Claude review: rust: drm: gem/shmem: Add DmaResvGuard helper

Claude review: rust: drm: gem/shmem: Add DmaResvGuard helper

[Claude Code Review Bot]

Patch Review

**Status: Minor concerns**

The guard pattern is correct in structure:

```rust
+struct DmaResvGuard<'a, T: DriverObject>(&'a Object<T>);
+
+impl<'a, T: DriverObject> DmaResvGuard<'a, T> {
+    fn new(obj: &'a Object<T>) -> Self {
+        unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
+        Self(obj)
+    }
+}
```

**Concern 1**: `dma_resv_lock()` returns `int`, but the return value is silently discarded. With `ctx = NULL`, `ww_mutex_lock` reduces to a simple mutex lock that in practice always returns 0, but ignoring the return value is not ideal. Since this is called from `Drop` contexts (via `SGTableMap::drop` in patch 6) where returning errors isn't possible, at minimum a `WARN_ON` or `BUG_ON` on failure would be appropriate — or a comment explaining why the return is ignored.

**Concern 2**: The `#[expect(unused)]` attribute is removed in patch 6, which is correct for the series as a whole, but it means patch 5 in isolation adds dead code. This is a minor bisectability concern — consider squashing patches 5 and 6, or keeping `#[expect(unused)]` in patch 5 and removing it in patch 6.

The TODO comment about replacing with WwMutex is appropriate.

---
Generated by Claude Code Patch Reviewer

[PATCH v16 0/6] Rust bindings for gem shmem

[Lyude Paul]

Most of this patch series has already been pushed upstream, this is just
the second half of the patch series that has not been pushed yet + some
additional changes which were required to implement changes requested by
the mailing list. This patch series is originally from Asahi, previously
posted by Daniel Almeida.

The previous version of the patch series can be found here:

	https://patchwork.freedesktop.org/series/164580/

Branch with patches applied available here:

	https://gitlab.freedesktop.org/lyudess/linux/-/commits/rust/gem-shmem

This patch series applies on top of drm-rust-next

Patch-series wide changes since V15:
* Fix some major rebasing errors I somehow didn't notice :(
* Drop the dependency on LazyInit, use the trick that Alice suggested
  instead.
* Fix dependency ordering so that Tyr can get the vmap stuff first
  without the other bits.

Lyude Paul (6):
  rust: drm: gem/shmem: Add DmaResvGuard helper
  rust: drm: gem: Add vmap functions to shmem bindings
  rust: sync: Add SetOnce::reset()
  rust: gem: shmem: Fix Default implementation for ObjectConfig
  rust: faux: Allow retrieving a bound Device
  rust: drm: gem: Introduce shmem::Object::sg_table()

 rust/kernel/drm/gem/shmem.rs | 507 ++++++++++++++++++++++++++++++++++-
 rust/kernel/faux.rs          |   7 +-
 rust/kernel/sync/set_once.rs |  60 ++++-
 3 files changed, 552 insertions(+), 22 deletions(-)


base-commit: b78dab829760aee9b83f5cf15550a0fe36c6f4b0
-- 
2.54.0

[PATCH v16 1/6] rust: drm: gem/shmem: Add DmaResvGuard helper

[Lyude Paul]

Just a temporary holdover to make locking/unlocking the dma_resv lock much
easier.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Co-authored-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/drm/gem/shmem.rs | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 35d7523e164ff..066a820b0bffc 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -27,7 +27,10 @@
         Deref,
         DerefMut, //
     },
-    ptr::NonNull,
+    ptr::{
+        self,
+        NonNull, //
+    },
 };
 use gem::{
     BaseObjectPrivate,
@@ -224,3 +227,29 @@ impl<T: DriverObject> driver::AllocImpl for Object<T> {
         dumb_map_offset: None,
     };
 }
+
+/// Private helper-type for holding the `dma_resv` object for a GEM shmem object.
+///
+/// When this is dropped, the `dma_resv` lock is dropped as well.
+///
+// TODO: This should be replace with a WwMutex equivalent once we have such bindings in the kernel.
+struct DmaResvGuard<'a, T: DriverObject>(&'a Object<T>);
+
+impl<'a, T: DriverObject> DmaResvGuard<'a, T> {
+    #[inline(always)]
+    #[expect(unused)]
+    fn new(obj: &'a Object<T>) -> Self {
+        // SAFETY: This lock is initialized throughout the lifetime of `object`.
+        unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
+
+        Self(obj)
+    }
+}
+
+impl<'a, T: DriverObject> Drop for DmaResvGuard<'a, T> {
+    #[inline(always)]
+    fn drop(&mut self) {
+        // SAFETY: We are releasing the lock grabbed during the creation of this object.
+        unsafe { bindings::dma_resv_unlock(self.0.raw_dma_resv()) };
+    }
+}
-- 
2.54.0

[PATCH v16 2/6] rust: drm: gem: Add vmap functions to shmem bindings

[Lyude Paul]

One of the more obvious use cases for gem shmem objects is the ability to
create mappings into their contents. So, let's hook this up in our rust
bindings.

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

---
V7:
* Switch over to the new iosys map bindings that use the Io trait
V8:
* Get rid of iosys_map bindings for now, only support non-iomem types
* s/as_shmem()/as_raw_shmem()
V9:
* Get rid of some outdated comments I missed
* Add missing SIZE check to raw_vmap()
* Add a proper unit test that ensures that we actually validate SIZE at
  compile-time.
  Turns out it takes only 34 lines to make a boilerplate DRM driver for a
  kunit test :)
* Add unit tests
* Add some missing #[inline]s
V10:
* Correct issue with iomem error path
  We previously called raw_vunmap() if we got an iomem allocation, but
  raw_vunmap() was written such that it assumed all allocations were sysmem
  allocations. Fix this by just making raw_vunmap() accept a iosys_map.
V11:
* Use Alexandre's clever solution to remove the macros we were using for
  maintaining two different VMap types.
* Change the order of items in Object<T> to ensure that sgt_res is always
  dropped before obj.
* Fix typo in Object.raw_vmap()
* s/raw_vmap()/make_vmap()/
  Deduplicate code a bit more as well by using more generics here
V15:
* Add these patches back
* We only have one VMap type now!
* Use ObjectConfig::default() in unit tests since we unbroke it.
V16:
* Fix huge rebase error I made and did not notice that squashed 1.5 patches
  together that were definitely not supposed to be squashed
* Update old commit message

