Re: [PATCH v1] rust: conditionally compile dma abstraction based on CONFIG_HAS_DMA

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Apr 09, 2025 at 02:55:01PM +0900, FUJITA Tomonori wrote:
> Make dma abstraction conditional on CONFIG_HAS_DMA.

I'm not convinced that this is the correct fix. This would require drivers to
depend on CONFIG_HAS_DMA if they want to use the DMA API. The C side does not do
this either.

Instead, for functions like dma_alloc_attrs(), there is an empty implementation
if CONFIG_HAS_DMA is not set [1].

However, this definition is not picked up by bindgen. Hence, I think the actual
fix would be to introduce the corresponding Rust helpers.

[1] https://elixir.bootlin.com/linux/v6.13.7/source/include/linux/dma-mapping.h#L189

> This fixes the following build error on UML:
> 
> error[E0425]: cannot find function `dma_alloc_attrs` in crate `bindings`
>      --> rust/kernel/dma.rs:171:23
>       |
> 171   |               bindings::dma_alloc_attrs(
>       |                         ^^^^^^^^^^^^^^^ help: a function with a similar name exists: `dma_alloc_pages`
>       |
>      ::: /home/fujita/build/um/rust/bindings/bindings_generated.rs:44568:5
>       |
> 44568 | /     pub fn dma_alloc_pages(
> 44569 | |         dev: *mut device,
> 44570 | |         size: usize,
> 44571 | |         dma_handle: *mut dma_addr_t,
> 44572 | |         dir: dma_data_direction,
> 44573 | |         gfp: gfp_t,
> 44574 | |     ) -> *mut page;
>       | |___________________- similarly named function `dma_alloc_pages` defined here
> 
> error[E0425]: cannot find function `dma_free_attrs` in crate `bindings`
>      --> rust/kernel/dma.rs:293:23
>       |
> 293   |               bindings::dma_free_attrs(
>       |                         ^^^^^^^^^^^^^^ help: a function with a similar name exists: `dma_free_pages`
>       |
>      ::: /home/fujita/build/um/rust/bindings/bindings_generated.rs:44577:5
>       |
> 44577 | /     pub fn dma_free_pages(
> 44578 | |         dev: *mut device,
> 44579 | |         size: usize,
> 44580 | |         page: *mut page,
> 44581 | |         dma_handle: dma_addr_t,
> 44582 | |         dir: dma_data_direction,
> 44583 | |     );
>       | |______- similarly named function `dma_free_pages` defined here
> 
> Fixes: ad2907b4e308 ("rust: add dma coherent allocator abstraction")
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/kernel/lib.rs | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index de07aadd1ff5..9d743af69dc8 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -46,6 +46,7 @@
>  pub mod device;
>  pub mod device_id;
>  pub mod devres;
> +#[cfg(CONFIG_HAS_DMA)]
>  pub mod dma;
>  pub mod driver;
>  pub mod error;
> 
> base-commit: c59026c0570a2a97ce2e7d5ae5e9c48fc841542b
> -- 
> 2.43.0
>