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

[Alexandre Courbot]

Hi Abdiel,

On Thu Mar 27, 2025 at 5:11 AM 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 | 87 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 87 insertions(+)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 24a6f10370c4..24025ec602ff 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -218,6 +218,93 @@ 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.
> +    ///
> +    /// Due to the safety requirements of slice, the caller should consider that the region could
> +    /// be modified by the device at anytime. For ringbuffer type of r/w access or use-cases where
> +    /// the pointer to the live data is needed, `start_ptr()` or `start_ptr_mut()` could be
> +    /// used instead.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * Callers must ensure that no hardware operations that involve the buffer are currently
> +    ///   taking place 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.
> +    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 {

IIUC this should be `if end > self.count`.

> +            return Err(EINVAL);
> +        }
> +        // 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.
> +        // - `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.
> +        Ok(unsafe { core::slice::from_raw_parts(self.cpu_addr.add(offset), count) })
> +    }
> +
> +    /// Performs the same functionality as [`CoherentAllocation::as_slice`], except that a mutable
> +    /// slice is returned.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * Callers must ensure that no hardware operations that involve the buffer are currently
> +    ///   taking place while the returned slice is live.
> +    /// * Callers must ensure that this call does not race with a read or write to the same region
> +    ///   while the returned slice is live.
> +    pub unsafe fn as_slice_mut(&self, offset: usize, count: usize) -> Result<&mut [T]> {
> +        let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
> +        if end >= self.count {

Ditto.

> +            return Err(EINVAL);
> +        }
> +        // 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.
> +        // - `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.
> +        Ok(unsafe { core::slice::from_raw_parts_mut(self.cpu_addr.add(offset), count) })
> +    }
> +
> +    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
> +    /// number of bytes.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * Callers must ensure that no hardware operations that involve the buffer overlaps with
> +    ///   this write.
> +    /// * Callers must ensure that this call does not race with a read or write to the same region
> +    ///   that overlaps with this write.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// # fn test(alloc: &mut kernel::dma::CoherentAllocation<u8>) -> Result {
> +    /// let somedata: [u8; 4] = [0xf; 4];
> +    /// let buf: &[u8] = &somedata;
> +    /// // SAFETY: No hw operation on the device and no other r/w access to the region at this point.
> +    /// unsafe { alloc.write(buf, 0)?; }
> +    /// # Ok::<(), Error>(()) }
> +    /// ```
> +    pub unsafe fn write(&self, src: &[T], offset: usize) -> Result {
> +        let end = offset.checked_add(src.len()).ok_or(EOVERFLOW)?;
> +        if end >= self.count {

Ditto.

> +            return Err(EINVAL);
> +        }
> +        // SAFETY:
> +        // - The pointer is valid due to type invariant on `CoherentAllocation`
> +        // and we've just checked that the range and index is within bounds.
> +        // - `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.
> +        unsafe {
> +            core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
> +        };

How about leveraging as_slice_mut() so this unsafe block can be removed?

Cheers,
Alex.