Rust bindings for gem shmem

rust/kernel/drm/gem/shmem.rs new file +6 -0

+
+        // SAFETY: We checked above that `sgt` is not an error pointer, so it must be a valid
+        // pointer to a scatterlist
+        Ok(unsafe { scatterlist::SGTable::from_raw(sgt) })
+    }
+}

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.

Similar to how we handle SGTables, we make sure there's two different types of mappings: owned mappings (kernel::drm::gem::shmem::VMap) and borrowed mappings (kernel::drm::gem::shmem::VMapRef).

Signed-off-by Lyude Paul <[email protected]>

---
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* Get rid of some outdated comments I missed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

Signed-off-by Lyude Paul <[email protected]>

---
  rust/kernel/drm/gem/shmem.rs | 358 ++++++++++++++++++++++++++++++++++-
  1 file changed, 356 insertions(+), 2 deletions(-)

rust/kernel/drm/gem/shmem.rs +292 -2

index 3fab5d76c197b..5254338bad394 100644
@@ -2,7 +2,7 @@
 
  //! DRM GEM shmem helper objects
  //!
-//! C header: [`include/linux/drm/drm_gem_shmem_helper.h`](srctree/include/linux/drm/drm_gem_shmem_helper.h)
+//! C header: [`include/linux/drm/drm_gem_shmem_helper.h`](srctree/include/drm/drm_gem_shmem_helper.h)
 
  // TODO:
  // - There are a number of spots here that manually acquire/release the DMA reservation lock using

@@ -21,6 +21,11 @@
          from_err_ptr,
          to_result, //
      },
+    io::{
+        Io,
+        IoCapable,
+        IoKnownSize, //
+    },
      prelude::*,
      scatterlist,
      types::{

@@ -29,13 +34,22 @@
      }, //
  };
  use core::{
+    ffi::c_void,
+    mem::{
+        self,
+        MaybeUninit, //
+    },
      ops::{
          Deref,
          DerefMut, //
      },
-    ptr::NonNull,
+    ptr::{
+        self,
+        NonNull, //
+    },
  };
  use gem::{
+    BaseObject,
      BaseObjectPrivate,
      DriverObject,
      IntoGEMObject, //

@@ -217,6 +231,88 @@ pub fn owned_sg_table(&self) -> Result<SGTable<T>> {
              _owner: self.into(),
          })
      }
+
+    /// Attempt to create a vmap from the gem object, and confirm the size of said vmap.
+    fn raw_vmap(&self, min_size: usize) -> Result<*mut c_void> {
+        if self.size() < min_size {
+            return Err(ENOSPC);
+        }
+
+        let mut map: MaybeUninit<bindings::iosys_map> = MaybeUninit::uninit();
+
+        // SAFETY: drm_gem_shmem_vmap can be called with the DMA reservation lock held
+        to_result(unsafe {
+            // TODO: see top of file
+            bindings::dma_resv_lock(self.raw_dma_resv(), ptr::null_mut());
+            let ret = bindings::drm_gem_shmem_vmap_locked(self.as_raw_shmem(), map.as_mut_ptr());
+            bindings::dma_resv_unlock(self.raw_dma_resv());
+            ret
+        })?;
+
+        // SAFETY: The call to drm_gem_shmem_vunmap_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, making it safe to call vunmap
+            // - We checked that this is an iomem allocation, making it safe to read vaddr_iomem
+            unsafe { self.raw_vunmap(map.__bindgen_anon_1.vaddr_iomem) };
+
+            Err(ENOTSUPP)
+        } else {
+            // SAFETY: We checked that this is not an iomem allocation, making it safe to read vaddr
+            Ok(unsafe { map.__bindgen_anon_1.vaddr })
+        }
+    }
+
+    /// Unmap a vmap from the gem object.
+    ///
+    /// # Safety
+    ///
+    /// - The caller promises that addr came from a prior call to [`Self::raw_vmap`] on this gem
+    ///   object.
+    /// - The caller promises that the memory pointed to by addr will no longer be accesed through
+    ///   this instance.
+    unsafe fn raw_vunmap(&self, vaddr: *mut c_void) {
+        let resv = self.raw_dma_resv();
+        let mut map = bindings::iosys_map {
+            is_iomem: false,
+            __bindgen_anon_1: bindings::iosys_map__bindgen_ty_1 { vaddr },
+        };
+
+        // 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 {
+            // TODO: see top of file
+            bindings::dma_resv_lock(resv, ptr::null_mut());
+            bindings::drm_gem_shmem_vunmap_locked(self.as_raw_shmem(), &mut map);
+            bindings::dma_resv_unlock(resv);
+        }
+    }
+
+    /// Creates and returns a virtual kernel memory mapping for this object.
+    #[inline]
+    pub fn vmap<const SIZE: usize>(&self) -> Result<VMapRef<'_, T, SIZE>> {
+        Ok(VMapRef {
+            // INVARIANT: `raw_vmap()` checks that the gem object is at least as large as `SIZE`.
+            addr: self.raw_vmap(SIZE)?,
+            owner: self,
+        })
+    }
+
+    /// 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<VMap<T, SIZE>> {
+        Ok(VMap {
+            // INVARIANT: `raw_vmap()` checks that the gem object is at least as large as `SIZE`.
+            addr: self.raw_vmap(SIZE)?,
+            owner: self.into(),
+        })
+    }
  }
 
