Re: [PATCH v9 2/3] rust: add dma coherent allocator abstraction.

Abdiel Janulgue
2025-01-23T13:37:55Z

On 21/01/2025 21:56, Boqun Feng wrote:
> On Tue, Jan 21, 2025 at 10:47:46AM +0200, Abdiel Janulgue wrote:
> [...]
>> +
>> + /// Reads 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 data returned should be regarded by the
>> + /// caller as a snapshot of the region when this function is called, as 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.
>> + pub unsafe fn read(&self, offset: usize, count: usize) -> Result<&[T]> {
>
> I don't think `read()` is a proper name here since this function only
> provides a slice for the caller to read. How about `as_slice()`? Or you
> can change the function signature to:
>
> read(&self, offset, usize, count: usize, dst: &mut [T]) -> Result
>

Thanks for the feedback! I've now changed this in v10 onwards.

/Abdiel

> Regards,
> Boqun
>
>> + if offset + count >= 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.
>> + Ok(unsafe { core::slice::from_raw_parts(self.cpu_addr.wrapping_add(offset), count) })
>> + }
>> +
> …