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

["Danilo Krummrich" <dakr@kernel.org>]

On Mon Aug 18, 2025 at 2:22 PM CEST, Alice Ryhl wrote:
> 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@kernel.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)]

What's your suggestion for whateveryouwant?

And I mean this in general, not only for this case of dma::DataDirection. If we
pick u32 than things break if a negative enum variant is added on the C side. If
we pick i32, it can happen that we overflow.

> enum DataDirection {
>     Bidirectional = bindings::dma_data_direction_DMA_BIDIRECTIONAL as _,

We have a clippy lint that warns about `as _` casts, but I guess you meant
`as whateveryouwant`. However, if bindgen picks whateveryouwant too, we also get
a warning.

Ultimately, we'd need to suppress a warning.

> }