  impl<T: DriverObject> Deref for Object<T> {

@@ -268,6 +364,147 @@ impl<T: DriverObject> driver::AllocImpl for Object<T> {
      };
  }
 
+macro_rules! impl_vmap_io_capable {
+    ($impl:ident, $ty:ty $(, $lifetime:lifetime )?) => {
+        impl<$( $lifetime ,)? D: DriverObject, const SIZE: usize> IoCapable<$ty>
+            for $impl<$( $lifetime ,)? D, SIZE>
+        {
+            #[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 $ty, and is properly
+                // aligned.
+                unsafe { ptr::write(ptr, value) }
+            }
+        }
+    };
+}
+
+// Implement various traits common to both VMap types
+macro_rules! impl_vmap_common {
+    ($impl:ident $(, $lifetime:lifetime )?) => {
+        impl<$( $lifetime ,)? D, const SIZE: usize> $impl<$( $lifetime ,)? D, SIZE>
+        where
+            D: DriverObject,
+        {
+            /// Borrows a reference to the object that owns this virtual mapping.
+            #[inline(always)]
+            pub fn owner(&self) -> &Object<D> {
+                &self.owner
+            }
+        }
+
+        impl<$( $lifetime ,)? D, const SIZE: usize> Drop for $impl<$( $lifetime ,)? D, SIZE>
+        where
+            D: DriverObject,
+        {
+            #[inline(always)]
+            fn drop(&mut self) {
+                // SAFETY: Our existence is proof that this map was previously created using
+                // self.owner
+                unsafe { self.owner.raw_vunmap(self.addr) };
+            }
+        }
+
+        impl<$( $lifetime ,)? D, const SIZE: usize> Io for $impl<$( $lifetime ,)? D, SIZE>
+        where
+            D: DriverObject,
+        {
+            #[inline(always)]
+            fn addr(&self) -> usize {
+                self.addr as usize
+            }
+
+            #[inline(always)]
+            fn maxsize(&self) -> usize {
+                self.owner.size()
+            }
+        }
+
+        impl<$( $lifetime ,)? D, const SIZE: usize> IoKnownSize for $impl<$( $lifetime ,)? D, SIZE>
+        where
+            D: DriverObject,
+        {
+            const MIN_SIZE: usize = SIZE;
+        }
+
+        impl_vmap_io_capable!($impl, u8 $( , $lifetime )?);
+        impl_vmap_io_capable!($impl, u16 $( , $lifetime )?);
+        impl_vmap_io_capable!($impl, u32 $( , $lifetime )?);
+        #[cfg(CONFIG_64BIT)]
+        impl_vmap_io_capable!($impl, u64 $( , $lifetime )?);
+    };
+}
+
+/// An owned reference to a virtual mapping for a 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: DriverObject, const SIZE: usize = 0> {
+    addr: *mut c_void,
+    owner: ARef<Object<D>>,
+}
+
+impl_vmap_common!(VMap);
+
+impl<D: DriverObject, const SIZE: usize> Clone for VMap<D, SIZE> {
+    #[inline]
+    fn clone(&self) -> Self {
+        // SAFETY: We have a successful vmap already, so this can't fail
+        unsafe { self.owner.owned_vmap().unwrap_unchecked() }
+    }
+}
+
+impl<'a, D: DriverObject, const SIZE: usize> From<VMapRef<'a, D, SIZE>> for VMap<D, SIZE> {
+    #[inline]
+    fn from(value: VMapRef<'a, D, SIZE>) -> Self {
+        let this = Self {
+            addr: value.addr,
+            owner: value.owner.into(),
+        };
+
+        mem::forget(value);
+        this
+    }
+}
+
+// SAFETY: addr is guaranteed to be valid and accessible for the lifetime of VMap, ensuring its
+// safe to send across threads.
+unsafe impl<D: DriverObject, const SIZE: usize> Send for VMap<D, SIZE> {}
+// SAFETY: addr is guaranteed to be valid and accessible for the lifetime of VMap, ensuring its
+// safe to send across threads.
+unsafe impl<D: DriverObject, const SIZE: usize> Sync for VMap<D, SIZE> {}
+
+/// A borrowed reference to a virtual mapping for a shmem-based GEM object in kernel address space.
+pub struct VMapRef<'a, D: DriverObject, const SIZE: usize = 0> {
+    addr: *mut c_void,
+    owner: &'a Object<D>,
+}
+
+impl_vmap_common!(VMapRef, 'a);
+
+impl<'a, D: DriverObject, const SIZE: usize> Clone for VMapRef<'a, D, SIZE> {
+    #[inline]
+    fn clone(&self) -> Self {
+        // SAFETY: We have a successful vmap already, so this can't fail
+        unsafe { self.owner.vmap().unwrap_unchecked() }
+    }
+}
+
  /// An owned reference to a scatter-gather table of DMA address spans for a GEM shmem object.
  ///
  /// This object holds an owned reference to the underlying GEM shmem object, ensuring that the

