[PATCH v10 5/5] rust: drm: gem: Add vmap functions to shmem bindings

Lyude Paul
2026-04-09T00:16:36Z

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 <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* Get rid of some outdated comments I missed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.

rust/kernel/drm/gem/shmem.rs | 354 +++++++++++++++++++++++++++++++++++
1 file changed, 354 insertions(+)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 111be446213df..6c186d83f528e 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -26,6 +26,11 @@
from_err_ptr, //
to_result,
},
+ io::{
+ Io,
+ IoCapable,
+ IoKnownSize, //
+ },
prelude::*,
scatterlist,
types::{
@@ -35,6 +40,11 @@
};
use core::{
cell::UnsafeCell,
+ ffi::c_void,
+ mem::{
+ self,
+ MaybeUninit, //
+ },
ops::{
Deref,
DerefMut, //
@@ -45,6 +55,7 @@
},
};
use gem::{
+ BaseObject,
BaseObjectPrivate,
DriverObject,
IntoGEMObject, //
@@ -288,6 +299,84 @@ pub fn owned_sg_table(&self, dev: &device::Device<Bound>) -> Result<SGTable<T>>
// `Some(Devres<SGTableMap<T>>)`.
Ok(SGTable(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, 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 {
+ // 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 `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 resv = self.raw_dma_resv();
+
+ // 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> {
@@ -386,6 +475,154 @@ unsafe impl<T: DriverObject> Send for SGTableMap<T> {}
// it points to is guaranteed to be thread-safe.
unsafe impl<T: DriverObject> Sync for SGTableMap<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 $impl<$( $lifetime ,)? D, SIZE>
+ where
+ D: DriverObject,
+ {
+ #[inline(always)]
+ fn drop(&mut self) {
+ // SAFETY:
+ // - Our existence is proof that this map was previously created using self.owner.
+ // - Since we are in Drop, we are guaranteed that no one will access the memory
+ // through this mapping after calling this.
+ unsafe {
+ self.owner.raw_vunmap(bindings::iosys_map {
+ is_iomem: false,
+ __bindgen_anon_1: bindings::iosys_map__bindgen_ty_1 { vaddr: self.addr }
+ })
+ };
+ }
+ }
+
+ impl<$( $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
@@ -413,3 +650,120 @@ fn deref(&self) -> &Self::Target {
unsafe { (*self.0.sgt_res.get()).as_ref().unwrap_unchecked() }
}
}
+
+#[kunit_tests(rust_drm_gem_shmem)]
+mod tests {
+ use super::*;
+ use crate::{
+ drm,
+ faux,
+ page::PAGE_SIZE, //
+ };
+
+ // The bare minimum needed to create a fake drm driver for kunit
+
+ #[pin_data]
+ struct KunitData {}
+ struct KunitDriver;
+ struct KunitFile;
+ #[pin_data]
+ struct KunitObject {}
+
+ const INFO: drm::DriverInfo = drm::DriverInfo {
+ major: 0,
+ minor: 0,
+ patchlevel: 0,
+ name: c"kunit",
+ desc: c"Kunit",
+ };
+
+ impl drm::file::DriverFile for KunitFile {
+ type Driver = KunitDriver;
+
+ fn open(_dev: &drm::Device<KunitDriver>) -> Result<Pin<KBox<Self>>> {
+ Ok(KBox::new(Self, GFP_KERNEL)?.into())
+ }
+ }
+
+ impl gem::DriverObject for KunitObject {
+ type Driver = KunitDriver;
+ type Args = ();
+
+ fn new(
+ _dev: &drm::Device<KunitDriver>,
+ _size: usize,
+ _args: Self::Args,
+ ) -> impl PinInit<Self, Error> {
+ try_pin_init!(KunitObject {})
+ }
+ }
+
+ #[vtable]
+ impl drm::Driver for KunitDriver {
+ type Data = KunitData;
+ type File = KunitFile;
+ type Object = Object<KunitObject>;
+
+ const INFO: drm::DriverInfo = INFO;
+ const IOCTLS: &'static [drm::ioctl::DrmIoctlDescriptor] = &[];
+ }
+
+ fn create_drm_dev() -> Result<(faux::Registration, ARef<drm::Device<KunitDriver>>)> {
+ // Create a faux DRM device so we can test gem object creation.
+ let data = try_pin_init!(KunitData {});
+ let dev = faux::Registration::new(c"Kunit", None)?;
+ let drm = drm::Device::<KunitDriver>::new(dev.as_ref(), data)?;
+
+ Ok((dev, drm))
+ }
+
+ #[test]
+ fn compile_time_vmap_sizes() -> Result {
+ let (_dev, drm) = create_drm_dev()?;
+
+ // 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(())
+ }
+}
--
2.53.0