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

Alice Ryhl
2025-01-27T09:45:20Z

On Mon, Jan 27, 2025 at 7:16 AM Petr Tesařík <petr@tesarici.cz> wrote:
>
> 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().

The as_slice() method is unsafe as currently defined. The purpose of
this proposal is so we can have a method that is not unsafe.

Alice