Re: [PATCH v9] rust: transmute: Add methods for FromBytes trait

["Alexandre Courbot" <acourbot@nvidia.com>]

On Tue Aug 12, 2025 at 6:38 AM JST, Christian S. Lima wrote:
> The two methods added take a slice of bytes and return those bytes in a
> specific type. These methods are useful when we need to transform the
> stream of bytes into specific type.
>
> The `Frombytessized` trait was added to make it easier to implement other

typo: FromBytesSized.

> user defined types within the codebase. With the current implementation,
> there's no way to interact without implementing `from_bytes` and
> `from_mut_bytes` for every new type, and this would end up generating a lot
> of duplicate code. By using FromBytesSized as a proxy trait, we can avoid
> this without generating a direct dependency. If necessary, the user can
> simply implement `FromBytes` if needed. For more context, please check the
> [1] and [2].
>
> [1] https://lore.kernel.org/rust-for-linux/DANSZ6Q476EC.3GY00K717QVUL@nvidia.com/
> [2] https://lore.kernel.org/rust-for-linux/DAOESYD6F287.3U3M64X0S1WN5@nvidia.com/
>
> Link: https://github.com/Rust-for-Linux/linux/issues/1119
> Suggested-by: Alexandre Courbot <acourbot@nvidia.com>
> Signed-off-by: Christian S. Lima <christiansantoslima21@gmail.com>

I mentioned it on v8 [1] and v7 [2], but the tests that break due to
this change need to be updated by this patch as well. This includes:

* The doctests in `rust/kernel/dma.rs`,
* The `samples/rust/rust_dma.rs` sample,
* The example for `FromBytes` introduced by this patch which uses `?` without
defining a function.

Please make sure to compile with `CONFIG_RUST_KERNEL_DOCTESTS` and
`CONFIG_SAMPLE_RUST_DMA` to see the failures.

Also, now that we are on 6.17-rc1, the types in `nova-core` that implement
`FromBytes` will also fail to build unless they are switched to
`FromBytesSized`. Which I guess speaks in favor of taking this into the Nova
tree if there are not other planned user for this cycle?

[1]
https://lore.kernel.org/rust-for-linux/DB5KEWX9EJ2Q.3CX5EGS66OVHH@nvidia.com/
[2]
https://lore.kernel.org/rust-for-linux/DANSZ6Q476EC.3GY00K717QVUL@nvidia.com/

In case this helps, here is the diff I needed to apply to get the tests
to build:

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index 1801836f31455..3797c70c13040 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -597,7 +597,7 @@ unsafe impl<T: AsBytes + FromBytes + Send> Send for CoherentAllocation<T> {}
 /// struct MyStruct { field: u32, }
 ///
 /// // SAFETY: All bit patterns are acceptable values for `MyStruct`.
-/// unsafe impl kernel::transmute::FromBytes for MyStruct{};
+/// unsafe impl kernel::transmute::FromBytesSized for MyStruct{};
 /// // SAFETY: Instances of `MyStruct` have no uninitialized portions.
 /// unsafe impl kernel::transmute::AsBytes for MyStruct{};
 ///
@@ -641,7 +641,7 @@ macro_rules! dma_read {
 /// struct MyStruct { member: u32, }
 ///
 /// // SAFETY: All bit patterns are acceptable values for `MyStruct`.
-/// unsafe impl kernel::transmute::FromBytes for MyStruct{};
+/// unsafe impl kernel::transmute::FromBytesSized for MyStruct{};
 /// // SAFETY: Instances of `MyStruct` have no uninitialized portions.
 /// unsafe impl kernel::transmute::AsBytes for MyStruct{};
 ///
diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index ba21fe49e4f07..e7bd698ec99cd 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -16,6 +16,7 @@
 /// ```
 /// use kernel::transmute::FromBytes;
 ///
+/// # fn test() -> Option<()> {
 /// let foo = [1, 2, 3, 4];
 ///
 /// let result = u32::from_bytes(&foo)?;
@@ -25,6 +26,7 @@
 ///
 /// #[cfg(target_endian = "big")]
 /// assert_eq!(*result, 0x1020304);
+/// # Some(()) }
 /// ```
 ///
 /// # Safety
diff --git a/samples/rust/rust_dma.rs b/samples/rust/rust_dma.rs
index 9fb3ae1dd8587..36ad877d4804d 100644
--- a/samples/rust/rust_dma.rs
+++ b/samples/rust/rust_dma.rs
@@ -65,7 +65,7 @@ fn new(h: u32, b: u32) -> Self {
 // SAFETY: All bit patterns are acceptable values for `MyStruct`.
 unsafe impl kernel::transmute::AsBytes for MyStruct {}
 // SAFETY: Instances of `MyStruct` have no uninitialized portions.
-unsafe impl kernel::transmute::FromBytes for MyStruct {}
+unsafe impl kernel::transmute::FromBytesSized for MyStruct {}

 kernel::pci_device_table!(
     PCI_TABLE,

-- end of diff --

Otherwise the patch in itself looks good to me. It is used in nova-core (and a
critical dependency for this cycle :)), so although I cannot give my
Reviewed-by until the tests are fixed, it is at least

Tested-by: Alexandre Courbot <acourbot@nvidia.com>