 rust/kernel/drm/gem/shmem.rs | 303 ++++++++++++++++++++++++++++++++++-
 1 file changed, 302 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 066a820b0bffc..843869056b1d7 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -18,11 +18,18 @@
         Device, //
     },
     error::to_result,
+    io::{
+        Io,
+        IoCapable,
+        IoKnownSize, //
+    },
     prelude::*,
     sync::aref::ARef,
     types::Opaque, //
 };
 use core::{
+    ffi::c_void,
+    mem::MaybeUninit, //
     ops::{
         Deref,
         DerefMut, //
@@ -33,6 +40,7 @@
     },
 };
 use gem::{
+    BaseObject,
     BaseObjectPrivate,
     DriverObject,
     IntoGEMObject, //
@@ -177,6 +185,80 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
         // SAFETY: We're recovering the Kbox<> we created in gem_create_object()
         let _ = unsafe { KBox::from_raw(this) };
     }
+
+    /// Attempt to create a vmap from the gem object, and confirm the size of said vmap.
+    fn make_vmap<'a, R, const SIZE: usize>(&'a self) -> Result<VMap<T, R, SIZE>>
+    where
+        R: Deref<Target = Self> + From<&'a Self>,
+    {
+        // INVARIANT: We check here that the gem object is at least as large as `SIZE`.
+        if self.size() < SIZE {
+            return Err(ENOSPC);
+        }
+
+        let mut map: MaybeUninit<bindings::iosys_map> = MaybeUninit::uninit();
+        let guard = DmaResvGuard::new(self);
+
+        // SAFETY: drm_gem_shmem_vmap can be called with the DMA reservation lock held
+        to_result(unsafe {
+            bindings::drm_gem_shmem_vmap_locked(self.as_raw_shmem(), map.as_mut_ptr())
+        })?;
+
+        // Drop the guard explicitly here, since we may need to call raw_vunmap() (which re-acquires
+        // the lock).
+        drop(guard);
+
+        // SAFETY: The call to drm_gem_shmem_vmap_locked succeeded above, so we are guaranteed that
+        // map is properly initialized.
+        let map = unsafe { map.assume_init() };
+
+        // XXX: We don't currently support iomem allocations
+        if map.is_iomem {
+            // SAFETY:
+            // - The vmap operation above succeeded, guaranteeing that `map` points to a valid
+            //   memory mapping.
+            // - We checked that this is an iomem allocation, making it safe to read vaddr_iomem
+            unsafe { self.raw_vunmap(map) };
+
+            Err(ENOTSUPP)
+        } else {
+            Ok(VMap {
+                // SAFETY: We checked that this is not an iomem allocation, making it safe to read
+                // vaddr
+                addr: unsafe { map.__bindgen_anon_1.vaddr },
+                owner: self.into(),
+            })
+        }
+    }
+
+    /// Unmap a vmap from the gem object.
+    ///
+    /// # Safety
+    ///
+    /// - The caller promises that `map` is a valid vmap on this gem object.
+    /// - The caller promises that the memory pointed to by map will no longer be accesed through
+    ///   this instance.
+    unsafe fn raw_vunmap(&self, mut map: bindings::iosys_map) {
+        let _guard = DmaResvGuard::new(self);
+
+        // SAFETY:
+        // - This function is safe to call with the DMA reservation lock held.
+        // - Our `ARef` is proof that the underlying gem object here is initialized and thus safe to
+        //   dereference.
+        unsafe { bindings::drm_gem_shmem_vunmap_locked(self.as_raw_shmem(), &mut map) };
+    }
+
+    /// Creates and returns a virtual kernel memory mapping for this object.
+    #[inline]
+    pub fn vmap<const SIZE: usize>(&self) -> Result<VMapRef<'_, T, SIZE>> {
+        self.make_vmap()
+    }
+
+    /// Creates and returns an owned reference to a virtual kernel memory mapping for this object.
+    #[inline]
+    pub fn owned_vmap<const SIZE: usize>(&self) -> Result<VMapOwned<T, SIZE>> {
+        self.make_vmap()
+    }
 }
 
 impl<T: DriverObject> Deref for Object<T> {
@@ -237,7 +319,6 @@ impl<T: DriverObject> driver::AllocImpl for Object<T> {
 
