Re: [PATCH v5 6/8] rust: drm: gem: Introduce shmem::SGTable

Daniel Almeida
2025-11-24T15:43:24Z

> On 23 Oct 2025, at 18:22, Lyude Paul <lyude@redhat.com> wrote:
>
> Currently we expose the ability to retrieve an SGTable for an shmem gem
> object using gem::shmem::Object::<T>::sg_table(). However, this only gives
> us a borrowed reference. This being said - retrieving an SGTable is a
> fallible operation, and as such it's reasonable that a driver may want to
> hold onto an SGTable for longer then a reference would allow in order to
> avoid having to deal with fallibility every time they want to access the
> SGTable. One such driver with this usecase is the Asahi driver.
>
> So to support this, let's introduce shmem::SGTable - which both holds a
> pointer to the SGTable and a reference to its respective GEM object in
> order to keep the GEM object alive for as long as the shmem::SGTable. The
> type can be used identically to a normal SGTable.
>
> 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
>
> rust/kernel/drm/gem/shmem.rs | 50 ++++++++++++++++++++++++++++++++++++
> 1 file changed, 50 insertions(+)
>
> diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
> index 45b95d60a3ec7..21ccb6c1824be 100644
> --- a/rust/kernel/drm/gem/shmem.rs
> +++ b/rust/kernel/drm/gem/shmem.rs
> @@ -173,6 +173,25 @@ pub fn sg_table(&self) -> Result<&scatterlist::SGTable> {
> // pointer to a scatterlist
> Ok(unsafe { scatterlist::SGTable::from_raw(sgt) })
> }
> +
> + /// Creates (if necessary) and returns an owned reference to a scatter-gather table of DMA pages
> + /// for this object.
> + ///
> + /// This is the same as [`sg_table`](Self::sg_table), except that it instead returns an
> + /// [`shmem::SGTable`] which holds a reference to the associated gem object, instead of a
> + /// reference to an [`scatterlist::SGTable`].
> + ///
> + /// This will pin the object in memory.
> + ///
> + /// [`shmem::SGTable`]: SGTable
> + pub fn owned_sg_table(&self) -> Result<SGTable<T>> {
> + Ok(SGTable {
> + sgt: self.sg_table()?.into(),
> + // INVARIANT: We take an owned refcount to `self` here, ensuring that `sgt` remains
> + // valid for as long as this `SGTable`.
> + _owner: self.into(),
> + })
> + }
> }
>
> impl<T: DriverObject> Deref for Object<T> {
> @@ -223,3 +242,34 @@ impl<T: DriverObject> driver::AllocImpl for Object<T> {
> dumb_map_offset: None,
> };
> }
> +
> +/// 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
> +/// [`scatterlist::SGTable`] referenced by this type remains valid for the lifetime of this object.
> +///
> +/// # Invariants
> +///
> +/// - `sgt` is kept alive by `_owner`, ensuring it remains valid for as long as `Self`.
> +/// - `sgt` corresponds to the owned object in `_owner`.
> +/// - This object is only exposed in situations where we know the underlying `SGTable` will not be
> +/// modified for the lifetime of this object. Thus, it is safe to send/access this type across
> +/// threads.
> +pub struct SGTable<T: DriverObject> {
> + sgt: NonNull<scatterlist::SGTable>,
> + _owner: ARef<Object<T>>,
> +}
> +
> +// SAFETY: This object is thread-safe via our type invariants.
> +unsafe impl<T: DriverObject> Send for SGTable<T> {}
> +// SAFETY: This object is thread-safe via our type invariants.
> +unsafe impl<T: DriverObject> Sync for SGTable<T> {}
> +
> +impl<T: DriverObject> Deref for SGTable<T> {
> + type Target = scatterlist::SGTable;
> +
> + fn deref(&self) -> &Self::Target {
> + // SAFETY: Creating an immutable reference to this is safe via our type invariants.
> + unsafe { self.sgt.as_ref() }
> + }
> +}
> --
> 2.51.0
>

Same comment as last patch.

— Daniel