Re: [PATCH v4 01/20] rust: dma: expose the count and size of CoherentAllocation

[Danilo Krummrich <dakr@kernel.org>]

On Wed, May 21, 2025 at 03:44:56PM +0900, Alexandre Courbot wrote:
> These properties are very useful to have and should be accessible.
> 
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  rust/kernel/dma.rs | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 605e01e35715667f93297fd9ec49d8e7032e0910..2a60eefa47dfc1f836c30ee342e26c6ff3e9b13a 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -129,6 +129,10 @@ pub mod attrs {
>  //
>  // Hence, find a way to revoke the device resources of a `CoherentAllocation`, but not the
>  // entire `CoherentAllocation` including the allocated memory itself.
> +//
> +// # Invariants
> +//
> +// The size in bytes of the allocation is equal to `size_of::<T> * count()`.

I think this also needs an invariant comment whenever self.count is set.

>  pub struct CoherentAllocation<T: AsBytes + FromBytes> {
>      dev: ARef<Device>,
>      dma_handle: bindings::dma_addr_t,
> @@ -201,6 +205,20 @@ pub fn alloc_coherent(
>          CoherentAllocation::alloc_attrs(dev, count, gfp_flags, Attrs(0))
>      }
>  
> +    /// Returns the number of elements `T` in this allocation.
> +    ///
> +    /// Note that this is not the size of the allocation in bytes, which is provided by
> +    /// [`Self::size`].
> +    pub fn count(&self) -> usize {
> +        self.count
> +    }
> +
> +    /// Returns the size in bytes of this allocation.
> +    pub fn size(&self) -> usize {
> +        // As per the invariants of `CoherentAllocation`.
> +        self.count * core::mem::size_of::<T>()
> +    }
> +
>      /// Returns the base address to the allocated region in the CPU's virtual address space.
>      pub fn start_ptr(&self) -> *const T {
>          self.cpu_addr
> 
> -- 
> 2.49.0
>