linux-kernel - [PATCH v5 01/23] rust: dma: expose the count and size of CoherentAllocation

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Message-Id: <20250612-nova-frts-v5-1-14ba7eaf166b@nvidia.com>
Date: Thu, 12 Jun 2025 23:01:29 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
 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>, 
 Benno Lossin <lossin@...nel.org>
Cc: 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, Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v5 01/23] rust: dma: expose the count and size of
 CoherentAllocation

These properties are very useful to have (and to be used by nova-core)
and should be accessible.

Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
 rust/kernel/dma.rs | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index a33261c62e0c2d3c2c9e92a4c058faab594e5355..1a6fc800256500ae04099fbf4f9a1bd1115ce202 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -114,9 +114,11 @@ pub mod attrs {
 ///
 /// # Invariants
 ///
-/// For the lifetime of an instance of [`CoherentAllocation`], the `cpu_addr` is a valid pointer
-/// to an allocated region of consistent memory and `dma_handle` is the DMA address base of
-/// the region.
+/// - For the lifetime of an instance of [`CoherentAllocation`], the `cpu_addr` is a valid pointer
+///   to an allocated region of consistent memory and `dma_handle` is the DMA address base of the
+///   region.
+/// - The size in bytes of the allocation is equal to `size_of::<T> * count`.
+/// - `size_of::<T> * count` fits into a `usize`.
 // TODO
 //
 // DMA allocations potentially carry device resources (e.g.IOMMU mappings), hence for soundness
@@ -179,9 +181,12 @@ pub fn alloc_attrs(
         if ret.is_null() {
             return Err(ENOMEM);
         }
-        // INVARIANT: We just successfully allocated a coherent region which is accessible for
-        // `count` elements, hence the cpu address is valid. We also hold a refcounted reference
-        // to the device.
+        // INVARIANT:
+        // - We just successfully allocated a coherent region which is accessible for
+        //   `count` elements, hence the cpu address is valid. We also hold a refcounted reference
+        //   to the device.
+        // - The allocated `size` is equal to `size_of::<T> * count`.
+        // - The allocated `size` fits into a `usize`.
         Ok(Self {
             dev: dev.into(),
             dma_handle,
@@ -201,6 +206,21 @@ 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 {
+        // INVARIANT: The type invariant of `Self` guarantees that size_of::<T> * count` fits into
+        // a `usize`.
+        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