[PATCH v17 0/6] Rust bindings for gem shmem

[PATCH v17 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.
Patch-series wide changes since V16:
* Fix ordering one more time (SetOnce::reset() doesn't need to come
  before adding vmap functions)
* Rebase against the latest DeviceContext changes from me that got
  pushed.

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

 rust/kernel/drm/gem/shmem.rs | 518 ++++++++++++++++++++++++++++++++++-
 rust/kernel/faux.rs          |   7 +-
 rust/kernel/sync/set_once.rs |  60 +++-
 3 files changed, 563 insertions(+), 22 deletions(-)


base-commit: 723bd79ca9e492cc91850094a2892bde0345c51a
-- 
2.54.0

[PATCH v17 1/6] rust: drm: 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>

---
V17:
* Rebase
* Fix format of commit message subject

 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 34af402899a0e..084b798ce795b 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -42,7 +42,6 @@
 ///
 /// 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, C: DeviceContext = Registered> {
     /// Whether to set the write-combine map flag.
     pub map_wc: bool,
@@ -53,6 +52,16 @@ pub struct ObjectConfig<'a, T: DriverObject, C: DeviceContext = Registered> {
     pub parent_resv_obj: Option<&'a Object<T, C>>,
 }
 
+impl<'a, T: DriverObject, C: DeviceContext> Default for ObjectConfig<'a, T, C> {
+    #[inline(always)]
+    fn default() -> Self {
+        Self {
+            map_wc: false,
+            parent_resv_obj: None,
+        }
+    }
+}
+
 /// A shmem-backed GEM object.
 ///
 /// # Invariants
-- 
2.54.0

Re: [PATCH v17 1/6] rust: drm: gem: shmem: Fix Default implementation for ObjectConfig

[Alice Ryhl]

On Wed, Jun 03, 2026 at 03:42:30PM -0400, Lyude Paul wrote:
> 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>

This one is so simple I went ahead and applied it to drm-rust-next right
away.

Alice

[PATCH v17 2/6] 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>

---
V17:
* Fix format of commit message title

 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 084b798ce795b..650c34dd8b7a4 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -30,7 +30,10 @@
         Deref,
         DerefMut, //
     },
-    ptr::NonNull, //
+    ptr::{
+        self,
+        NonNull, //
+    },
 };
 use gem::{
     BaseObjectPrivate,
@@ -244,3 +247,29 @@ impl<T: DriverObject, C: DeviceContext> driver::AllocImpl for Object<T, C> {
         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, C: DeviceContext = Registered>(&'a Object<T, C>);
+
+impl<'a, T: DriverObject, C: DeviceContext> DmaResvGuard<'a, T, C> {
+    #[inline(always)]
+    #[expect(unused)]
+    fn new(obj: &'a Object<T, C>) -> 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, C: DeviceContext> Drop for DmaResvGuard<'a, T, C> {
+    #[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 v17 3/6] rust: drm: gem: shmem: Add vmap functions

[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
V17:
* Rebase
* Fix format of commit message title

 rust/kernel/drm/gem/shmem.rs | 312 ++++++++++++++++++++++++++++++++++-
 1 file changed, 311 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 650c34dd8b7a4..1f05a5bc5fe66 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -20,12 +20,19 @@
         Registered, //
     },
     error::to_result,
+    io::{
+        Io,
+        IoCapable,
+        IoKnownSize, //
+    },
     prelude::*,
     sync::aref::ARef,
     types::Opaque, //
 };
 use core::{
+    ffi::c_void,
     marker::PhantomData,
+    mem::MaybeUninit, //
     ops::{
         Deref,
         DerefMut, //
@@ -36,6 +43,7 @@
     },
 };
 use gem::{
+    BaseObject,
     BaseObjectPrivate,
     DriverObject,
     IntoGEMObject, //
@@ -197,6 +205,80 @@ 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, C, 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, C, 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, C, SIZE>> {
+        self.make_vmap()
+    }
 }
 
 impl<T: DriverObject, C: DeviceContext> Deref for Object<T, C> {
@@ -257,7 +339,6 @@ impl<T: DriverObject, C: DeviceContext> driver::AllocImpl for Object<T, C> {
 
