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

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Message-ID: <aKMa7YzO-PwEv9AT@google.com>
Date: Mon, 18 Aug 2025 12:22:05 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
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 01:27:44PM +0200, Danilo Krummrich wrote:
> 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
> 	    }
> 	}

My suggestion is to cast on the Rust-side.

#[repr(whateveryouwant)]
enum DataDirection {
    Bidirectional = bindings::dma_data_direction_DMA_BIDIRECTIONAL as _,
}