Re: [PATCH 08/13] rust: drm/gem: add GEM object query helpers for debugfs

["Danilo Krummrich" <dakr@kernel.org>]

On Thu Mar 26, 2026 at 7:53 AM CET, Alvin Sun wrote:
> Add name(), refcount(), is_imported(), is_exported(), and mmap_offset()
> to BaseObject so drivers can expose GEM state in debugfs.
>
> Signed-off-by: Alvin Sun <alvin.sun@linux.dev>
> ---
>  rust/kernel/drm/gem/mod.rs | 49 +++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 46 insertions(+), 3 deletions(-)
>
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index 276ba3c53475d..b6188c8873ece 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -21,9 +21,13 @@
>      },
>      error::to_result,
>      prelude::*,
> -    sync::aref::{
> -        ARef,
> -        AlwaysRefCounted, //
> +    sync::{
> +        aref::{
> +            ARef,
> +            AlwaysRefCounted, //
> +        },
> +        atomic::Relaxed,
> +        Refcount, //
>      },
>      types::Opaque,
>  };
> @@ -177,6 +181,45 @@ fn size(&self) -> usize {
>          unsafe { (*self.as_raw()).size }
>      }
>  
> +    /// Returns the name of the object.
> +    #[inline]
> +    fn name(&self) -> i32 {
> +        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `struct drm_gem_object`.
> +        unsafe { (*self.as_raw()).name }
> +    }
> +
> +    /// Returns the reference count of the object.
> +    #[inline]
> +    fn refcount(&self) -> u32 {
> +        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `struct drm_gem_object`.
> +        let raw_refcount = unsafe { &raw mut (*self.as_raw()).refcount }.cast::<Refcount>();
> +        // SAFETY: `raw_refcount` has the same layout as `Refcount`.
> +        let refcount = unsafe { &*raw_refcount };
> +
> +        refcount.as_atomic().load(Relaxed) as u32
> +    }

How is this reference count useful in debugfs? In any case, please don't expose
object reference counts, it motivates abuse.

Can't we just prove a common Debug impl for GEM objects instead of providing all
those accessors?

> +
> +    /// Returns true if the object is imported.
> +    #[inline]
> +    fn is_imported(&self) -> bool {
> +        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `struct drm_gem_object`.
> +        !unsafe { (*self.as_raw()).import_attach }.is_null()
> +    }
> +
> +    /// Returns true if the object is exported.
> +    #[inline]
> +    fn is_exported(&self) -> bool {
> +        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `struct drm_gem_object`.
> +        !unsafe { (*self.as_raw()).dma_buf }.is_null()
> +    }
> +
> +    /// Returns the offset for mmap, or 0 if no offset has been allocated.
> +    #[inline]
> +    fn mmap_offset(&self) -> u64 {
> +        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `struct drm_gem_object`.
> +        unsafe { (*self.as_raw()).vma_node.vm_node.start }
> +    }
> +
>      /// Creates a new handle for the object associated with a given `File`
>      /// (or returns an existing one).
>      fn create_handle<D, F>(&self, file: &drm::File<F>) -> Result<u32>
>
> -- 
> 2.43.0