 impl<'a, T: DriverObject> DmaResvGuard<'a, T> {
     #[inline(always)]
-    #[expect(unused)]
     fn new(obj: &'a Object<T>) -> Self {
         // SAFETY: This lock is initialized throughout the lifetime of `object`.
         unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
@@ -253,3 +334,223 @@ fn drop(&mut self) {
         unsafe { bindings::dma_resv_unlock(self.0.raw_dma_resv()) };
     }
 }
+
+macro_rules! impl_vmap_io_capable {
+    ($impl:ident, $ty:ty) => {
+        impl<D, R, const SIZE: usize> IoCapable<$ty> for $impl<D, R, SIZE>
+        where
+            D: DriverObject,
+            R: Deref<Target = Object<D>>,
+        {
+            #[inline(always)]
+            unsafe fn io_read(&self, address: usize) -> $ty {
+                let ptr = address as *mut $ty;
+
+                // SAFETY: The safety contract of `io_read` guarantees that address is a valid
+                // address within the bounds of `Self` of at least the size of $ty, and is properly
+                // aligned.
+                unsafe { ptr::read(ptr) }
+            }
+
+            #[inline(always)]
+            unsafe fn io_write(&self, value: $ty, address: usize) {
+                let ptr = address as *mut $ty;
+
+                // SAFETY: The safety contract of `io_write` guarantees that address is a valid
+                // address within the bounds of `Self` of at least the size of $ty, and is properly
+                // aligned.
+                unsafe { ptr::write(ptr, value) }
+            }
+        }
+    };
+}
+
+/// A reference to a virtual mapping for an shmem-based GEM object in kernel address space.
+///
+/// # Invariants
+///
+/// - The size of `owner` is >= SIZE.
+/// - The memory pointed to by addr remains valid at least until this object is dropped.
+pub struct VMap<D, R, const SIZE: usize = 0>
+where
+    D: DriverObject,
+    R: Deref<Target = Object<D>>,
+{
+    addr: *mut c_void,
+    owner: R,
+}
+
+/// An alias type for a reference to a shmem-based GEM object's VMap.
+pub type VMapRef<'a, D, const SIZE: usize = 0> = VMap<D, &'a Object<D>, SIZE>;
+
+/// An alias type for an owned reference to a shmem-based GEM object's VMap.
+pub type VMapOwned<D, const SIZE: usize = 0> = VMap<D, ARef<Object<D>>, SIZE>;
+
+impl<D, R, const SIZE: usize> VMap<D, R, SIZE>
+where
+    D: DriverObject,
+    R: Deref<Target = Object<D>>,
+{
+    /// Borrows a reference to the object that owns this virtual mapping.
+    #[inline(always)]
+    pub fn owner(&self) -> &Object<D> {
+        &self.owner
+    }
+}
+
+impl<D, R, const SIZE: usize> Drop for VMap<D, R, SIZE>
+where
+    D: DriverObject,
+    R: Deref<Target = Object<D>>,
+{
+    #[inline(always)]
+    fn drop(&mut self) {
+        // SAFETY:
+        // - Our existence is proof that this map was previously created using self.owner.
+        // - Since we are in Drop, we are guaranteed that no one will access the memory
+        //   through this mapping after calling this.
+        unsafe {
+            self.owner.raw_vunmap(bindings::iosys_map {
+                is_iomem: false,
+                __bindgen_anon_1: bindings::iosys_map__bindgen_ty_1 { vaddr: self.addr },
+            })
+        };
+    }
+}
+
+impl<D, R, const SIZE: usize> Io for VMap<D, R, SIZE>
+where
+    D: DriverObject,
+    R: Deref<Target = Object<D>>,
+{
+    #[inline(always)]
+    fn addr(&self) -> usize {
+        self.addr as usize
+    }
+
+    #[inline(always)]
+    fn maxsize(&self) -> usize {
+        self.owner.size()
+    }
+}
+
+impl<D, R, const SIZE: usize> IoKnownSize for VMap<D, R, SIZE>
+where
+    D: DriverObject,
+    R: Deref<Target = Object<D>>,
+{
+    const MIN_SIZE: usize = SIZE;
+}
+
+impl_vmap_io_capable!(VMap, u8);
+impl_vmap_io_capable!(VMap, u16);
+impl_vmap_io_capable!(VMap, u32);
+#[cfg(CONFIG_64BIT)]
+impl_vmap_io_capable!(VMap, u64);
+
+#[kunit_tests(rust_drm_gem_shmem)]
+mod tests {
+    use super::*;
+    use crate::{
+        drm,
+        faux,
+        page::PAGE_SIZE, //
+    };
+
+    // The bare minimum needed to create a fake drm driver for kunit
+
+    #[pin_data]
+    struct KunitData {}
+    struct KunitDriver;
+    struct KunitFile;
+    #[pin_data]
+    struct KunitObject {}
+
+    const INFO: drm::DriverInfo = drm::DriverInfo {
+        major: 0,
+        minor: 0,
+        patchlevel: 0,
+        name: c"kunit",
+        desc: c"Kunit",
+    };
+
+    impl drm::file::DriverFile for KunitFile {
+        type Driver = KunitDriver;
+
+        fn open(_dev: &drm::Device<KunitDriver>) -> Result<Pin<KBox<Self>>> {
+            Ok(KBox::new(Self, GFP_KERNEL)?.into())
+        }
+    }
+
+    impl gem::DriverObject for KunitObject {
+        type Driver = KunitDriver;
+        type Args = ();
+
+        fn new(
+            _dev: &drm::Device<KunitDriver>,
+            _size: usize,
+            _args: Self::Args,
+        ) -> impl PinInit<Self, Error> {
+            try_pin_init!(KunitObject {})
+        }
+    }
+
+    #[vtable]
+    impl drm::Driver for KunitDriver {
+        type Data = KunitData;
+        type File = KunitFile;
+        type Object = Object<KunitObject>;
+
+        const INFO: drm::DriverInfo = INFO;
+        const IOCTLS: &'static [drm::ioctl::DrmIoctlDescriptor] = &[];
+    }
+
+    fn create_drm_dev() -> Result<(faux::Registration, ARef<drm::Device<KunitDriver>>)> {
+        // Create a faux DRM device so we can test gem object creation.
+        let data = try_pin_init!(KunitData {});
+        let dev = faux::Registration::new(c"Kunit", None)?;
+        let drm = drm::Device::<KunitDriver>::new(dev.as_ref(), data)?;
+
+        Ok((dev, drm))
+    }
+
+    #[test]
+    fn compile_time_vmap_sizes() -> Result {
+        let (_dev, drm) = create_drm_dev()?;
+
+        let obj = Object::<KunitObject>::new(&drm, PAGE_SIZE, ObjectConfig::default(), ())?;
+
+        // Try creating a normal vmap
+        obj.vmap::<PAGE_SIZE>()?;
+
+        // Try creating a vmap that's smaller then the size we specified
+        obj.vmap::<{ PAGE_SIZE - 100 }>()?;
+
+        // Make sure creating a vmap that's too large fails
+        assert!(obj.vmap::<{ PAGE_SIZE + 200 }>().is_err());
+
+        Ok(())
+    }
+
+    #[test]
+    fn vmap_io() -> Result {
+        let (_dev, drm) = create_drm_dev()?;
+
+        let obj = Object::<KunitObject>::new(&drm, PAGE_SIZE, ObjectConfig::default(), ())?;
+
+        let vmap = obj.vmap::<PAGE_SIZE>()?;
+
+        vmap.write8(0xDE, 0x0);
+        assert_eq!(vmap.read8(0x0), 0xDE);
+        vmap.write32(0xFFFFFFFF, 0x20);
+
+        assert_eq!(vmap.read32(0x20), 0xFFFFFFFF);
+
+        assert_eq!(vmap.read8(0x20), 0xFF);
+        assert_eq!(vmap.read8(0x21), 0xFF);
+        assert_eq!(vmap.read8(0x22), 0xFF);
+        assert_eq!(vmap.read8(0x23), 0xFF);
+
+        Ok(())
+    }
+}
-- 
2.54.0

