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

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Oct 30, 2024 at 3:06 PM Rob Herring <robh@kernel.org> wrote:
>
> 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.

Abstracting over a set of C functions that matches on the different
integer types seems like it'll be pretty common in the kernel. I think
it's perfectly fine for you to add a trait for that purpose.

> > [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.

Having bool separate seems fine to me.

Alice