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

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Message-ID: <aD1t34em7jd7vFek@pollux>
Date: Mon, 2 Jun 2025 11:24:47 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	John Hubbard <jhubbard@...dia.com>, Ben Skeggs <bskeggs@...dia.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Timur Tabi <ttabi@...dia.com>, Alistair Popple <apopple@...dia.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4 01/20] rust: dma: expose the count and size of
 CoherentAllocation

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 <acourbot@...dia.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?