Re: [PATCH drm-rust-next] rust: dma_fence: gate abstractions on CONFIG_DMA_SHARED_BUFFER

[Philipp Stanner <phasta@mailbox.org>]

On Wed, 2026-09-09 at 13:23 -0700, Edwin Peer wrote:
> The dma_fence abstractions added by commit 3282d5916019 ("rust: Add
> dma_fence abstractions") are compiled unconditionally, but the C
> `dma_fence_*` symbols they reference live in
> drivers/dma-buf/dma-fence.c, which is only built when
> CONFIG_DMA_SHARED_BUFFER=y. Kernels with CONFIG_RUST=y and
> CONFIG_RUST_KERNEL_DOCTESTS=y but CONFIG_DMA_SHARED_BUFFER=n fail
> to link:
> 
>   ld.lld: error: undefined symbol: dma_fence_release
>   >>> referenced by usercopy_64.c
>   >>>               vmlinux.o:(rust_helper_dma_fence_put)
>   ... dma_fence_signal{,_locked}, dma_fence_context_alloc,
>       dma_fence_init, dma_fence_{add,remove}_callback ...
> 
> Wrap the dma_fence.c helper include in the same
> `pub mod dma_buf;` in rust/kernel/lib.rs with a matching #[cfg].

Oh dear, yes. Sorry about that, thx for fixing!

> 
> Fixes: 3282d5916019 ("rust: Add dma_fence abstractions")
> Signed-off-by: Edwin Peer <epeer@nvidia.com>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>

Reviewed-by: Philipp Stanner <phasta@kernel.org>

> ---
>  rust/helpers/helpers.c | 2 +-
>  rust/kernel/lib.rs     | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 609689fba3dd..2c5eecc091c4 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -57,8 +57,8 @@
>  #include "cred.c"
>  #include "device.c"
>  #include "dma.c"
> -#include "dma_fence.c"
>  #ifdef CONFIG_DMA_SHARED_BUFFER
> +#include "dma_fence.c"
>  #include "dma-resv.c"
>  #endif
>  #include "drm.c"
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 72f5527ba66c..f9ef36217bb5 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -67,6 +67,7 @@
>  pub mod device_id;
>  pub mod devres;
>  pub mod dma;
> +#[cfg(CONFIG_DMA_SHARED_BUFFER)]
>  pub mod dma_buf;
>  pub mod driver;
>  #[cfg(CONFIG_DRM = "y")]