From c2437c43cfb1f7f461669ba3cd74936fe345640b Mon Sep 17 00:00:00 2001 From: Danilo Krummrich Date: Thu, 28 Aug 2025 15:32:14 +0200 Subject: [PATCH] rust: dma: implement DataDirection Add the `DataDirection` struct, a newtype wrapper around the C `enum dma_data_direction`. This provides a type-safe Rust interface for specifying the direction of DMA transfers. Reviewed-by: Alice Ryhl Reviewed-by: Alexandre Courbot Reviewed-by: Daniel Almeida Reviewed-by: Lyude Paul Link: https://lore.kernel.org/r/20250828133323.53311-2-dakr@kernel.org Signed-off-by: Danilo Krummrich --- rust/bindings/bindings_helper.h | 1 + rust/kernel/dma.rs | 68 +++++++++++++++++++++++++++++++++ 2 files changed, 69 insertions(+) diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index 0e140e07758bf..c2cc52ee9945b 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -47,6 +47,7 @@ #include #include #include +#include #include #include #include diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs index 2bc8ab51ec280..27b25f041f32d 100644 --- a/rust/kernel/dma.rs +++ b/rust/kernel/dma.rs @@ -244,6 +244,74 @@ pub mod attrs { pub const DMA_ATTR_PRIVILEGED: Attrs = Attrs(bindings::DMA_ATTR_PRIVILEGED); } +/// DMA data direction. +/// +/// Corresponds to the C [`enum dma_data_direction`]. +/// +/// [`enum dma_data_direction`]: srctree/include/linux/dma-direction.h +#[derive(Copy, Clone, PartialEq, Eq, Debug)] +#[repr(u32)] +pub enum DataDirection { + /// The DMA mapping is for bidirectional data transfer. + /// + /// This is used when the buffer can be both read from and written to by the device. + /// The cache for the corresponding memory region is both flushed and invalidated. + Bidirectional = Self::const_cast(bindings::dma_data_direction_DMA_BIDIRECTIONAL), + + /// The DMA mapping is for data transfer from memory to the device (write). + /// + /// The CPU has prepared data in the buffer, and the device will read it. + /// The cache for the corresponding memory region is flushed before device access. + ToDevice = Self::const_cast(bindings::dma_data_direction_DMA_TO_DEVICE), + + /// The DMA mapping is for data transfer from the device to memory (read). + /// + /// The device will write data into the buffer for the CPU to read. + /// The cache for the corresponding memory region is invalidated before CPU access. + FromDevice = Self::const_cast(bindings::dma_data_direction_DMA_FROM_DEVICE), + + /// The DMA mapping is not for data transfer. + /// + /// This is primarily for debugging purposes. With this direction, the DMA mapping API + /// will not perform any cache coherency operations. + None = Self::const_cast(bindings::dma_data_direction_DMA_NONE), +} + +impl DataDirection { + /// Casts the bindgen-generated enum type to a `u32` at compile time. + /// + /// This function will cause a compile-time error if the underlying value of the + /// C enum is out of bounds for `u32`. + const fn const_cast(val: bindings::dma_data_direction) -> u32 { + // CAST: The C standard allows compilers to choose different integer types for enums. + // To safely check the value, we cast it to a wide signed integer type (`i128`) + // which can hold any standard C integer enum type without truncation. + let wide_val = val as i128; + + // Check if the value is outside the valid range for the target type `u32`. + // CAST: `u32::MAX` is cast to `i128` to match the type of `wide_val` for the comparison. + if wide_val < 0 || wide_val > u32::MAX as i128 { + // Trigger a compile-time error in a const context. + build_error!("C enum value is out of bounds for the target type `u32`."); + } + + // CAST: This cast is valid because the check above guarantees that `wide_val` + // is within the representable range of `u32`. + wide_val as u32 + } +} + +impl From for bindings::dma_data_direction { + /// Returns the raw representation of [`enum dma_data_direction`]. + fn from(direction: DataDirection) -> Self { + // CAST: `direction as u32` gets the underlying representation of our `#[repr(u32)]` enum. + // The subsequent cast to `Self` (the bindgen type) assumes the C enum is compatible + // with the enum variants of `DataDirection`, which is a valid assumption given our + // compile-time checks. + direction as u32 as Self + } +} + /// An abstraction of the `dma_alloc_coherent` API. /// /// This is an abstraction around the `dma_alloc_coherent` API which is used to allocate and map -- 2.47.3