Re: [PATCH v12 2/3] rust: io: mem: add a generic iomem abstraction

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Jul 10, 2025 at 10:58:13AM -0300, Daniel Almeida wrote:
> Hi Alice,
> 
> >> +impl<const SIZE: usize> IoMem<SIZE> {
> >> +    fn ioremap(resource: &Resource) -> Result<Self> {
> >> +        let size = resource.size();
> >> +        if size == 0 {
> >> +            return Err(EINVAL);
> >> +        }
> >> +
> >> +        let res_start = resource.start();
> >> +
> >> +        let addr = if resource
> >> +            .flags()
> >> +            .contains(io::resource::flags::IORESOURCE_MEM_NONPOSTED)
> >> +        {
> >> +            // SAFETY:
> >> +            // - `res_start` and `size` are read from a presumably valid `struct resource`.
> >> +            // - `size` is known not to be zero at this point.
> >> +            unsafe { bindings::ioremap_np(res_start, size as usize) }
> > 
> > Here you cast from ResourceSize to usize. Are you sure that is correct?
> > I thought those types could be different.
> 
> This seems to what C is doing as well, i.e.:
> 
> static void __iomem *__devm_ioremap(struct device *dev, resource_size_t offset,
> 				    resource_size_t size, <---------
> 				    enum devm_ioremap_type type)
> {
> 
>         […]
> 
> 	case DEVM_IOREMAP_NP:
> 		addr = ioremap_np(offset, size);
> 		break;
> 	}
> 
> 
> Where:
> 
> `static inline void __iomem *ioremap_np(phys_addr_t offset, size_t size)`
> 
> IOW: this stems from the mix and match of types used the C API itself.
> 
> What do you suggest here? Maybe a try_into() then?

What a mess. It looks like there aren't any 32-bit architectures that
define ioremap_np. This means that sometimes this cast will be lossy,
but in those cases the function body just returns NULL and doesn't read
the size.

I would probably cast to an underscore instead of explicitly mentioning
the target type and make a comment about it.

Alice