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

[Lyude Paul <lyude@redhat.com>]

On Wed, 2026-04-29 at 08:19 +0000, Alice Ryhl wrote:
> >       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.

We do actually need the mutex, see the discussions on the older versions of
the patch series that I had with Alexandre. The only alternative is the
ability to pass a impl Init<_, Error> to SetOnce::populate. Whcih is because
Devres setup can fail, and we can only have one Devres resource for SGTableMap
at a time for a given gem object because dropping the Devres also releases the
sgt table. So trying to do this without a lock would imply creating a Devres,
attempting to pass it to populate (it could have been populated between
creation and populating the devres), and then destroying a potentially
redundant Devres that will drop the SGT resources that the prior Devres
instance would be responsible for unmapping.

-- 
Cheers,
 Lyude Paul (she/her)
 Senior Software Engineer at Red Hat

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