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

Petr Tesařík
2025-01-27T06:16:34Z

On Fri, 24 Jan 2025 08:27:36 +0100
Alice Ryhl <aliceryhl@google.com> wrote:

> On Thu, Jan 23, 2025 at 11:43 AM Abdiel Janulgue
> <abdiel.janulgue@gmail.com> wrote:
> >
> > Add a simple dma coherent allocator rust abstraction. Based on
> > Andreas Hindborg's dma abstractions from the rnvme driver, which
> > was also based on earlier work by Wedson Almeida Filho.
> >
> > Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
>
> Overall I think it looks pretty good, but a follow-up from our
> discussion the other day.
>
> > + /// Returns the device, base address, dma handle, attributes and the size of the
> > + /// allocated region.
> > + ///
> > + /// The caller takes ownership of the returned resources, i.e., will have the responsibility
> > + /// in calling `bindings::dma_free_attrs`. The allocated region is valid as long as
> > + /// the returned device exists.
> > + pub fn into_parts(
> > + self,
> > + ) -> (
> > + ARef<Device>,
> > + *mut T,
> > + bindings::dma_addr_t,
> > + crate::ffi::c_ulong,
> > + usize,
> > + ) {
> > + let size = self.count * core::mem::size_of::<T>();
> > + let ret = (
> > + // SAFETY: `&self.dev` is valid for reads.
> > + unsafe { core::ptr::read(&self.dev) },
>
> This safety comment should explain why this will not lead to
> decrementing the refcount too many times. Which is because this
> doesn't actually duplicate the ARef due to the use of mem::forget.
>
> That said, I think wrapping self in ManuallyDrop is slightly easier to
> read than using mem::forget. I refer you to this pattern from Tokio
> that solves a similar problem:
> https://github.com/tokio-rs/tokio/blob/ee19b0ed7371b069112b9c9ef9280b81f3438d26/tokio/src/sync/rwlock/owned_write_guard.rs#L38-L51
>
> > + self.cpu_addr,
> > + self.dma_handle,
> > + self.dma_attrs.as_raw(),
>
> Why not return the Attrs type instead of an integer?
>
> > + size,
> > + );
> > + core::mem::forget(self);
> > + ret
> > + }
>
> > + /// 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?
>
> 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);
>
> 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.

I don't have a strong opinion, but maybe you could elaborate a bit on
what exactly you dislike about as_slice() / as_slice_mut().

Petr T