 impl<'a, T: DriverObject, C: DeviceContext> DmaResvGuard<'a, T, C> {
     #[inline(always)]
-    #[expect(unused)]
     fn new(obj: &'a Object<T, C>) -> Self {
         // SAFETY: This lock is initialized throughout the lifetime of `object`.
         unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
@@ -273,3 +354,232 @@ fn drop(&mut self) {
         unsafe { bindings::dma_resv_unlock(self.0.raw_dma_resv()) };
     }
 }
+
+macro_rules! impl_vmap_io_capable {
+    ($impl:ident, $ty:ty) => {
+        impl<D, R, C, const SIZE: usize> IoCapable<$ty> for $impl<D, R, C, SIZE>
+        where
+            D: DriverObject,
+            C: DeviceContext,
+            R: Deref<Target = Object<D, C>>,
+        {
+            #[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, C = Registered, const SIZE: usize = 0>
+where
+    D: DriverObject,
+    C: DeviceContext,
+    R: Deref<Target = Object<D, C>>,
+{
+    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, C, const SIZE: usize = 0> = VMap<D, &'a Object<D, C>, C, SIZE>;
+
+/// An alias type for an owned reference to a shmem-based GEM object's VMap.
+pub type VMapOwned<D, C, const SIZE: usize = 0> = VMap<D, ARef<Object<D, C>>, C, SIZE>;
+
+impl<D, R, C, const SIZE: usize> VMap<D, R, C, SIZE>
+where
+    D: DriverObject,
+    C: DeviceContext,
+    R: Deref<Target = Object<D, C>>,
+{
+    /// Borrows a reference to the object that owns this virtual mapping.
+    #[inline(always)]
+    pub fn owner(&self) -> &Object<D, C> {
+        &self.owner
+    }
+}
+
+impl<D, R, C, const SIZE: usize> Drop for VMap<D, R, C, SIZE>
+where
+    D: DriverObject,
+    C: DeviceContext,
+    R: Deref<Target = Object<D, C>>,
+{
+    #[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, C, const SIZE: usize> Io for VMap<D, R, C, SIZE>
+where
+    D: DriverObject,
+    C: DeviceContext,
+    R: Deref<Target = Object<D, C>>,
+{
+    #[inline(always)]
+    fn addr(&self) -> usize {
+        self.addr as usize
+    }
+
+    #[inline(always)]
+    fn maxsize(&self) -> usize {
+        self.owner.size()
+    }
+}
+
+impl<D, R, C, const SIZE: usize> IoKnownSize for VMap<D, R, C, SIZE>
+where
+    D: DriverObject,
+    C: DeviceContext,
+    R: Deref<Target = Object<D, C>>,
+{
+    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::{
+            self,
+            UnregisteredDevice, //
+        },
+        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<C: DeviceContext>(
+            _dev: &drm::Device<KunitDriver, C>,
+            _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<Ctx: DeviceContext> = Object<KunitObject, Ctx>;
+
+        const INFO: drm::DriverInfo = INFO;
+        const IOCTLS: &'static [drm::ioctl::DrmIoctlDescriptor] = &[];
+    }
+
+    fn create_drm_dev() -> Result<(faux::Registration, UnregisteredDevice<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 = UnregisteredDevice::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 v17 4/6] 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

Re: [PATCH v17 4/6] rust: faux: Allow retrieving a bound Device

[Danilo Krummrich]

On Wed Jun 3, 2026 at 9:42 PM CEST, Lyude Paul wrote:
> 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.

Yes, it should be, but the reasons are non-trivial and should be part of the
safety comment below.

  - faux_match() always returns 1, and probe runs synchronously
    (PROBE_FORCE_SYNCHRONOUS)

  - suppress_bind_attrs = true on faux_driver prevents userspace-triggered
    unbind via sysfs

  - mem::forget(Registration) is not a problem; if the Registration is leaked,
    the faux device stays bound forever

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

I can pick this through the driver-core tree already, as it seems this it not
needed by this series. Feel free to resend as individual patch for thus purpose.