@@ -298,3 +535,120 @@ fn deref(&self) -> &Self::Target {
          unsafe { self.sgt.as_ref() }
      }
  }
+
+#[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>;

rust/kernel/drm/gem/shmem.rs +54 -0

+
+        Ok((dev, drm))
+    }
+
+    #[test]
+    fn compile_time_vmap_sizes() -> Result {
+        let (_dev, drm) = create_drm_dev()?;
+
+        // Create a gem object to test with
+        let cfg_ = ObjectConfig::<KunitObject> {
+            map_wc: false,
+            parent_resv_obj: None,
+        };
+        let obj = Object::<KunitObject>::new(&drm, PAGE_SIZE, cfg_, ())?;
+
+        // 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()?;
+
+        // Create a gem object to test with
+        let cfg_ = ObjectConfig::<KunitObject> {
+            map_wc: false,
+            parent_resv_obj: None,
+        };
+        let obj = Object::<KunitObject>::new(&drm, PAGE_SIZE, cfg_, ())?;
+
+        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(())
+    }
+}

Lyude Paul (5):
rust: drm: Add gem::impl_aref_for_gem_obj!
rust: gem: Introduce DriverObject::Args

Asahi Lina (2):
rust: drm: gem: shmem: Add DRM shmem helper abstraction

Quoted 3 lines · Lyude Paul

Lyude Paul (5):
rust: drm: Add gem::impl_aref_for_gem_obj!
rust: gem: Introduce DriverObject::Args

Applied to drm-rust-next, thanks!

Quoted 2 lines

Asahi Lina (2):
rust: drm: gem: shmem: Add DRM shmem helper abstraction

I was about to pick this one up as well, but did run into quite some build errors and warnings. I fixed them all up, but I consider this too excessive to actually apply the patch. This is the changelog I came up with:

[ * DRM_GEM_SHMEM_HELPER is a tristate; when a module driver selects it,
it becomes =m. The Rust kernel crate and its C helpers are always built into vmlinux and can't reference symbols from a module, causing link errors.

Thus, add RUST_DRM_GEM_SHMEM_HELPER bool Kconfig that selects DRM_GEM_SHMEM_HELPER, forcing it built-in when Rust drivers need it; use cfg(CONFIG_RUST_DRM_GEM_SHMEM_HELPER) for the shmem module.

* Add cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), expect(unused)) on pub(crate) use impl_aref_for_gem_obj and BaseObjectPrivate, so that unused warnings are suppressed when shmem is not enabled.

* Enable const_refs_to_static (stabilized in 1.83) to prevent build errors with older compilers.

* Use &raw const for bindings::drm_gem_shmem_vm_ops and add #[allow(unused_unsafe, reason = "Safe since Rust 1.82.0")].

* Fix incorrect C Header path and minor spelling and formatting issues.

* Drop shmem::Object::sg_table() as the current implementation is unsound.

