On Thu, May 22, 2025 at 02:29:35PM +0900, Alexandre Courbot wrote:
> 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 <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()`.
As you propose below.
"... and does fit into a `usize`"
You then need an '// INVARIANT' comment where we actually ensure this, i.e. in
Self::alloc_attrs().
> >> 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`.
"The type invariant of `Self` guarantees that [...]" or similar.
> >> + 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?
