Re: [PATCH 2/4] rust: dma: generalize `dma_{read,write}` macro

["Benno Lossin" <lossin@kernel.org>]

On Sat Feb 14, 2026 at 11:46 AM CET, Gary Guo wrote:
> On 2026-02-14 10:04, Benno Lossin wrote:
>> On Sat Feb 14, 2026 at 6:33 AM CET, Gary Guo wrote:
>>> +    (@parse [$dma:expr] [$($proj:tt)*] [.$field:tt $($rest:tt)*]) => {
>>> +        $crate::dma_write!(@parse [$dma] [$($proj)* .$field] [$($rest)*])
>>> +    };
>>> +    (@parse [$dma:expr] [$($proj:tt)*] [[$index:expr]? $($rest:tt)*]) => {
>>> +        $crate::dma_write!(@parse [$dma] [$($proj)* [$index]?] [$($rest)*])
>>> +    };
>>> +    (@parse [$dma:expr] [$($proj:tt)*] [[$index:expr] $($rest:tt)*]) => {
>>> +        $crate::dma_write!(@parse [$dma] [$($proj)* [$index]] [$($rest)*])
>>> +    };
>>> +    ($dma:expr, $($rest:tt)*) => {
>>> +        $crate::dma_write!(@parse [$dma] [] [$($rest)*])
>>>      };
>> 
>> I'm wondering if this also works:
>>     
>>     ($dma:expr, $($(.$field:ident)? $([$index:expr])?)*, $val:expr) => {{
>>         let dma = &$dma;
>>         let ptr = $crate::project_pointer!(
>>             mut $crate::dma::CoherentAllocation::as_mut_ptr(dma),
>>             $($(.$field)? $([$index])?)*,
>>         );
>>         let val = $val;
>>         // SAFETY: pointer created by projection is within DMA region.
>>         unsafe { $crate::dma::CoherentAllocation::field_write(dma, ptr, val) }
>>     }}
>
> Rust would complain that the outer repetition can match empty token tree.

Ah right.

>>> @@ -91,17 +91,17 @@ fn drop(self: Pin<&mut Self>) {
>>>          dev_info!(self.pdev, "Unload DMA test driver.\n");
>>>  
>>>          for (i, value) in TEST_VALUES.into_iter().enumerate() {
>>> -            let val0 = kernel::dma_read!(self.ca[i].h);
>>> -            let val1 = kernel::dma_read!(self.ca[i].b);
>>> -            assert!(val0.is_ok());
>>> -            assert!(val1.is_ok());
>>> +            let result = (|| -> Result<_> {
>>> +                let val0 = kernel::dma_read!(self.ca, [i]?.h);
>>> +                let val1 = kernel::dma_read!(self.ca, [i]?.b);
>>>  
>>> -            if let Ok(val0) = val0 {
>>>                  assert_eq!(val0, value.0);
>>> -            }
>>> -            if let Ok(val1) = val1 {
>>>                  assert_eq!(val1, value.1);
>>> -            }
>>> +
>>> +                Ok(())
>>> +            })();
>> 
>> I dislike that we have to reintroduce the budget-try block here. Ideally
>> we could add something like `try` at the beginning of the macro and then
>> automatically add the try block. Feel free to make that a future series.
>
> I don't think this is an issue. It's visible inside the samples because
> we are testing the values, but in practice most users would propagate the
> errors out.

Maybe we should just have a function that returns a Result in this test
that's called from drop.

> I also dislike that the budget-try block that we have inside `dma_read!`
> currently hard-codes the error type.

Yeah me too.

Cheers,
Benno