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

[Danilo Krummrich <dakr@kernel.org>]

On Fri, Jan 24, 2025 at 08:27:36AM +0100, Alice Ryhl wrote:
> On Thu, Jan 23, 2025 at 11:43 AM Abdiel Janulgue
> <abdiel.janulgue@gmail.com> 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 as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
> 
> You were asked to rename this function because it returns a slice, but
> I wonder if it's better to take an `&mut [T]` argument and to have
> this function copy data into that argument. That way, we could make
> the function itself safe. Perhaps the actual copy needs to be
> volatile?

Why do we consider the existing one unsafe?

Surely, it's not desirable that the contents of the buffer are modified by the
HW unexpectedly, but is this a concern in terms of Rust safety requirements?

And if so, how does this go away with the proposed approach?

> 
> Well ... I understand that we did this previously and that we want to
> avoid it because it causes too much reading if T is a struct and we
> just want to read one of its fields. How about an API like this?
> 
> dma_read!(my_alloc[7].foo)
> 
> which expands to something that reads the value of the foo field of
> the 7th element, and
> 
> dma_write!(my_alloc[7].foo = 13);

I really like how this turns out.

> 
> That expands to something that writes 13 to field foo of the 7th element.
> 
> Thoughts? I'm proposing this to avoid going in circles between the
> same solutions.
> 
> Alice