- Danilo ]

Please always consider [1] and [2].

[1] https://drm.pages.freedesktop.org/maintainer-tools/committer/committer-drm-rust.html#submit-checklist
[2] https://rust-for-linux.com/contributing#submit-checklist-addendum

(@Deborah: I assume you were testing this with Tyr built-in?)

@Lyude, Alice, Miguel: Please have a look at what I came up with below.

Quoted 3 lines · Lyude Paul

Lyude Paul (5):
rust: drm: Add gem::impl_aref_for_gem_obj!
rust: gem: Introduce DriverObject::Args

Applied to drm-rust-next, thanks!

Quoted 2 lines

Asahi Lina (2):
rust: drm: gem: shmem: Add DRM shmem helper abstraction

I was about to pick this one up as well, but did run into quite some build errors and warnings. I fixed them all up, but I consider this too excessive to actually apply the patch. This is the changelog I came up with:

[ * DRM_GEM_SHMEM_HELPER is a tristate; when a module driver selects it,
it becomes =m. The Rust kernel crate and its C helpers are always built into vmlinux and can't reference symbols from a module, causing link errors.

Thus, add RUST_DRM_GEM_SHMEM_HELPER bool Kconfig that selects DRM_GEM_SHMEM_HELPER, forcing it built-in when Rust drivers need it; use cfg(CONFIG_RUST_DRM_GEM_SHMEM_HELPER) for the shmem module.

* Add cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), expect(unused)) on pub(crate) use impl_aref_for_gem_obj and BaseObjectPrivate, so that unused warnings are suppressed when shmem is not enabled.

* Enable const_refs_to_static (stabilized in 1.83) to prevent build errors with older compilers.

* Use &raw const for bindings::drm_gem_shmem_vm_ops and add #[allow(unused_unsafe, reason = "Safe since Rust 1.82.0")].

* Fix incorrect C Header path and minor spelling and formatting issues.

* Drop shmem::Object::sg_table() as the current implementation is unsound.

- Danilo ]

Please always consider [1] and [2].

[1] https://drm.pages.freedesktop.org/maintainer-tools/committer/committer-drm-rust.html#submit-checklist
[2] https://rust-for-linux.com/contributing#submit-checklist-addendum

(@Deborah: I assume you were testing this with Tyr built-in?)

@Lyude, Alice, Miguel: Please have a look at what I came up with below.

commit 2dc69d77944dbd1494d2b10a4b134b7fead1c8e7

Author: Asahi Lina <[email protected]>
Date:   Mon Mar 16 17:16:13 2026 -0400

rust: drm: gem: shmem: Add DRM shmem helper abstraction

The DRM shmem helper includes common code useful for drivers which allocate GEM objects as anonymous shmem. Add a Rust abstraction for this. Drivers can choose the raw GEM implementation or the shmem layer, depending on their needs.

Signed-off-by Asahi Lina <[email protected]>

Signed-off-by Daniel Almeida <[email protected]>

Reviewed-by Daniel Almeida <[email protected]>

Signed-off-by Lyude Paul <[email protected]>

Tested-by Deborah Brouwer <[email protected]>

Link https://patch.msgid.link/[email protected] open here

[ * DRM_GEM_SHMEM_HELPER is a tristate; when a module driver selects it,
it becomes =m. The Rust kernel crate and its C helpers are always built into vmlinux and can't reference symbols from a module, causing link errors.

Thus, add RUST_DRM_GEM_SHMEM_HELPER bool Kconfig that selects DRM_GEM_SHMEM_HELPER, forcing it built-in when Rust drivers need it; use cfg(CONFIG_RUST_DRM_GEM_SHMEM_HELPER) for the shmem module.

* Add cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), expect(unused)) on pub(crate) use impl_aref_for_gem_obj and BaseObjectPrivate, so that unused warnings are suppressed when shmem is not enabled.

* Enable const_refs_to_static (stabilized in 1.83) to prevent build errors with older compilers.

* Use &raw const for bindings::drm_gem_shmem_vm_ops and add #[allow(unused_unsafe, reason = "Safe since Rust 1.82.0")].

* Fix incorrect C Header path and minor spelling and formatting issues.

* Drop shmem::Object::sg_table() as the current implementation is unsound.

- Danilo ]

Signed-off-by Danilo Krummrich <[email protected]>

@Lyude, Alice, Miguel: Please have a look at what I came up with below.

