Re: [PATCH v3 13/14] rust: drm: gem: Add export() callback

[Lyude Paul <lyude@redhat.com>]

On Fri, 2025-09-05 at 12:09 -0300, Daniel Almeida wrote:
> > +impl<T: IntoGEMObject> Drop for DmaBuf<T> {
> > +    #[inline]
> > +    fn drop(&mut self) {
> > +        // SAFETY:
> > +        // - `dma_buf::DmaBuf` is guaranteed to have an identical layout to `struct dma_buf`
> > +        //   by its type invariants.
> > +        // - We hold the last reference to this `DmaBuf`, making it safe to destroy.
> 
> How can we be sure of this?

DmaBuf objects created with drm_gem_dmabuf_export() are unique, e.g. if you
call the function twice you have two DmaBufs - not two references to the same
DmaBuf. Since we don't implement Clone on DmaBuf, we're also the only one who
could hold a reference to the respective dma_buf::DmaBuf.

Note that this is unlike SGTables with shmem, where you only have references
to a single shared SGTable for each gem object that's created dynamically.

> 
> > +        unsafe { bindings::drm_gem_dmabuf_release(self.0.cast().as_ptr()) }
> > +    }
> > +}
> > +
> > +impl<T: IntoGEMObject> DmaBuf<T> {
> > +    /// Leak the reference for this [`DmaBuf`] and return a raw pointer to it.
> > +    #[inline]
> > +    pub(crate) fn into_raw(self) -> *mut bindings::dma_buf {
> 
> Then this should perhaps be called leak()? At least if we’re following the std nomenclature.

Nope, into_raw() is correct actually! FWIW: I had to double check this against
the std, it goes like this:

https://doc.rust-lang.org/std/boxed/struct.Box.html#method.into_raw
into_raw() provides Box<T, A> → *mut T

https://doc.rust-lang.org/std/boxed/struct.Box.html#method.leak
leak<'a>() provides Box<T, A> -> &'a mut T

Since we're returning *mut here, it should be into_raw().

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

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