Re: [PATCH v13 4/5] rust: drm: gem: Introduce shmem::SGTable

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Apr 28, 2026 at 03:03:44PM -0400, Lyude Paul wrote:
> 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. We store this in an UnsafeCell and protect

Outdated? No longer UnsafeCell.

> access to it using the dma_resv lock that we already have from the shmem
> gem object, which is the same lock that currently protects
> drm_gem_object_shmem->sgt.
> 
> We also provide two different methods for acquiring an sg table:
> self.sg_table(), and self.owned_sg_table(). The first function is for
> short-term uses of mapped SGTables, the second is for callers that need to
> hold onto the mapped SGTable for an extended period of time. The second
> variant uses Devres of course, whereas the first simply relies on rust's
> borrow checker to prevent driver-unbind when using the mapped SGTable.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>

>      obj: Opaque<bindings::drm_gem_shmem_object>,
>      /// Parent object that owns this object's DMA reservation object.
>      parent_resv_obj: Option<ARef<Object<T>>>,
> +    /// Devres object for unmapping any SGTable on driver-unbind.
> +    /// TODO: Drop the mutex once we can use Init with SetOnce.
> +    #[pin]
> +    sgt_res: Mutex<SetOnce<Devres<SGTableMap<T>>>>,

As far as I can tell, you don't need this Mutex. Also, it causes
problems like requiring the reference transmute below.

Alice