[PATCH v16 3/6] rust: sync: Add SetOnce::reset()

[Lyude Paul]

This function simply drops the contents of the SetOnce, given a mutable
reference - since that proves we have exclusive access to the SetOnce.
Additionally, update the invariants for SetOnce to make it clear as to why
this is safe.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/sync/set_once.rs | 60 +++++++++++++++++++++++++++++++-----
 1 file changed, 52 insertions(+), 8 deletions(-)

diff --git a/rust/kernel/sync/set_once.rs b/rust/kernel/sync/set_once.rs
index 139cef05e935f..d6e4fc2695673 100644
--- a/rust/kernel/sync/set_once.rs
+++ b/rust/kernel/sync/set_once.rs
@@ -15,7 +15,7 @@
 ///
 /// # Invariants
 ///
-/// - `init` may only increase in value.
+/// - `init` may only increase in value, unless modified through a mutable reference.
 /// - `init` may only assume values in the range `0..=2`.
 /// - `init == 0` if and only if `value` is uninitialized.
 /// - `init == 1` if and only if there is exactly one thread with exclusive
@@ -110,17 +110,61 @@ pub fn copy(&self) -> Option<T>
     {
         self.as_ref().copied()
     }
+
+    /// # Safety
+    ///
+    /// If this function returns `true`, `self` must be freed or `init` must be reset to `0`.
+    unsafe fn drop_val(&mut self) -> bool {
+        if *self.init.get_mut() != 2 {
+            return false;
+        }
+
+        let value = self.value.get_mut();
+        // SAFETY: Via our type invariants, `init` == 2 means `value` is initialized.
+        unsafe { value.assume_init_drop() };
+
+        true
+    }
+
+    /// Unset the [`SetOnce`].
+    ///
+    /// After this function is called, the [`SetOnce`] is empty and uninitialized. This function is
+    /// mainly intended for usage in destructors.
+    ///
+    /// Returns `true` if `self` was previously initialized.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use kernel::sync::SetOnce;
+    /// let mut value = SetOnce::new();
+    /// assert_eq!(value.populate(67), true);
+    ///
+    /// assert_eq!(value.reset(), true);
+    /// assert!(value.as_ref().is_none());
+    /// assert_eq!(value.reset(), false);
+    ///
+    /// assert_eq!(value.populate(69), true);
+    /// ```
+    pub fn reset(&mut self) -> bool {
+        // SAFETY: We write `0` to init below if this returns true.
+        let dropped = unsafe { self.drop_val() };
+        if dropped {
+            // INVARIANT:
+            // - We set `init` back to 0 through a mutable reference.
+            // - We dropped `value` above.
+            *self.init.get_mut() = 0;
+        }
+
+        dropped
+    }
 }
 
 impl<T> Drop for SetOnce<T> {
+    #[inline(always)]
     fn drop(&mut self) {
-        if *self.init.get_mut() == 2 {
-            let value = self.value.get_mut();
-            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
-            // contains a valid value. We have exclusive access, as we hold a `mut` reference to
-            // `self`.
-            unsafe { value.assume_init_drop() };
-        }
+        // SAFETY: We are dropping this value.
+        unsafe { self.drop_val() };
     }
 }
 
-- 
2.54.0

[PATCH v16 4/6] rust: gem: shmem: Fix Default implementation for ObjectConfig

[Lyude Paul]

I completely forgot when coming up with this type that #[derive(Default)]
only works if all generics mentioned in the type implement Default (and T
usually doesn't). This being said: We don't use `T` for anything besides
using it for a reference type, so whether or not it implements `Default`
shouldn't actually need to matter.

So, fix this by just manually implementing Default instead of deriving it.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/drm/gem/shmem.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 843869056b1d7..d8bda8c8e2fb2 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -50,7 +50,6 @@
 ///
 /// This is used with [`Object::new()`] to control various properties that can only be set when
 /// initially creating a shmem-backed GEM object.
-#[derive(Default)]
 pub struct ObjectConfig<'a, T: DriverObject> {
     /// Whether to set the write-combine map flag.
     pub map_wc: bool,
@@ -61,6 +60,16 @@ pub struct ObjectConfig<'a, T: DriverObject> {
     pub parent_resv_obj: Option<&'a Object<T>>,
 }
 
+impl<'a, T: DriverObject> Default for ObjectConfig<'a, T> {
+    #[inline(always)]
+    fn default() -> Self {
+        Self {
+            map_wc: false,
+            parent_resv_obj: None,
+        }
+    }
+}
+
 /// A shmem-backed GEM object.
 ///
 /// # Invariants
-- 
2.54.0

[PATCH v16 5/6] rust: faux: Allow retrieving a bound Device

[Lyude Paul]

When writing up some rust code that used faux devices for unit testing, I
noticed that we never actually added the Bound device context to
faux::Registration's AsRef<device::Device> implementation. This being said:
the Registration object itself is proof that a driver is bound to the
device - so this should be safe.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/faux.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/faux.rs b/rust/kernel/faux.rs
index 43b4974f48cd2..e0856b2964a2c 100644
--- a/rust/kernel/faux.rs
+++ b/rust/kernel/faux.rs
@@ -25,7 +25,8 @@
 ///
 /// # Invariants
 ///
-/// `self.0` always holds a valid pointer to an initialized and registered [`struct faux_device`].
+/// - `self.0` always holds a valid pointer to an initialized and registered [`struct faux_device`].
+/// - This object is proof that the object described by this `Registration` is bound to a device.
 ///
 /// [`struct faux_device`]: srctree/include/linux/device/faux.h
 pub struct Registration(NonNull<bindings::faux_device>);
@@ -59,8 +60,8 @@ fn as_raw(&self) -> *mut bindings::faux_device {
     }
 }
 
