linux-kernel - Re: [PATCH 1/4] rust: dma: implement DataDirection

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Message-Id: <DC5INEG2DXU5.DM4JIICEQ2PC@kernel.org>
Date: Mon, 18 Aug 2025 13:27:44 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: <akpm@...ux-foundation.org>, <ojeda@...nel.org>,
 <alex.gaynor@...il.com>, <boqun.feng@...il.com>, <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, <lossin@...nel.org>, <a.hindborg@...nel.org>,
 <tmgross@...ch.edu>, <abdiel.janulgue@...il.com>, <acourbot@...dia.com>,
 <jgg@...pe.ca>, <lyude@...hat.com>, <robin.murphy@....com>,
 <daniel.almeida@...labora.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] rust: dma: implement DataDirection

On Mon Aug 18, 2025 at 11:34 AM CEST, Alice Ryhl wrote:
> On Fri, Aug 15, 2025 at 07:10:02PM +0200, Danilo Krummrich wrote:
>> 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.
>> 
>> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>
>> +/// 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)]
>> +pub struct DataDirection(bindings::dma_data_direction);
>
> Perhaps this should be a real Rust enum so that you can do an exhaustive
> match?

	/// 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(i32)]

Does bindgen ever pick another type than i32 for C enums? If so, it'd be a
downside that we'd have to mess with the type either in the `repr` or by casting
the variants.

	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 = 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.
	    ToDevice = 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 = 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 = bindings::dma_data_direction_DMA_NONE,
	}
	
	impl From<DataDirection> for bindings::dma_data_direction {
	    /// Returns the raw representation of [`enum dma_data_direction`].
	    fn from(direction: DataDirection) -> Self {
	        direction as Self
	    }
	}