Re: [PATCH v4 3/3] rust: dma: add as_slice/write functions for CoherentAllocation

[Abdiel Janulgue]

On 02/06/2025 16:05, Alexandre Courbot wrote:
> On Mon Jun 2, 2025 at 5:53 PM JST, Abdiel Janulgue wrote:
> > Add unsafe accessors for the region for reading or writing large
> > blocks of data.
> >
> > Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> > Signed-off-by: Abdiel Janulgue <abdiel.janulgue@xxxxxxxxx>
>
> A couple remaining nits/questions below, but FWIW:
>
> Reviewed-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
> > +
> > +    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
> > +    /// number of bytes.
>
> Reading this sentence it occured to me that `offset` may be ambiguous
> here, as in my mind it rings as being in bytes unit. How about using
> `index` throughout the file?

Thanks! I don't have any strong opinion about this, I think it's enough 
that the subsequent paragraph makes it clear that the unit is in bytes 
unit? In any case, this could this be updated later after the merge?

> > +    /// ```
> > +    pub unsafe fn write(&self, src: &[T], offset: usize) -> Result {
>
> Can this function be written by leveraging `as_slice_mut` and
> `clone_from_slice`?

using `slice::clone_from_slice` would enforce the length of the coherent 
allocation to be always the same as src data. Not sure if that is what 
we want. Also, instead of just a straight memcpy, this would go through 
a 2-step layer (a call to `slice::from_raw_parts_mut` and then the 
`slice::clone_from_slice` itself)?.


Regards,
Abdiel