-impl AsRef<device::Device> for Registration {
-    fn as_ref(&self) -> &device::Device {
+impl AsRef<device::Device<device::Bound>> for Registration {
+    fn as_ref(&self) -> &device::Device<device::Bound> {
         // SAFETY: The underlying `device` in `faux_device` is guaranteed by the C API to be
         // a valid initialized `device`.
         unsafe { device::Device::from_raw(addr_of_mut!((*self.as_raw()).dev)) }
-- 
2.54.0

[PATCH v16 6/6] rust: drm: gem: Introduce shmem::Object::sg_table()

[Lyude Paul]

In order to do this, we need to be careful to ensure that any interface we
expose for scatterlists ensures that any mappings created from one are
destroyed on driver-unbind. To do this, we introduce a Devres resource into
shmem::Object that we use in order to ensure that we release any SGTable
mappings on driver-unbind.

There's some other slightly unfortunate caveats of this:

* Drivers don't have explicit control at the moment over when unmapping
  happens (which is exactly the same as the C side atm, so it might not be
  a problem).
* We can't just return `SGTableMap` to the user through an Arc to attempt
  to fix the last caveat - because that implies the gem object would need
  to hold a reference count to the scatterlist mapping, which just leaves
  us with the same problem.

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

---
V3:
* Rename OwnedSGTable to shmem::SGTable. Since the current version of the
  SGTable abstractions now has a `Owned` and `Borrowed` variant, I think
  renaming this to shmem::SGTable makes things less confusing.
  We do however, keep the name of owned_sg_table() as-is.
V4:
* Clarify safety comments for SGTable to explain why the object is
  thread-safe.
* Rename from SGTableRef to SGTable
V10:
* Use Devres in order to ensure that SGTables are revocable, and are
  unmapped on driver-unbind.
V11:
* s/create_sg_table()/get_sg_table()
* Get rid of extraneous `ret = ` in shmem::Object::get_sg_table()
V12:
* Actually move sgt_res in this patch and not the next one
V13:
* Use DmaResvGuard suggestion from Alexander
* Use Alexander's (much better) solution for get_sg_table()
* Use SetOnce instead of UnsafeCell
* s/SGTableRef/SGTableMap
* Fix typo in SGTableMap documentation
* Create fallible constructor for SGTableMap
* Don't reuse dma_resv lock for protecting Object contents, just use Mutex
  + SetOnce
* Drop use of drm_gem_shmem_get_pages_sgt_locked(), since we don't need to
  hold the dma_resv lock ourselves for anything but this function.
* Check that the device we receive in the bounds for sg_table() and
  owned_sg_table() that said Device is in fact, the correct device.
* Remove redundant docs in owned_sg_table(), just point it back to
  sg_table().
* Implement Deborah's suggestion to fix double-free in
  free_callback()
* Restore original order of Object<T>
* Fix doc typo for SGTableMap
V14:
* Use new InitOnce container over the Mutex/SetOnce horror show we had
  before.
* Start using LazyInit container for storing Devres for sgt unmap
* Add some kunit tests for sg_table (not sure why I didn't do this before)
  using some of the boilerplate code leftover from the vmap bindings
* Get rid of the owned SGTable variant for now, we'll add it back in a
  future patch if people actually need it.
* Use new LazyInit container from me to get rid of the horrid
  Mutex<SetOnce<>> mess.
* Add the best we can do for unit tests w/r/t SGTable at the moment
V16:
* Get rid of LazyInit, go back to SetOnce, use trick that Alice recommended
  that is a lot cleaner.
* Fix horrid rebasing mistake

 rust/kernel/drm/gem/shmem.rs | 164 +++++++++++++++++++++++++++++++++--
 1 file changed, 155 insertions(+), 9 deletions(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index d8bda8c8e2fb2..5f27e8d59e765 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -11,20 +11,34 @@
 
 use crate::{
     container_of,
+    device::{
+        self,
+        Bound, //
+    },
+    devres::*,
     drm::{
         driver,
         gem,
         private::Sealed,
         Device, //
     },
-    error::to_result,
+    error::{
+        from_err_ptr,
+        to_result, //
+    },
     io::{
         Io,
         IoCapable,
         IoKnownSize, //
     },
     prelude::*,
-    sync::aref::ARef,
+    scatterlist,
+    sync::{
+        aref::ARef,
+        new_mutex,
+        Mutex,
+        SetOnce, //
+    },
     types::Opaque, //
 };
 use core::{
@@ -83,6 +97,11 @@ pub struct Object<T: DriverObject> {
     obj: Opaque<bindings::drm_gem_shmem_object>,
     /// Parent object that owns this object's DMA reservation object.
     parent_resv_obj: Option<ARef<Object<T>>>,
+    /// Devres object for unmapping any SGTable on driver-unbind.
+    sgt_res: SetOnce<Devres<SGTableMap<T>>>,
+    #[pin]
+    /// Lock for protecting initialization of `sgt_res`.
+    sgt_lock: Mutex<()>,
     #[pin]
     inner: T,
 }
@@ -135,6 +154,8 @@ pub fn new(
             try_pin_init!(Self {
                 obj <- Opaque::init_zeroed(),
                 parent_resv_obj: config.parent_resv_obj.map(|p| p.into()),
+                sgt_res: SetOnce::new(),
+                sgt_lock <- new_mutex!(()),
                 inner <- T::new(dev, size, args),
             }),
             GFP_KERNEL,
@@ -178,18 +199,23 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
         // - DRM always passes a valid gem object here
         // - We used drm_gem_shmem_create() in our create_gem_object callback, so we know that
         //   `obj` is contained within a drm_gem_shmem_object
-        let this = unsafe { container_of!(obj, bindings::drm_gem_shmem_object, base) };
-
-        // SAFETY:
-        // - We're in free_callback - so this function is safe to call.
-        // - We won't be using the gem resources on `this` after this call.
-        unsafe { bindings::drm_gem_shmem_release(this) };
+        let base = unsafe { container_of!(obj, bindings::drm_gem_shmem_object, base) };
 
