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

[Danilo Krummrich]

On Mon, Jan 27, 2025 at 11:43:39AM +0100, Alice Ryhl wrote:
> On Mon, Jan 27, 2025 at 11:37 AM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxx> 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?
> 
> In Rust, it is undefined behavior if the value behind an immutable
> reference changes (unless the type uses UnsafeCell / Opaque or
> similar). That is, any two consecutive reads of the same immutable
> reference must return the same byte value no matter what happened in
> between those reads.

Undefined as in the sense of anything is allowed to happen? I thought undefined
as in you might still see the old value on two consecutive reads.

Do you have a pointer to the corresponding docs?

> 
> If we manually perform the read as a volatile read, then it is
> arguably allowed for the value to be modified by the hardware while we
> read the value.

>From read_volatile() [1]: "In particular, a race between a read_volatile and any
write operation to the same location is undefined behavior."

Also, what if the hardware put a value that is invalid for the type?

[1] https://doc.rust-lang.org/std/ptr/fn.read_volatile.html

> 
> > > 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.
> 
> Yes, I think it would be a nice API.
> 
> Alice