Merge tag 'rust-io-7.4-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/driver-core/driver-core into drm-rust-next

rust: io: support register projections and remove relative registers

Currently registers work for all untyped I/O regions, which is not ideal.
It allows registers defined for device A to work for another device B and
there is no safeguarding at all.

Change this by requiring a base type for registers. `register!` can still
define registers on untyped `Region`s, although users would need to do so
explicitly and supply a concrete type.

This change makes it possible to use projection for relative registers;
relative registers can be implemented by defining new types for the I/O
subregions and just define registers for these subregion types like normal
registers.

This actually results in more ergnomic code for users of relative registers
(currently only nova-core), because non-array registers can be written to
with

    #[...]
    struct Subregion(...);

    register! {
        base: MyBase;
        SUBREGION: Subregion @ ...;
    }

    register! {
        base: Subregion;
        REG(u32) @ .. { .. }
    }

    let subregion = io_project!(bar, build: SUBREGION);
    subregion.read(REG)
    subregion.write_reg(reg)

instead of

    struct SubregionType;
    struct Subregion;
    impl RegisterBase<SubregionType> for Subregion {
        const BASE: usize = ...;
    }

    register! {
        REG(u32) @ Subregion + .. { .. }
    }

    bar.read(REG::of::<Subregion>())
    bar.write(WithBase::of::<Subregion>(), reg)

This also allows a lot more code sharing between I/O projection and
`register!` macro.

The expressiveness power of Rust declarative macros is becoming limiting,
so it is converted to a proc macro before new features being added to it.

This is a stable tag for other trees to merge.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>

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