         // SAFETY:
         // - We verified above that `obj` is valid, which makes `this` valid
         // - This function is set in AllocOps, so we know that `this` is contained within a
         //   `Object<T>`
-        let this = unsafe { container_of!(Opaque::cast_from(this), Self, obj) }.cast_mut();
+        let this = unsafe { container_of!(Opaque::cast_from(base), Self, obj) }.cast_mut();
+
+        // We need to drop `sgt_res` first, since doing so requires that the GEM object is still
+        // alive.
+        // SAFETY: We verified above that `this` is valid.
+        unsafe { &mut (*this).sgt_res }.reset();
+
+        // SAFETY:
+        // - We're in free_callback - so this function is safe to call.
+        // - We won't be using the gem resources on `this` after this call.
+        unsafe { bindings::drm_gem_shmem_release(base) };
 
         // SAFETY: We're recovering the Kbox<> we created in gem_create_object()
         let _ = unsafe { KBox::from_raw(this) };
@@ -268,6 +294,45 @@ pub fn vmap<const SIZE: usize>(&self) -> Result<VMapRef<'_, T, SIZE>> {
     pub fn owned_vmap<const SIZE: usize>(&self) -> Result<VMapOwned<T, SIZE>> {
         self.make_vmap()
     }
+
+    /// Creates (if necessary) and returns an immutable reference to a scatter-gather table of DMA
+    /// pages for this object.
+    ///
+    /// This will pin the object in memory. It is expected that `dev` should be a pointer to the
+    /// same [`device::Device`] which `self` belongs to, otherwise this function will return
+    /// `Err(EINVAL)`.
+    pub fn sg_table<'a>(
+        &'a self,
+        dev: &'a device::Device<Bound>,
+    ) -> Result<&'a scatterlist::SGTable> {
+        if dev.as_raw() != self.dev().as_ref().as_raw() {
+            return Err(EINVAL);
+        }
+
+        let sgt_res = 'out: {
+            // Fast path: sgt_res is already initialized
+            if let Some(sgt_res) = self.sgt_res.as_ref() {
+                break 'out sgt_res;
+            }
+
+            // Slow path: Grab the lock and see if we need to initialize sgt_res.
+            let _guard = self.sgt_lock.lock();
+
+            // If someone initialized it while we were waiting, we can exit early.
+            if let Some(sgt_res) = self.sgt_res.as_ref() {
+                break 'out sgt_res;
+            }
+
+            // If not, finish initializing and return.
+            self.sgt_res
+                .populate(Devres::new(dev, SGTableMap::new(self))?);
+
+            // SAFETY: We just populated sgt_res above.
+            unsafe { self.sgt_res.as_ref().unwrap_unchecked() }
+        };
+
+        Ok(sgt_res.access(dev)?)
+    }
 }
 
 impl<T: DriverObject> Deref for Object<T> {
@@ -457,6 +522,63 @@ impl<D, R, const SIZE: usize> IoKnownSize for VMap<D, R, SIZE>
 #[cfg(CONFIG_64BIT)]
 impl_vmap_io_capable!(VMap, u64);
 
+/// A reference to a GEM object that is known to have a mapped [`SGTable`].
+///
+/// This is used by the Rust bindings with [`Devres`] in order to ensure that mappings for SGTables
+/// on GEM shmem objects are revoked on driver-unbind.
+///
+/// # Invariants
+///
+/// - `self.obj` always points to a valid GEM object.
+/// - This object is proof that `self.obj.owner.sgt` has an initialized and valid
+///   [`scatterlist::SGTable`].
+pub struct SGTableMap<T: DriverObject> {
+    obj: NonNull<Object<T>>,
+}
+
+impl<T: DriverObject> Deref for SGTableMap<T> {
+    type Target = scatterlist::SGTable;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY:
+        // - The NonNull is guaranteed to be valid via our type invariants.
+        // - The sgt field is guaranteed to be initialized and valid via our type invariants.
+        unsafe { scatterlist::SGTable::from_raw((*self.obj.as_ref().as_raw_shmem()).sgt) }
+    }
+}
+
+impl<T: DriverObject> Drop for SGTableMap<T> {
+    fn drop(&mut self) {
+        // SAFETY: `obj` is always valid via our type invariants
+        let obj = unsafe { self.obj.as_ref() };
+        let _lock = DmaResvGuard::new(obj);
+
+        // SAFETY: We acquired the lock needed for calling this function above
+        unsafe { bindings::__drm_gem_shmem_free_sgt_locked(obj.as_raw_shmem()) };
+    }
+}
+
+impl<T: DriverObject> SGTableMap<T> {
+    fn new(obj: &Object<T>) -> impl Init<Self, Error> {
+        // INVARIANT:
+        // - We call drm_gem_shmem_get_pages_sgt_locked below and check whether or not it
+        //   succeeds, fulfilling the invariant of SGTableMap that the object's `sgt` field is
+        //   initialized.
+        // SAFETY:
+        // - `obj` is fully initialized, making this function safe to call.
+        from_err_ptr(unsafe { bindings::drm_gem_shmem_get_pages_sgt(obj.as_raw_shmem()) })?;
+
+        Ok(Self { obj: obj.into() })
+    }
+}
+
+// SAFETY: The NonNull in SGTableMap is guaranteed valid by our type invariants, and the GEM object
+// it points to is guaranteed to be thread-safe.
+unsafe impl<T: DriverObject> Send for SGTableMap<T> {}
+// SAFETY: The NonNull in SGTableMap is guaranteed valid by our type invariants, and the GEM object
+// it points to is guaranteed to be thread-safe.
+unsafe impl<T: DriverObject> Sync for SGTableMap<T> {}
+
 #[kunit_tests(rust_drm_gem_shmem)]
 mod tests {
     use super::*;
@@ -562,4 +684,28 @@ fn vmap_io() -> Result {
 
