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

[Deborah Brouwer <deborah.brouwer@collabora.com>]

On Wed, Apr 29, 2026 at 08:19:41AM +0000, Alice Ryhl wrote:
> 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 Outdated?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.

I think it is also causing a lockdep warning pointing to this sgt_res mutex.
It only happens when KUnit tests are enabled.
If the KUnit tests register a lock class for this mutex first, then when Tyr
later locks it via sg_table(), lockdep complains about the same key being used
for a different class.

> 
> Alice