.e. it must be ensured that the reference count of the C `struct drm_device` `ptr` points
> +    ///   to can't drop to zero, for the duration of this function call and the entire duration when
> +    ///   the returned reference exists.
> +    /// * Additionally, callers must ensure that the `struct device`, `ptr` is pointing to, is

I'd drop "additionally", it is a listing anyways.

> +    ///   embedded in `Self`.
> +    /// * Callers promise that any type invariants of `C` will be upheld.

What do you mean by "type invariants of C"?

>  impl<T: Driver> Registration<T> {
>      /// Creates a new [`Registration`] and registers it.
> -    fn new(drm: &drm::Device<T>, flags: usize) -> Result<Self> {
> +    fn new(drm: drm::UnregisteredDevice<T>, flags: usize) -> Result<Self> {
>          // SAFETY: `drm.as_raw()` is valid by the invariants of `drm::Device`.
>          to_result(unsafe { bindings::drm_dev_register(drm.as_raw(), flags) })?;
>  
> -        Ok(Self(drm.into()))
> +        // SAFETY: We just called `drm_dev_register` above
> +        let new = NonNull::from(unsafe { drm.assume_ctx() });
> +
> +        // Leak the ARef from UnregisteredDevice in preparation for transferring its ownership.
> +        mem::forget(drm);
> +
> +        // SAFETY: `drm`'s `Drop` constructor was never called, ensuring that there remains at least
> +        // one reference to the device - which we take ownership over here.
> +        let new = unsafe { ARef::from_raw(new) };
> +
> +        Ok(Self(new))
>      }
>  
> -    /// Same as [`Registration::new`}, but transfers ownership of the [`Registration`] to
> +    /// Same as [`Registration::new`], but transfers ownership of the [`Registration`] to
>      /// [`devres::register`].
> -    pub fn new_foreign_owned(
> -        drm: &drm::Device<T>,
> -        dev: &device::Device<device::Bound>,
> +    pub fn new_foreign_owned<'a>(
> +        drm: drm::UnregisteredDevice<T>,
> +        dev: &'a device::Device<device::Bound>,
>          flags: usize,
> -    ) -> Result
> +    ) -> Result<&'a drm::Device<T>>
>      where
>          T: 'static,
>      {
> -        if drm.as_ref().as_raw() != dev.as_raw() {
> +        let this_dev: &device::Device = drm.as_ref();
> +        if this_dev.as_raw() != dev.as_raw() {

this_dev is misleading, since it actually is the parent device. Besides that,
the change seems unnecessary, no?

>              return Err(EINVAL);
>          }
>  
>          let reg = Registration::<T>::new(drm, flags)?;
> +        let drm = NonNull::from(reg.device());
> +
> +        devres::register(dev, reg, GFP_KERNEL)?;
>  
> -        devres::register(dev, reg, GFP_KERNEL)
> +        // SAFETY: Since `reg` was passed to devres::register(), the device now owns the lifetime
> +        // of the DRM registration - ensuring that this references lives for at least as long as 'a.
> +        Ok(unsafe { drm.as_ref() })
>      }

Re: [PATCH v2 2/3] rust/drm: Don't setup private driver data until registration

[Danilo Krummrich]

On Fri Jan 16, 2026 at 9:41 PM CET, Lyude Paul wrote:
> @@ -118,7 +120,7 @@ pub trait DeviceContext: Sealed + Send + Sync {}
>  ///
>  /// The device in `self.0` is guaranteed to be a newly created [`Device`] that has not yet been
>  /// registered with userspace until this type is dropped.
> -pub struct UnregisteredDevice<T: drm::Driver>(ARef<Device<T, Uninit>>, NotThreadSafe);
> +pub struct UnregisteredDevice<T: drm::Driver>(pub(crate) ARef<Device<T, Uninit>>, NotThreadSafe);
>  
>  impl<T: drm::Driver> Deref for UnregisteredDevice<T> {
>      type Target = Device<T, Uninit>;
> @@ -165,7 +167,7 @@ impl<T: drm::Driver> UnregisteredDevice<T> {
>      /// Create a new `UnregisteredDevice` for a `drm::Driver`.
>      ///
>      /// This can be used to create a [`Registration`](kernel::drm::Registration).
> -    pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<Self> {
> +    pub fn new(dev: &device::Device) -> Result<Self> {
>          // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
>          // compatible `Layout`.
>          let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
> @@ -184,22 +186,6 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<S
>          .cast();
>          let raw_drm = NonNull::new(from_err_ptr(raw_drm)?).ok_or(ENOMEM)?;
>  
> -        // SAFETY: `raw_drm` is a valid pointer to `Self`.
> -        let raw_data = unsafe { ptr::addr_of_mut!((*raw_drm.as_ptr()).data) };
> -
> -        // SAFETY:
> -        // - `raw_data` is a valid pointer to uninitialized memory.
> -        // - `raw_data` will not move until it is dropped.
> -        unsafe { data.__pinned_init(raw_data) }.inspect_err(|_| {
> -            // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
> -            // successful.
> -            let drm_dev = unsafe { Device::into_drm_device(raw_drm) };
> -
> -            // SAFETY: `__drm_dev_alloc()` was successful, hence `drm_dev` must be valid and the
> -            // refcount must be non-zero.
> -            unsafe { bindings::drm_dev_put(drm_dev) };
> -        })?;
> -
>          // SAFETY: The reference count is one, and now we take ownership of that reference as a
>          // `drm::Device`.
>          // INVARIANT: We just created the device above, but have yet to call `drm_dev_register`.
> @@ -231,7 +217,15 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<S
>  #[repr(C)]
>  pub struct Device<T: drm::Driver, C: DeviceContext = Registered> {
>      dev: Opaque<bindings::drm_device>,
> -    data: T::Data,
> +
> +    /// Keeps track of whether we've initialized the device data yet.
> +    pub(crate) data_is_init: AtomicBool,

Please use a kernel atomic, i.e. kernel::sync::atomic::Atomic<bool>. See also [1].

[1] https://lore.kernel.org/all/20251230093718.1852322-1-fujita.tomonori@gmail.com/

> +    /// The Driver's private data.
> +    ///
> +    /// This must only be written to from [`drm::Registration::new`].
> +    pub(crate) data: UnsafeCell<MaybeUninit<T::Data>>,

Why not just Opaque<T::Data>?

> +
>      _ctx: PhantomData<C>,
>  }