         Ok(())
     }
+
+    // TODO: I would love to actually test the success paths of sg_table(), but that would require
+    // also implementing dummy dma_ops so that trying to create a mapping doesn't explode. So, leave
+    // that for someone else.
+
+    // Ensures that passing the wrong device to sg_table() fails as we expect, and also ensure it
+    // skips initializing `sgt_res` since we could otherwise create `sgt_res` with the wrong device
+    // bound to it.
+    #[test]
+    fn fail_sg_table_on_wrong_dev() -> Result {
+        let (_dev, drm) = create_drm_dev()?;
+        let wrong_dev = faux::Registration::new(c"EvilKunit", None)?;
+
+        let obj = Object::<KunitObject>::new(&drm, PAGE_SIZE, ObjectConfig::default(), ())?;
+
+        assert_eq!(obj.sg_table(wrong_dev.as_ref()).err().unwrap(), EINVAL);
+
+        // If sgt_res was not initialized mistakenly with the wrong device, this should still fail.
+        assert_eq!(obj.sg_table(wrong_dev.as_ref()).err().unwrap(), EINVAL);
+
+        // TODO: Someday, we should test that creating an sg_table here still succeeds.
+
+        Ok(())
+    }
 }
-- 
2.54.0

Claude review: Rust bindings for gem shmem

[Claude Code Review Bot]

Overall Series Review

Subject: Rust bindings for gem shmem
Author: Lyude Paul <lyude@redhat.com>
Patches: 7
Reviewed: 2026-06-04T12:29:53.038433

---

This is v16 of the Rust GEM shmem bindings series from Lyude Paul, building on work originally from Asahi Lina and Daniel Almeida. The series adds DMA reservation locking helpers, vmap support, `SetOnce::reset()`, a `Default` fix for `ObjectConfig`, faux device bound-context support, and scatter-gather table integration. The code is well-structured with clear safety documentation and thoughtful RAII patterns.

The overall architecture is sound: `DmaResvGuard` for scoped locking, `VMap` for typed memory mappings with compile-time size validation, and `SGTableMap` wrapped in `Devres` to ensure revocation on driver unbind. The double-checked locking pattern for lazy SGT initialization is correct with proper acquire/release ordering.

Key concerns:
1. Patch 6 references `__drm_gem_shmem_free_sgt_locked`, which does not exist in the current drm-next tree. This likely depends on a C-side export that should be in drm-rust-next but should be verified.
2. The import restructuring in patch 6 (changing `device` from `crate::drm::device` to `crate::device`) changes the meaning of the `dev()` return type. This may be correct on the intended base tree (drm-rust-next, base-commit b78dab829) but warrants attention.
3. Minor style and safety-comment nits throughout.

---

---
Generated by Claude Code Patch Reviewer

Claude review: rust: drm: gem/shmem: Add DmaResvGuard helper

[Claude Code Review Bot]

Patch Review

A simple RAII guard for the DMA reservation lock. Clean and correct.

The return value of `dma_resv_lock` is discarded:
```rust
unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
```
With a NULL `ww_acquire_ctx`, `dma_resv_lock` degenerates to `mutex_lock` and always returns 0, so ignoring the return value is safe in practice. The TODO comment acknowledging this should be replaced with proper WwMutex bindings is appropriate.

The `#[expect(unused)]` on `new()` is fine since patch 2 removes it immediately. The `#[inline(always)]` on both `new()` and `drop()` is reasonable for a thin lock wrapper.

No issues.

---
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Claude review: rust: drm: gem: Add vmap functions to shmem bindings

[Claude Code Review Bot]

Patch Review

This is the largest patch and adds the core vmap functionality. The design is well thought out.

**`make_vmap` guard lifetime**: The explicit `drop(guard)` before the iomem error path is correct — `raw_vunmap` re-acquires the lock internally:
```rust
let guard = DmaResvGuard::new(self);
to_result(unsafe {
    bindings::drm_gem_shmem_vmap_locked(self.as_raw_shmem(), map.as_mut_ptr())
})?;
drop(guard);
```
Good defensive pattern.

**VMap::Drop reconstruction**: The `drop` reconstructs an `iosys_map` to pass to `raw_vunmap`:
```rust
self.owner.raw_vunmap(bindings::iosys_map {
    is_iomem: false,
    __bindgen_anon_1: bindings::iosys_map__bindgen_ty_1 { vaddr: self.addr },
})
```
This works because `make_vmap` already validated `!is_iomem` before constructing the `VMap`. The invariant is maintained.

**IoCapable implementations**: The `impl_vmap_io_capable!` macro implements `ptr::read`/`ptr::write` through raw pointer casts:
```rust
unsafe fn io_read(&self, address: usize) -> $ty {
    let ptr = address as *mut $ty;
    unsafe { ptr::read(ptr) }
}
```
The safety is delegated to the `Io` trait's `io_read`/`io_write` contract (alignment, bounds). This is correct.

**Minor**: The `VMap` struct's `addr` field is `*mut c_void` — this raw pointer doesn't implement `Send`/`Sync`, but `VMap` doesn't explicitly implement them either. Since the GEM object backing the mapping is thread-safe, and the mapping itself is valid for the lifetime of the owner, this might need explicit `Send`/`Sync` impls if callers need to share the mapping across threads. However, this may be intentionally restrictive for now.

**Tests**: Good coverage — `compile_time_vmap_sizes` validates the size check and `vmap_io` tests read/write through the mapping. The byte-level verification of a `write32` is a nice touch.

---
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Claude review: rust: sync: Add SetOnce::reset()

[Claude Code Review Bot]

Patch Review

Adds `reset()` which takes `&mut self` (proving exclusive access), drops the value, and resets `init` to 0.

**Invariant update**: The change from "init may only increase in value" to "init may only increase in value, unless modified through a mutable reference" is correct — `&mut self` guarantees no concurrent observers.

**`drop_val` safety contract**: The `unsafe fn drop_val` has a critical safety requirement:
```rust
/// If this function returns `true`, `self` must be freed or `init` must be reset to `0`.
```
Between `drop_val()` returning `true` and the caller resetting `init`, there's a window where `init == 2` but the value is dropped. This is safe because `&mut self` ensures exclusive access, but the contract is correctly documented. Both callers (`reset` and `Drop`) satisfy it.

