rust: drm: ensure kmalloc() compatible Layout

drm::Device is allocated through __drm_dev_alloc() (which uses
kmalloc()) and the driver private data, <T as drm::Driver>::Data, is
initialized in-place.

Due to the order of fields in drm::Device

  pub struct Device<T: drm::Driver> {
     dev: Opaque<bindings::drm_device>,
     data: T::Data,
  }

even with an arbitrary large alignment requirement of T::Data it can't
happen that the size of Device is smaller than its alignment requirement.

However, let's not rely on this subtle circumstance and create a proper
kmalloc() compatible Layout.

Fixes: 1e4b889 ("rust: drm: add device abstraction")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250731154919.4132-3-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Danilo Krummrich

rust/kernel/drm/device.rs

@@ -5,14 +5,15 @@
 //! C header: [`include/linux/drm/drm_device.h`](srctree/include/linux/drm/drm_device.h)
 
 use crate::{
+    alloc::allocator::Kmalloc,
     bindings, device, drm,
     drm::driver::AllocImpl,
     error::from_err_ptr,
     error::Result,
     prelude::*,
     types::{ARef, AlwaysRefCounted, Opaque},
 };
-use core::{mem, ops::Deref, ptr, ptr::NonNull};
+use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
 
 #[cfg(CONFIG_DRM_LEGACY)]
 macro_rules! drm_legacy_fields {
@@ -96,14 +97,18 @@ impl<T: drm::Driver> Device<T> {
 
     /// Create a new `drm::Device` for a `drm::Driver`.
     pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<ARef<Self>> {
+        // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
+        // compatible `Layout`.
+        let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
+
         // 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 {
             bindings::__drm_dev_alloc(
                 dev.as_raw(),
                 &Self::VTABLE,
-                mem::size_of::<Self>(),
+                layout.size(),
                 mem::offset_of!(Self, dev),
             )
         }