> ---
>  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

Re: [PATCH v17 4/6] rust: faux: Allow retrieving a bound Device

[lyude]

I mentioned this already to Danilo via alternate channels, but to
clarify - this patch -is- actually needed, as we make use of
`AsRef<Device<Bound>>` in the kunit test for SGTable

On Thu, 2026-06-04 at 15:25 +0200, Danilo Krummrich wrote:
> On Wed Jun 3, 2026 at 9:42 PM CEST, Lyude Paul wrote:
> > 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.
> 
> Yes, it should be, but the reasons are non-trivial and should be part
> of the
> safety comment below.
> 
>   - faux_match() always returns 1, and probe runs synchronously
>     (PROBE_FORCE_SYNCHRONOUS)
> 
>   - suppress_bind_attrs = true on faux_driver prevents userspace-
> triggered
>     unbind via sysfs
> 
>   - mem::forget(Registration) is not a problem; if the Registration
> is leaked,
>     the faux device stays bound forever
> 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> 
> I can pick this through the driver-core tree already, as it seems
> this it not
> needed by this series. Feel free to resend as individual patch for
> thus purpose.
> 
> > ---
> >  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 v17 5/6] 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

Re: [PATCH v17 5/6] rust: sync: Add SetOnce::reset()

[Alice Ryhl]

On Wed, Jun 03, 2026 at 03:42:34PM -0400, Lyude Paul wrote:
> 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>

This isn't needed, you can just do:

unsafe { (*this).sgt_res = SetOnce::new() };

Re: [PATCH v17 5/6] rust: sync: Add SetOnce::reset()

[lyude]

On Thu, 2026-06-04 at 11:58 +0000, Alice Ryhl wrote:
> On Wed, Jun 03, 2026 at 03:42:34PM -0400, Lyude Paul wrote:
> > 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>
> 
> This isn't needed, you can just do:
> 
> unsafe { (*this).sgt_res = SetOnce::new() };

I realized after sending this I could also just be using ManuallyDrop
here instead of adding a reset function, which might be a bit clearer
anyhow

[PATCH v17 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
V17:
* Rebase
* Fix missing safety comment in free_callback() (we forgot to justify why
  &mut is safe in `unsafe { &mut (*this).sgt_res }.reset()`)

 rust/kernel/drm/gem/shmem.rs | 166 +++++++++++++++++++++++++++++++++--
 1 file changed, 157 insertions(+), 9 deletions(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 1f05a5bc5fe66..dff90771eb34a 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -11,6 +11,11 @@
 
 use crate::{
     container_of,
+    device::{
+        self,
+        Bound, //
+    },
+    devres::*,
     drm::{
         driver,
         gem,
@@ -19,14 +24,23 @@
         DeviceContext,
         Registered, //
     },
-    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::{
@@ -87,6 +101,11 @@ pub struct Object<T: DriverObject, C: DeviceContext = Registered> {
     obj: Opaque<bindings::drm_gem_shmem_object>,
     /// Parent object that owns this object's DMA reservation object.
     parent_resv_obj: Option<ARef<Object<T, C>>>,
+    /// Devres object for unmapping any SGTable on driver-unbind.
+    sgt_res: SetOnce<Devres<SGTableMap<T, C>>>,
+    #[pin]
+    /// Lock for protecting initialization of `sgt_res`.
+    sgt_lock: Mutex<()>,
     #[pin]
     inner: T,
     _ctx: PhantomData<C>,
@@ -145,6 +164,8 @@ 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),
                 _ctx: PhantomData::<C>,
             }),
@@ -189,18 +210,25 @@ 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, C>`
-        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.
+        // - We are in free_callback, guaranteeing we have exclusive access to `this`.
+        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) };
@@ -279,6 +307,45 @@ pub fn vmap<const SIZE: usize>(&self) -> Result<VMapRef<'_, T, C, SIZE>> {
     pub fn owned_vmap<const SIZE: usize>(&self) -> Result<VMapOwned<T, C, 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, C: DeviceContext> Deref for Object<T, C> {
@@ -474,6 +541,63 @@ impl<D, R, C, const SIZE: usize> IoKnownSize for VMap<D, R, C, 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, C: DeviceContext> {
+    obj: NonNull<Object<T, C>>,
+}
+
+impl<T: DriverObject, C: DeviceContext> Deref for SGTableMap<T, C> {
+    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, C: DeviceContext> Drop for SGTableMap<T, C> {
+    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, C: DeviceContext> SGTableMap<T, C> {
+    fn new(obj: &Object<T, C>) -> 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, C: DeviceContext> Send for SGTableMap<T, C> {}
+// 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, C: DeviceContext> Sync for SGTableMap<T, C> {}
+
 #[kunit_tests(rust_drm_gem_shmem)]
 mod tests {
     use super::*;
@@ -582,4 +706,28 @@ 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