Re: [PATCH v3 3/3] rust: dma: add as_slice/write functions for CoherentAllocation

[Alexandre Courbot]

Hi Abdiel,

On Fri Apr 25, 2025 at 4:35 PM JST, Abdiel Janulgue wrote:
> Add unsafe accessors for the region for reading or writing large
> blocks of data.
>
> Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@xxxxxxxxx>
> ---
>  rust/kernel/dma.rs | 85 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 85 insertions(+)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index a61da5eeb017..e01bb7e7c440 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -218,6 +218,91 @@ pub fn dma_handle(&self) -> bindings::dma_addr_t {
>          self.dma_handle
>      }
>  
> +    /// Returns the data from the region starting from `offset` as a slice.
> +    /// `offset` and `count` are in units of `T`, not the number of bytes.
> +    ///
> +    /// For ringbuffer type of r/w access or use-cases where the pointer to the live data is needed,
> +    /// [`CoherentAllocation::start_ptr`] or [`CoherentAllocation::start_ptr_mut`] could be used instead.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * Callers must ensure that the device does not read/write to/from memory while the returned
> +    ///   slice is live.
> +    /// * Callers must ensure that this call does not race with a write to the same region while
> +    ///   while the returned slice is live.

"while" is repeated twice in the second bullet point.

> +    pub unsafe fn as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
> +        let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
> +        if end > self.count {
> +            return Err(EINVAL);
> +        }

Since `end` is not subsequently used, how about reworking this to avoid
declaring it as a variable that is visible for the whole method:

    offset.checked_add(count)
      .ok_or(EOVERFLOW)
      .and_then(|end| if end > self.count { Err(EINVAL) } else { Ok(()) })?;

Or maybe better, move this into a private helper method:

    fn validate_range(&self, offset: usize, count: usize) -> Result<()>

that you can call from all three methods requiring it instead of
duplicating the same code snippet (in that case, you can declare `end`
if you prefer this style as it will be useful for the entirety of the
method).

> +        // SAFETY:
> +        // - The pointer is valid due to type invariant on `CoherentAllocation`,
> +        //   we've just checked that the range and index is within bounds. The immutability of the
> +        //   of data is also guaranteed by the safety requirements of the function.

"of the of data" sounds like a typo.

> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
> +        //   that `self.count` won't overflow early in the constructor.

IIUC you rather need to state that `offset + count` (not just `offset`)
is smaller than `self.count`?

Same comments apply to `as_slice_mut` and `write`.

Cheers,
Alex.