Quoted 3 lines · Lyude Paul

Lyude Paul (5):
rust: drm: Add gem::impl_aref_for_gem_obj!
rust: gem: Introduce DriverObject::Args

Applied to drm-rust-next, thanks!

Quoted 2 lines

Asahi Lina (2):
rust: drm: gem: shmem: Add DRM shmem helper abstraction

I was about to pick this one up as well, but did run into quite some build errors and warnings. I fixed them all up, but I consider this too excessive to actually apply the patch. This is the changelog I came up with:

[ * DRM_GEM_SHMEM_HELPER is a tristate; when a module driver selects it,
it becomes =m. The Rust kernel crate and its C helpers are always built into vmlinux and can't reference symbols from a module, causing link errors.

Thus, add RUST_DRM_GEM_SHMEM_HELPER bool Kconfig that selects DRM_GEM_SHMEM_HELPER, forcing it built-in when Rust drivers need it; use cfg(CONFIG_RUST_DRM_GEM_SHMEM_HELPER) for the shmem module.

* Add cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), expect(unused)) on pub(crate) use impl_aref_for_gem_obj and BaseObjectPrivate, so that unused warnings are suppressed when shmem is not enabled.

* Enable const_refs_to_static (stabilized in 1.83) to prevent build errors with older compilers.

* Use &raw const for bindings::drm_gem_shmem_vm_ops and add #[allow(unused_unsafe, reason = "Safe since Rust 1.82.0")].

* Fix incorrect C Header path and minor spelling and formatting issues.

* Drop shmem::Object::sg_table() as the current implementation is unsound.

- Danilo ]

Please always consider [1] and [2].

[1] https://drm.pages.freedesktop.org/maintainer-tools/committer/committer-drm-rust.html#submit-checklist
[2] https://rust-for-linux.com/contributing#submit-checklist-addendum

(@Deborah: I assume you were testing this with Tyr built-in?)

@Lyude, Alice, Miguel: Please have a look at what I came up with below.

commit 2dc69d77944dbd1494d2b10a4b134b7fead1c8e7

Author: Asahi Lina <[email protected]>
Date:   Mon Mar 16 17:16:13 2026 -0400

rust: drm: gem: shmem: Add DRM shmem helper abstraction

The DRM shmem helper includes common code useful for drivers which allocate GEM objects as anonymous shmem. Add a Rust abstraction for this. Drivers can choose the raw GEM implementation or the shmem layer, depending on their needs.

Signed-off-by Asahi Lina <[email protected]>

Signed-off-by Daniel Almeida <[email protected]>

Reviewed-by Daniel Almeida <[email protected]>

Signed-off-by Lyude Paul <[email protected]>

Tested-by Deborah Brouwer <[email protected]>

Link https://patch.msgid.link/[email protected] open here

[ * DRM_GEM_SHMEM_HELPER is a tristate; when a module driver selects it,
it becomes =m. The Rust kernel crate and its C helpers are always built into vmlinux and can't reference symbols from a module, causing link errors.

Thus, add RUST_DRM_GEM_SHMEM_HELPER bool Kconfig that selects DRM_GEM_SHMEM_HELPER, forcing it built-in when Rust drivers need it; use cfg(CONFIG_RUST_DRM_GEM_SHMEM_HELPER) for the shmem module.

* Add cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), expect(unused)) on pub(crate) use impl_aref_for_gem_obj and BaseObjectPrivate, so that unused warnings are suppressed when shmem is not enabled.

* Enable const_refs_to_static (stabilized in 1.83) to prevent build errors with older compilers.

* Use &raw const for bindings::drm_gem_shmem_vm_ops and add #[allow(unused_unsafe, reason = "Safe since Rust 1.82.0")].

* Fix incorrect C Header path and minor spelling and formatting issues.

* Drop shmem::Object::sg_table() as the current implementation is unsound.

