Re: [PATCH RFC 2/3] rust: Add bindings for device properties

[Rob Herring <robh@kernel.org>]

On Wed, Oct 30, 2024 at 3:15 AM Alice Ryhl <aliceryhl@google.com> wrote:
>
> On Tue, Oct 29, 2024 at 8:35 PM Rob Herring <robh@kernel.org> wrote:
> >
> > On Tue, Oct 29, 2024 at 1:57 PM Miguel Ojeda
> > <miguel.ojeda.sandonis@gmail.com> wrote:
> > >
> > > On Tue, Oct 29, 2024 at 7:48 PM Alice Ryhl <aliceryhl@google.com> wrote:
> > > >
> > > > One option is to define a trait for integers:
> >
> > Yeah, but that doesn't feel like something I should do here. I imagine
> > other things might need the same thing. Perhaps the bindings for
> > readb/readw/readl for example. And essentially the crate:num already
> > has the trait I need. Shouldn't the kernel mirror that? I recall
> > seeing some topic of including crates in the kernel?
>
> You can design the trait to look similar to traits in external crates.
> We did that for FromBytes/AsBytes.
>
> I assume you're referring to the PrimInt trait [1]? That trait doesn't
> really let you get rid of the catch-all case, and it's not even
> unreachable due to the u128 type.

It was num::Integer which seems to be similar.

>
> [1]: https://docs.rs/num-traits/0.2.19/num_traits/int/trait.PrimInt.html
>
> > > +1, one more thing to consider is whether it makes sense to define a
> > > DT-only trait that holds all the types that can be a device property
> > > (like `bool` too, not just the `Integer`s).
> > >
> > > Then we can avoid e.g. `property_read_bool` and simply do it in `property_read`.
> >
> > Is there no way to say must have traitA or traitB?
>
> No. What should it do if you pass it something that implements both traits?
>
> If you want a single function name, you'll need one trait.

I'm not sure I want that actually.

DT boolean is a bit special. A property not present is false.
Everything else is true. For example, 'prop = <0>' or 'prop =
"string"' are both true. I'm moving things in the kernel to be
stricter so that those cases are errors. I recently introduced
(of|device)_property_present() for that reason. There's no type
information stored in DT.  At the DT level, it's all just byte arrays.
However, we now have all the type information for properties within
the schema. So eventually, I want to use that to warn on accessing
properties with the wrong type.

For example, I think I don't want this to work:

if dev.property_read(c_str!("test,i16-array"))? {
    // do something
}

But instead have:

if dev.property_present(c_str!("test,i16-array")) {
    // do something
}

To actually warn on property_read_bool, I'm going to have to rework
the underlying C implementation to separate device_property_present
and device_property_read_bool implementations.

Rob