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

[Alexandre Courbot]

On Wed May 21, 2025 at 9:43 PM JST, Boqun Feng wrote:
> 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 at 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()`.
>>  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>()
>
> I think we need a comment or even an invariant saying this multiply
> cannot overflow.

I'll add the following invariant:

    `size_of::<T> * count` fits into a `usize`.

and refer to it here. Does it work?