[PATCH v7 4/7] rust: drm: gem: shmem: Add DRM shmem helper abstraction

Lyude Paul
2026-02-06T22:35:45Z

From: Asahi Lina <lina@asahilina.net>

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 <lina@asahilina.net>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>

---

V2:
* Use the drm_gem_shmem_init() and drm_gem_shmem_release() that I extracted
so we can handle memory allocation in rust, which means we no longer have
to handle freeing rust members of the struct by hand and have a closer
implementation to the main gem object
(this also gets rid of gem_create_object)
* Get rid of GemObjectRef and UniqueGemObjectRef, we have ARef<T> at home.
* Use Device<T::Driver> in Object<T>
* Cleanup Object::<T>::new() a bit:
* Cleanup safety comment
* Use cast_mut()
* Just import container_of!(), we use it all over anyhow
* mut_shmem() -> as_shmem(), make it safe (there's no reason for being unsafe)
* Remove any *const and *muts in structs, just use NonNull
* Get rid of the previously hand-rolled sg_table bindings in shmem, use the
bindings from Abdiel's sg_table patch series
* Add a TODO at the top about DMA reservation APIs and a desire for WwMutex
* Get rid of map_wc() and replace it with a new ObjectConfig struct. While
it currently only specifies the map_wc flag, the idea here is that
settings like map_wc() and parent_resv_obj() shouldn't be exposed as
normal functions since the only place where it's safe to set them is
when we're still guaranteed unique access to the GEM object, e.g. before
returning it to the caller. Using a struct instead of individual
arguments here is mainly because we'll be adding at least one more
argument, and there's enough other gem shmem settings that trying to add
all of them as individual function arguments in the future would be a bit
messy.
* Get rid of vm_numa_fields!, Lina didn't like this macro much either and I
think that it's fine for us to just specify the #[cfg(…)] attributes by
hand since we only need to do it twice.
* Set drm_gem_object_funcs.vm_ops directly to drm_gem_shmem_vm_ops, don't
export the various shmem funcs. I'm not sure why this wasn't possible
before but it seems to work fine now.
V4:
* Switch from OpaqueObject to a normal Object<T> now that we've removed it
* Remove `dev` from Object, it's redundant as drm_gem_object already has a
device pointer we can use.
* Use &raw instead of addr_of!()
V5:
* Fix typo in shmem::Object::as_raw()
* Add type invariant around `obj` always being a valid
`drm_gem_shmem_object` for the duration of the lifetime of `Object`.
* Use Opaque::cast_from() instead of unrestricted casts
* Use IS_ENABLED() for gem shmem C helpers.
V7:
* Fix import style
* Don't forget to make Object<T> Send/Sync

rust/bindings/bindings_helper.h | 2 +
rust/helpers/drm.c | 56 ++++++-
rust/kernel/drm/gem/mod.rs | 5 +-
rust/kernel/drm/gem/shmem.rs | 249 ++++++++++++++++++++++++++++++++
4 files changed, 310 insertions(+), 2 deletions(-)
create mode 100644 rust/kernel/drm/gem/shmem.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index ecf31681df806..c93afbd095e87 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -33,6 +33,7 @@
#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>
@@ -61,6 +62,7 @@
#include <linux/fs.h>
#include <linux/i2c.h>
#include <linux/ioport.h>
+#include <linux/iosys-map.h>
#include <linux/jiffies.h>
#include <linux/jump_label.h>
#include <linux/mdio.h>
diff --git a/rust/helpers/drm.c b/rust/helpers/drm.c
index 450b406c6f273..53ec06879db3d 100644
--- a/rust/helpers/drm.c
+++ b/rust/helpers/drm.c
@@ -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
@@ -20,4 +21,57 @@ __u64 rust_helper_drm_vma_node_offset_addr(struct drm_vma_offset_node *node)
return drm_vma_node_offset_addr(node);
}

-#endif
+#if IS_ENABLED(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 */
diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index 972d50d4342dd..379ae3dfb02f5 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -3,6 +3,8 @@
//! DRM GEM API
//!
//! C header: [`include/drm/drm_gem.h`](srctree/include/drm/drm_gem.h)
+#[cfg(CONFIG_DRM_GEM_SHMEM_HELPER)]
+pub mod shmem;

use crate::{
alloc::flags::*,
@@ -50,6 +52,8 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) {
};
}

+pub(crate) use impl_aref_for_gem_obj;
+
/// A type alias for retrieving a [`Driver`]s [`DriverFile`] implementation from its
/// [`DriverObject`] implementation.
///
@@ -205,7 +209,6 @@ 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)]
pub(crate) trait BaseObjectPrivate: IntoGEMObject {
/// Return a pointer to this object's dma_resv.
fn raw_dma_resv(&self) -> *mut bindings::dma_resv {
diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
new file mode 100644
index 0000000000000..d9f1a4e95cedc
--- /dev/null
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -0,0 +1,249 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! DRM GEM shmem helper objects
+//!
+//! C header: [`include/linux/drm/drm_gem_shmem_helper.h`](srctree/include/linux/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::{
+ from_err_ptr,
+ to_result, //
+ },
+ prelude::*,
+ scatterlist,
+ 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,
+ // SAFETY: `drm_gem_shmem_vm_ops` is static const on the C side, so immutable references are
+ // safe here and such references shall be valid forever
+ vm_ops: unsafe { &bindings::drm_gem_shmem_vm_ops },
+ evict: None,
+ };
+
+ /// Return a raw pointer to the embedded drm_gem_shmem_object.
+ fn as_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_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_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) };
+ }
+
+ /// 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.
+ #[inline]
+ pub fn sg_table(&self) -> Result<&scatterlist::SGTable> {
+ // SAFETY:
+ // - drm_gem_shmem_get_pages_sgt is thread-safe.
+ // - drm_gem_shmem_get_pages_sgt returns either a valid pointer to a scatterlist, or an
+ // error pointer.
+ let sgt = from_err_ptr(unsafe { bindings::drm_gem_shmem_get_pages_sgt(self.as_shmem()) })?;
+
+ // 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) })
+ }
+}
+
+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,
+ };
+}
--
2.53.0