- Danilo ]

Signed-off-by Danilo Krummrich <[email protected]>

drivers/gpu/drm/Kconfig +7 -0

index 0d0657dd1b41..0f68446c9122 100644
@@ -258,6 +258,13 @@ config DRM_GEM_SHMEM_HELPER
 	help
 	  Choose this if you need the GEM shmem helper functions
 
+config RUST_DRM_GEM_SHMEM_HELPER
+	bool
+	depends on DRM && MMU
+	select DRM_GEM_SHMEM_HELPER
+	help
+	  Choose this if you need the GEM shmem helper functions In Rust
+
 config DRM_SUBALLOC_HELPER
 	tristate
 	depends on DRM

rust/bindings/bindings_helper.h +2 -0

index 563863d96d38..eda8f50d3a3c 100644
@@ -34,6 +34,7 @@
 #include <drm/drm_drv.h>
 #include <drm/drm_file.h>
 #include <drm/drm_gem.h>
+#include <drm/drm_gem_shmem_helper.h>
 #include <drm/drm_ioctl.h>
 #include <kunit/test.h>
 #include <linux/auxiliary_bus.h>

@@ -63,6 +64,7 @@
 #include <linux/interrupt.h>
 #include <linux/io-pgtable.h>
 #include <linux/ioport.h>
+#include <linux/iosys-map.h>
 #include <linux/jiffies.h>
 #include <linux/jump_label.h>
 #include <linux/mdio.h>

rust/helpers/drm.c +55 -1

index fe226f7b53ef..65f3f22b0e1d 100644
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #include <drm/drm_gem.h>
+#include <drm/drm_gem_shmem_helper.h>
 #include <drm/drm_vma_manager.h>
 
 #ifdef CONFIG_DRM

@@ -21,4 +22,57 @@ rust_helper_drm_vma_node_offset_addr(struct drm_vma_offset_node *node)
 	return drm_vma_node_offset_addr(node);
 }
 
-#endif
+#ifdef CONFIG_DRM_GEM_SHMEM_HELPER
+__rust_helper void
+rust_helper_drm_gem_shmem_object_free(struct drm_gem_object *obj)
+{
+	return drm_gem_shmem_object_free(obj);
+}
+
+__rust_helper void
+rust_helper_drm_gem_shmem_object_print_info(struct drm_printer *p, unsigned int indent,
+					    const struct drm_gem_object *obj)
+{
+	drm_gem_shmem_object_print_info(p, indent, obj);
+}
+
+__rust_helper int
+rust_helper_drm_gem_shmem_object_pin(struct drm_gem_object *obj)
+{
+	return drm_gem_shmem_object_pin(obj);
+}
+
+__rust_helper void
+rust_helper_drm_gem_shmem_object_unpin(struct drm_gem_object *obj)
+{
+	drm_gem_shmem_object_unpin(obj);
+}
+
+__rust_helper struct sg_table *
+rust_helper_drm_gem_shmem_object_get_sg_table(struct drm_gem_object *obj)
+{
+	return drm_gem_shmem_object_get_sg_table(obj);
+}
+
+__rust_helper int
+rust_helper_drm_gem_shmem_object_vmap(struct drm_gem_object *obj,
+				      struct iosys_map *map)
+{
+	return drm_gem_shmem_object_vmap(obj, map);
+}
+
+__rust_helper void
+rust_helper_drm_gem_shmem_object_vunmap(struct drm_gem_object *obj,
+					struct iosys_map *map)
+{
+	drm_gem_shmem_object_vunmap(obj, map);
+}
+
+__rust_helper int
+rust_helper_drm_gem_shmem_object_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
+{
+	return drm_gem_shmem_object_mmap(obj, vma);
+}
+
+#endif /* CONFIG_DRM_GEM_SHMEM_HELPER */
+#endif /* CONFIG_DRM */

rust/kernel/drm/gem/mod.rs +6 -1

index 527d86f4ce92..58eb0a3d5686 100644
@@ -26,6 +26,9 @@
     ptr::NonNull, //
 };
 
+#[cfg(CONFIG_RUST_DRM_GEM_SHMEM_HELPER)]
+pub mod shmem;
+
 /// A macro for implementing [`AlwaysRefCounted`] for any GEM object type.
 ///
 /// Since all GEM objects use the same refcounting scheme.

@@ -60,6 +63,8 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) {
         }
     };
 }
+#[cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), allow(unused))]
+pub(crate) use impl_aref_for_gem_obj;
 
 /// A type alias for retrieving a [`Driver`]s [`DriverFile`] implementation from its
 /// [`DriverObject`] implementation.

