Re: [PATCH v3] rust: dma: allow drivers to tune max segment size

[Robin Murphy <robin.murphy@arm.com>]

On 2026-01-28 1:53 pm, Beata Michalska wrote:
> Make dma_set_max_seg_size() available to Rust so drivers can perform
> standard DMA setup steps.

Yup, API-wise this goes hand-in-hand with dma_set_mask calls, so keeping 
all the requirements in sync is the right thing to do. FWIW,

Acked-by: Robin Murphy <robvin.murphy@arm.com>

> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> ---
> Apologies for delay.
> 
> v3:
> - Added __rust_helper
> 
> v2:
> - Aligned safety requirements
> 
>   rust/helpers/dma.c |  6 ++++++
>   rust/kernel/dma.rs | 17 +++++++++++++++++
>   2 files changed, 23 insertions(+)
> 
> diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
> index e7defeecda71..20232ac64850 100644
> --- a/rust/helpers/dma.c
> +++ b/rust/helpers/dma.c
> @@ -43,3 +43,9 @@ size_t rust_helper_dma_max_mapping_size(struct device *dev)
>   {
>   	return dma_max_mapping_size(dev);
>   }
> +
> +__rust_helper void rust_helper_dma_set_max_seg_size(struct device *dev,
> +						    unsigned int size)
> +{
> +	dma_set_max_seg_size(dev, size);
> +}
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index acc65b1e0f24..909d56fd5118 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -85,6 +85,23 @@ unsafe fn dma_set_mask_and_coherent(&self, mask: DmaMask) -> Result {
>               bindings::dma_set_mask_and_coherent(self.as_ref().as_raw(), mask.value())
>           })
>       }
> +
> +    /// Set the maximum size of a single DMA segment the device may request.
> +    ///
> +    /// This method is usually called once from `probe()` as soon as the device capabilities are
> +    /// known.
> +    ///
> +    /// # Safety
> +    ///
> +    /// This method must not be called concurrently with any DMA allocation or mapping primitives,
> +    /// such as [`CoherentAllocation::alloc_attrs`].
> +    unsafe fn dma_set_max_seg_size(&self, size: u32) {
> +        // SAFETY:
> +        // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
> +        // - The safety requirement of this function guarantees that there are no concurrent calls
> +        //   to DMA allocation and mapping primitives using this parameter.
> +        unsafe { bindings::dma_set_max_seg_size(self.as_ref().as_raw(), size) }
> +    }
>   }
>   
>   /// A DMA mask that holds a bitmask with the lowest `n` bits set.