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

[Benno Lossin <benno.lossin@proton.me>]

On Wed Mar 26, 2025 at 9:11 PM CET, Abdiel Janulgue wrote:
> +    /// 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 {
> +            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) })

I vaguely recall that there was some discussion on why this is OK (ie
the value behind the reference being modified by the device), but I
haven't followed it. Can you add the reasoning for why that is fine to
some comment here?

I also am not really fond of the phrase "hardware operations that
involve the buffer":
* what do you mean with "buffer"? `self`?
* what are "hardware operations"? (I no nothing about hardware, so that
  might be a knowledge gap on my part)
* what does "involve" mean?

---
Cheers,
Benno

> +    }