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 Reviewed-by: Janne Grunau j@jananu.net
--- 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
Signed-off-by: Lyude Paul lyude@redhat.com --- 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 6c77ace05d30a..3fab5d76c197b 100644 --- a/rust/kernel/drm/gem/shmem.rs +++ b/rust/kernel/drm/gem/shmem.rs @@ -198,6 +198,25 @@ 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> { @@ -248,3 +267,34 @@ 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: NonNullscatterlist::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() } + } +}