@@ -216,7 +221,7 @@ fn create_mmap_offset(&self) -> Result<u64> {
 impl<T: IntoGEMObject> BaseObject for T {}
 
 /// Crate-private base operations shared by all GEM object classes.
-#[expect(unused)]
+#[cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), expect(unused))]
 pub(crate) trait BaseObjectPrivate: IntoGEMObject {
     /// Return a pointer to this object's dma_resv.
     fn raw_dma_resv(&self) -> *mut bindings::dma_resv {

rust/kernel/drm/gem/shmem.rs new file +228 -0

new file mode 100644
index 000000000000..d025fb035195
@@ -0,0 +1,228 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! DRM GEM shmem helper objects
+//!
+//! C header: [`include/linux/drm/drm_gem_shmem_helper.h`](srctree/include/drm/drm_gem_shmem_helper.h)
+
+// TODO:
+// - There are a number of spots here that manually acquire/release the DMA reservation lock using
+//   dma_resv_(un)lock(). In the future we should add support for ww mutex, expose a method to
+//   acquire a reference to the WwMutex, and then use that directly instead of the C functions here.
+
+use crate::{
+    container_of,
+    drm::{
+        device,
+        driver,
+        gem,
+        private::Sealed, //
+    },
+    error::to_result,
+    prelude::*,
+    types::{
+        ARef,
+        Opaque, //
+    }, //
+};
+use core::{
+    ops::{
+        Deref,
+        DerefMut, //
+    },
+    ptr::NonNull,
+};
+use gem::{
+    BaseObjectPrivate,
+    DriverObject,
+    IntoGEMObject, //
+};
+
+/// A struct for controlling the creation of shmem-backed GEM objects.
+///
+/// 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,
+
+    /// Reuse the DMA reservation from another GEM object.
+    ///
+    /// The newly created [`Object`] will hold an owned refcount to `parent_resv_obj` if specified.
+    pub parent_resv_obj: Option<&'a Object<T>>,
+}
+
+/// A shmem-backed GEM object.
+///
+/// # Invariants
+///
+/// `obj` contains a valid initialized `struct drm_gem_shmem_object` for the lifetime of this
+/// object.
+#[repr(C)]
+#[pin_data]
+pub struct Object<T: DriverObject> {
+    #[pin]
+    obj: Opaque<bindings::drm_gem_shmem_object>,
+    /// Parent object that owns this object's DMA reservation object.
+    parent_resv_obj: Option<ARef<Object<T>>>,
+    #[pin]
+    inner: T,
+}
+
+super::impl_aref_for_gem_obj!(impl<T> for Object<T> where T: DriverObject);
+
+// SAFETY: All GEM objects are thread-safe.
+unsafe impl<T: DriverObject> Send for Object<T> {}
+
+// SAFETY: All GEM objects are thread-safe.
+unsafe impl<T: DriverObject> Sync for Object<T> {}
+
+impl<T: DriverObject> Object<T> {
+    /// `drm_gem_object_funcs` vtable suitable for GEM shmem objects.
+    const VTABLE: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
+        free: Some(Self::free_callback),
+        open: Some(super::open_callback::<T>),
+        close: Some(super::close_callback::<T>),
+        print_info: Some(bindings::drm_gem_shmem_object_print_info),
+        export: None,
+        pin: Some(bindings::drm_gem_shmem_object_pin),
+        unpin: Some(bindings::drm_gem_shmem_object_unpin),
+        get_sg_table: Some(bindings::drm_gem_shmem_object_get_sg_table),
+        vmap: Some(bindings::drm_gem_shmem_object_vmap),
+        vunmap: Some(bindings::drm_gem_shmem_object_vunmap),
+        mmap: Some(bindings::drm_gem_shmem_object_mmap),
+        status: None,
+        rss: None,
+        #[allow(unused_unsafe, reason = "Safe since Rust 1.82.0")]
+        // SAFETY: `drm_gem_shmem_vm_ops` is a valid, static const on the C side.
+        vm_ops: unsafe { &raw const bindings::drm_gem_shmem_vm_ops },
+        evict: None,
+    };
+
+    /// Return a raw pointer to the embedded drm_gem_shmem_object.
+    fn as_raw_shmem(&self) -> *mut bindings::drm_gem_shmem_object {
+        self.obj.get()
+    }
+
+    /// Create a new shmem-backed DRM object of the given size.
+    ///
+    /// Additional config options can be specified using `config`.
+    pub fn new(
+        dev: &device::Device<T::Driver>,
+        size: usize,
+        config: ObjectConfig<'_, T>,
+        args: T::Args,
+    ) -> Result<ARef<Self>> {
