On 5/21/2025 11:57 AM, Joel Fernandes wrote:
>
>
> On 5/21/2025 8:43 AM, 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 <acour...@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.
>>
> If there is a coding error (say large count passed to alloc_coherent()), then
> I
> don't think it can guaranteed. Maybe use
> self.count.checked_mul(core::mem::size_of::<T>())?
>
Nevermind, we already checking for overflow in alloc_coherent():
let size = count
.checked_mul(core::mem::size_of::<T>())
.ok_or(EOVERFLOW)?;
So maybe just a comment suffices, then.
thanks,
- Joel
