Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

Alice Ryhl
2025-01-28T11:29:05Z

On Tue, Jan 28, 2025 at 11:37 AM Daniel Almeida
<daniel.almeida@collabora.com> wrote:
>
> >
> > I mean, I imagine that you could make the syntax
> > `dma_read!(my_alloc[7])` read the entire struct. I'm not sure which
> > safe methods you are referring to, because right now there is only the
> > unsafe as_slice().
> >
> > Alice
>
> I mean this:
>
> + /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
> + /// number of bytes.
> + pub fn write(&self, src: &[T], offset: usize) -> Result {
> + if offset + src.len() >= self.count {
> + 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())
> + };
> + Ok(())
> + }
> +}
>
> …and the similar read() method that was apparently removed, i.e.:
>
> + /// 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]> {
> + 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) })
> + }
>
> I don’t think there’s anything wrong with these, so maybe your solution can come in as a convenience
> when one wants to save on the amount of copying? IMHO the two methods above co-exist with what
> you propose.
>
> The unsafe `as_slice()` and `as_slice_mut()` should also stay, in my opinion. They can be used, for example,
> in codec drivers.

Agreed, there is nothing wrong with the safe write that was here
previously, or the current unsafe functions.

Alice