+        let new: Pin<KBox<Self>> = KBox::try_pin_init(
+            try_pin_init!(Self {
+                obj <- Opaque::init_zeroed(),
+                parent_resv_obj: config.parent_resv_obj.map(|p| p.into()),
+                inner <- T::new(dev, size, args),
+            }),
+            GFP_KERNEL,
+        )?;
+
+        // SAFETY: `obj.as_raw()` is guaranteed to be valid by the initialization above.
+        unsafe { (*new.as_raw()).funcs = &Self::VTABLE };
+
+        // SAFETY: The arguments are all valid via the type invariants.
+        to_result(unsafe { bindings::drm_gem_shmem_init(dev.as_raw(), new.as_raw_shmem(), size) })?;
+
+        // SAFETY: We never move out of `self`.
+        let new = KBox::into_raw(unsafe { Pin::into_inner_unchecked(new) });
+
+        // SAFETY: We're taking over the owned refcount from `drm_gem_shmem_init`.
+        let obj = unsafe { ARef::from_raw(NonNull::new_unchecked(new)) };
+
+        // Start filling out values from `config`
+        if let Some(parent_resv) = config.parent_resv_obj {
+            // SAFETY: We have yet to expose the new gem object outside of this function, so it is
+            // safe to modify this field.
+            unsafe { (*obj.obj.get()).base.resv = parent_resv.raw_dma_resv() };
+        }
+
+        // SAFETY: We have yet to expose this object outside of this function, so we're guaranteed
+        // to have exclusive access - thus making this safe to hold a mutable reference to.
+        let shmem = unsafe { &mut *obj.as_raw_shmem() };
+        shmem.set_map_wc(config.map_wc);
+
+        Ok(obj)
+    }
+
+    /// Returns the `Device` that owns this GEM object.
+    pub fn dev(&self) -> &device::Device<T::Driver> {
+        // SAFETY: `dev` will have been initialized in `Self::new()` by `drm_gem_shmem_init()`.
+        unsafe { device::Device::from_raw((*self.as_raw()).dev) }
+    }
+
+    extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
+        // SAFETY:
+        // - 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) };
+
+        // 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();
+
+        // SAFETY: We're recovering the Kbox<> we created in gem_create_object()
+        let _ = unsafe { KBox::from_raw(this) };
+    }
+}
+
+impl<T: DriverObject> Deref for Object<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl<T: DriverObject> DerefMut for Object<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.inner
+    }
+}
+
+impl<T: DriverObject> Sealed for Object<T> {}
+
+impl<T: DriverObject> gem::IntoGEMObject for Object<T> {
+    fn as_raw(&self) -> *mut bindings::drm_gem_object {
+        // SAFETY:
+        // - Our immutable reference is proof that this is safe to dereference.
+        // - `obj` is always a valid drm_gem_shmem_object via our type invariants.
+        unsafe { &raw mut (*self.obj.get()).base }
+    }
+
+    unsafe fn from_raw<'a>(obj: *mut bindings::drm_gem_object) -> &'a Object<T> {
+        // SAFETY: The safety contract of from_gem_obj() guarantees that `obj` is contained within
+        // `Self`
+        unsafe {
+            let obj = Opaque::cast_from(container_of!(obj, bindings::drm_gem_shmem_object, base));
+
+            &*container_of!(obj, Object<T>, obj)
+        }
+    }
+}
+
+impl<T: DriverObject> driver::AllocImpl for Object<T> {
+    type Driver = T::Driver;
+
+    const ALLOC_OPS: driver::AllocOps = driver::AllocOps {
+        gem_create_object: None,
+        prime_handle_to_fd: None,
+        prime_fd_to_handle: None,
+        gem_prime_import: None,
+        gem_prime_import_sg_table: Some(bindings::drm_gem_shmem_prime_import_sg_table),
+        dumb_create: Some(bindings::drm_gem_shmem_dumb_create),
+        dumb_map_offset: None,
+    };
+}

rust/kernel/lib.rs +1 -0

index e0837ffc91bf..40de00ce4f97 100644
@@ -38,6 +38,7 @@
 #![feature(const_option)]
 #![feature(const_ptr_write)]
 #![feature(const_refs_to_cell)]
+#![feature(const_refs_to_static)]
 //
 // Stable since Rust 1.84.0.
 #![feature(strict_provenance)]

Asahi Lina (2):
rust: drm: gem: shmem: Add DRM shmem helper abstraction

Lyude Paul (5):
rust: drm: gem: Add raw_dma_resv() function