Re: [RFC PATCH 10/11] rust: add basic abstractions for iomem operations

[Danilo Krummrich <dakr@redhat.com>]

On Tue, May 21, 2024 at 11:18:04AM +0200, Miguel Ojeda wrote:
> On Tue, May 21, 2024 at 9:36 AM Philipp Stanner <pstanner@redhat.com> wrote:
> >
> > Justified questions – it is public because the Drop implementation for
> > pci::Bar requires the ioptr to pass it to pci_iounmap().
> >
> > The alternative would be to give pci::Bar a copy of ioptr (it's just an
> > integer after all), but that would also not be exactly beautiful.
> 
> If by copy you mean keeping an actual copy elsewhere, then you could
> provide an access method instead.

As mentioned earlier, given the context how we use IoMem, I think IoMem should
just be a trait. And given that, maybe we'd want to name this trait differently
then, something like `trait IoOps` maybe?

pub trait IoOps {
   // INVARIANT: The implementation must ensure that the returned value is
   // either an error code or a non-null and valid address suitable for  I/O
   // operations of the given offset and length.
   fn io_addr(&self, offset: usize, len: usize) -> Result<usize>;

   fn readb(&self, offset: usize) -> Result<u8> {
      let addr = self.io_addr(offset, 1)?;

      // SAFETY: `addr` is guaranteed to be valid as by the invariant required
      // by `io_addr`.
      Ok(unsafe { bindings::readb(addr as _) })
   }

   [...]
}

We can let the resource type (e.g. `pci::Bar`) track the base address and limit
instead and just let pci::Bar implement `IoMem::io_addr`.

As for the compile time size, this would be up the the actual resource then.
`pci::Bar` can't make use of this optimization, while others might be able to.

Does that sound reasonable?

- Danilo