[PATCH v2] rust: dma: allow drivers to tune max segment size

[PATCH v2] rust: dma: allow drivers to tune max segment size

[Beata Michalska]

Make dma_set_max_seg_size() available to Rust so drivers can perform
standard DMA setup steps.

Signed-off-by: Beata Michalska <beata.michalska@arm.com>
---

v2:
- Aligned safety requirements

 rust/helpers/dma.c |  5 +++++
 rust/kernel/dma.rs | 17 +++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
index 2afa32c21c94..220b9308830f 100644
--- a/rust/helpers/dma.c
+++ b/rust/helpers/dma.c
@@ -40,3 +40,8 @@ size_t rust_helper_dma_max_mapping_size(struct device *dev)
 {
 	return dma_max_mapping_size(dev);
 }
+
+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.
-- 
2.25.1

Re: [PATCH v2] rust: dma: allow drivers to tune max segment size

[Dirk Behme]

On 23.01.26 14:13, Beata Michalska wrote:
> Make dma_set_max_seg_size() available to Rust so drivers can perform
> standard DMA setup steps.
> 
> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> ---
> 
> v2:
> - Aligned safety requirements
> 
>  rust/helpers/dma.c |  5 +++++
>  rust/kernel/dma.rs | 17 +++++++++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
> index 2afa32c21c94..220b9308830f 100644
> --- a/rust/helpers/dma.c
> +++ b/rust/helpers/dma.c
> @@ -40,3 +40,8 @@ size_t rust_helper_dma_max_mapping_size(struct device *dev)
>  {
>  	return dma_max_mapping_size(dev);
>  }
> +
> +void rust_helper_dma_set_max_seg_size(struct device *dev, unsigned int size)
> +{
> +	dma_set_max_seg_size(dev, size);
> +}

I was about to ask "please add __rust_helper" [1]. But looking into
driver-core-next it looks like some of the function in dma.c have
__rust_helper (from [1]) and some don't (from [2]). Is this by
intention or was this an omission in [2]?

Best regards

Dirk

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/driver-core/driver-core.git/commit/rust/helpers/dma.c?h=driver-core-next&id=8a03afe94763108537e33b48378bbc7472b4cc9d

[2]
https://git.kernel.org/pub/scm/linux/kernel/git/driver-core/driver-core.git/commit/rust/helpers/dma.c?h=driver-core-next&id=d8932355f8c5673106eca49abd142f8fe0c1fe8b

Re: [PATCH v2] rust: dma: allow drivers to tune max segment size

[Gary Guo]

On Fri Jan 23, 2026 at 4:39 PM GMT, Dirk Behme wrote:
> On 23.01.26 14:13, Beata Michalska wrote:
>> Make dma_set_max_seg_size() available to Rust so drivers can perform
>> standard DMA setup steps.
>> 
>> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
>> ---
>> 
>> v2:
>> - Aligned safety requirements
>> 
>>  rust/helpers/dma.c |  5 +++++
>>  rust/kernel/dma.rs | 17 +++++++++++++++++
>>  2 files changed, 22 insertions(+)
>> 
>> diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
>> index 2afa32c21c94..220b9308830f 100644
>> --- a/rust/helpers/dma.c
>> +++ b/rust/helpers/dma.c
>> @@ -40,3 +40,8 @@ size_t rust_helper_dma_max_mapping_size(struct device *dev)
>>  {
>>  	return dma_max_mapping_size(dev);
>>  }
>> +
>> +void rust_helper_dma_set_max_seg_size(struct device *dev, unsigned int size)
>> +{
>> +	dma_set_max_seg_size(dev, size);
>> +}
>
> I was about to ask "please add __rust_helper" [1]. But looking into
> driver-core-next it looks like some of the function in dma.c have
> __rust_helper (from [1]) and some don't (from [2]). Is this by
> intention or was this an omission in [2]?

This is an omission. It's an also a relatively new addition which means that
it's not covered in Alice's series.

Best,
Gary