**Doctest**: The example is good and demonstrates the full reset/repopulate cycle:
```rust
assert_eq!(value.populate(67), true);
assert_eq!(value.reset(), true);
assert!(value.as_ref().is_none());
assert_eq!(value.reset(), false);
assert_eq!(value.populate(69), true);
```

No issues.

---
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Claude review: rust: gem: shmem: Fix Default implementation for ObjectConfig

[Claude Code Review Bot]

Patch Review

Straightforward fix. `#[derive(Default)]` requires `T: Default` for all generic parameters, but `ObjectConfig` only uses `T` in `Option<&'a Object<T>>` which defaults to `None` regardless of `T`'s `Default` impl. The manual implementation correctly removes the spurious bound:
```rust
impl<'a, T: DriverObject> Default for ObjectConfig<'a, T> {
    fn default() -> Self {
        Self {
            map_wc: false,
            parent_resv_obj: None,
        }
    }
}
```

No issues.

---
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Claude review: rust: faux: Allow retrieving a bound Device

[Claude Code Review Bot]

Patch Review

Changes `AsRef<device::Device>` to `AsRef<device::Device<device::Bound>>`:
```rust
-impl AsRef<device::Device> for Registration {
-    fn as_ref(&self) -> &device::Device {
+impl AsRef<device::Device<device::Bound>> for Registration {
+    fn as_ref(&self) -> &device::Device<device::Bound> {
```

The justification is correct — a `Registration` is proof of device binding, so returning `Device<Bound>` is sound. The added invariant bullet point documents this:
```
+/// - This object is proof that the object described by this `Registration` is bound to a device.
```

**Potential concern**: This is a breaking API change — any existing code calling `registration.as_ref()` expecting `&device::Device` (without `Bound`) will break. Since this is in the kernel tree and patch 6 is the consumer, this is fine, but downstream users of the faux crate should be aware.

No issues for this series.

---
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Claude review: rust: drm: gem: Introduce shmem::Object::sg_table()

[Claude Code Review Bot]

Patch Review

The most complex patch. Several observations:

**Missing C-side function**: `SGTableMap::drop` calls:
```rust
unsafe { bindings::__drm_gem_shmem_free_sgt_locked(obj.as_raw_shmem()) };
```
This function does not exist in the current drm-next tree. It presumably lives in the "first half" of the series already pushed to drm-rust-next. If it's missing from the base, this won't compile. This should be verified against the actual base tree.

**Import restructuring**: The patch changes `device` from `crate::drm::device` to `crate::device`:
```rust
+    device::{
+        self,
+        Bound, //
+    },
     drm::{
+        driver,
+        gem,
+        private::Sealed,
         Device, //
     },
```
This changes the meaning of `device::Device<T::Driver>` in the existing `dev()` method. On the actual drm-rust-next base, this may already be reconciled. Worth verifying.

**Double-checked locking in `sg_table()`**: The pattern is correct:
```rust
if let Some(sgt_res) = self.sgt_res.as_ref() {  // fast path (Acquire)
    break 'out sgt_res;
}
let _guard = self.sgt_lock.lock();               // slow path
if let Some(sgt_res) = self.sgt_res.as_ref() {  // re-check
    break 'out sgt_res;
}
self.sgt_res.populate(...);                       // Release
```
The `SetOnce` uses `Acquire` on read and `Release` on write, which provides the necessary happens-before guarantees for the double-checked locking pattern.

**SGTableMap lifetime safety**: `SGTableMap` holds `NonNull<Object<T>>` without an `ARef`:
```rust
pub struct SGTableMap<T: DriverObject> {
    obj: NonNull<Object<T>>,
}
```
This is safe because `SGTableMap` is stored inside the `Object<T>` itself (via `sgt_res`), so the object outlives the mapping. The `free_callback` explicitly resets `sgt_res` before releasing the GEM object:
```rust
unsafe { &mut (*this).sgt_res }.reset();   // drops SGTableMap first
unsafe { bindings::drm_gem_shmem_release(base) };  // then releases GEM
let _ = unsafe { KBox::from_raw(this) };   // then frees memory
```
This ordering is correct and critical.

**Device identity check**:
```rust
if dev.as_raw() != self.dev().as_ref().as_raw() {
    return Err(EINVAL);
}
```
Comparing raw device pointers to verify the caller passed the correct device is a good defensive check. The test `fail_sg_table_on_wrong_dev` verifies this works.

**`SGTableMap::new` discards the sg_table pointer**:
```rust
fn new(obj: &Object<T>) -> impl Init<Self, Error> {
    from_err_ptr(unsafe { bindings::drm_gem_shmem_get_pages_sgt(obj.as_raw_shmem()) })?;
    Ok(Self { obj: obj.into() })
}
```
The returned `sg_table*` is discarded. This is correct because `drm_gem_shmem_get_pages_sgt` stores the SGT internally in `shmem->sgt`, which is what `SGTableMap::deref` reads later.

**Minor style**: The `#[pin]` attribute appears before the doc comment on `sgt_lock`:
```rust
+    #[pin]
+    /// Lock for protecting initialization of `sgt_res`.
+    sgt_lock: Mutex<()>,
```
Convention is doc comments before attributes, though both orderings are valid Rust.

**Test coverage**: The `fail_sg_table_on_wrong_dev` test is good but limited — as the TODOs note, testing the success path requires mock DMA ops. The comment is honest about this limitation, which is appreciated.

---
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Claude review: rust: drm: gem/shmem: Add DmaResvGuard helper

[Claude Code Review Bot]

Patch Review

**Status: Minor observations**

The guard pattern is correct for wrapping `dma_resv_lock` / `dma_resv_unlock`. The return value of `dma_resv_lock` is discarded:

```rust
unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
```

With a NULL `ww_acquire_ctx`, this reduces to a plain mutex lock which cannot fail (just blocks), so discarding the return value is fine in practice. The `#[expect(unused)]` attribute is appropriate since it's only used starting in patch 3.

The TODO note about replacing this with proper WwMutex bindings is good forward-looking guidance.

---
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