[PATCH v4 04/16] rust: io: perform conversions using `AsRepr`

[Gary Guo <gary@garyguo.net>]

For types that are layout-compatible with an I/O capable type, we would
want the ability to use them directly for I/O operations. E.g.

    bitfield! {
        pub struct Foo(u32) {
            ...
        }
    }

    #[repr(C)]
    struct Bar {
        foo: Foo,
    }

    let mmio: Mmio<'_, Bar> = ...;
    io_read!(mmio, .foo)

Currently this feature is available from `register!()` macro but not
otherwise available with `io_read!`, `io_write!`. Support this by
performing conversions to I/O primitives via the `AsRepr`/`AsReprMut`
trait.

This makes the `IoLoc::IoType` and `Register::Storage` redundant; thus
remove them; also convert register methods to use the `read_val` and
`write_val` instead.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/kernel/bitfield.rs    |   9 ++++
 rust/kernel/io.rs          | 128 ++++++++++++++++++++++++---------------------
 rust/kernel/io/register.rs |  15 ------
 rust/macros/io/register.rs |   2 -
 4 files changed, 78 insertions(+), 76 deletions(-)

diff --git a/rust/kernel/bitfield.rs b/rust/kernel/bitfield.rs
index a0d089423f21..15c78790e151 100644
--- a/rust/kernel/bitfield.rs
+++ b/rust/kernel/bitfield.rs
@@ -346,6 +346,15 @@ fn from(val: $storage) -> $name {
                 Self::from_raw(val)
             }
         }
+
+        // SAFETY: `$name` is transparent over `$storage` and `$storage` has no interior mutability.
+        unsafe impl $crate::mem::AsRepr for $name {
+            // Normalize `$storage` to the canonical repr type in case it is signed.
+            type Repr = <$storage as $crate::mem::AsRepr>::Repr;
+        }
+
+        // SAFETY: `$name` is transparent over `$storage`.
+        unsafe impl $crate::mem::AsReprMut for $name {}
     };
 
     // Definitions requiring knowledge of individual fields: private and public field accessors,
diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index 5ce9fd129068..18c71bd765b2 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -11,6 +11,10 @@
 
 use crate::{
     bindings,
+    mem::{
+        AsRepr,
+        AsReprMut, //
+    },
     prelude::*,
     ptr::{
         Alignment,
@@ -226,6 +230,17 @@ fn io_view<'a, IO: Io<'a>, U>(
     Ok(unsafe { IO::Backend::project_view(view, projected_ptr) })
 }
 
+/// Returns the primitive view of a I/O view.
+#[inline]
+fn io_view_as_repr<'a, IO: Io<'a, Target = T>, T: AsRepr>(
+    this: IO,
+) -> <IO::Backend as IoBackend>::View<'a, T::Repr> {
+    let view = this.as_view();
+
+    // SAFETY: `AsRepr` guarantees layout compatibility.
+    unsafe { IO::Backend::project_view(view, IO::Backend::as_ptr(view).cast::<T::Repr>()) }
+}
+
 /// I/O backends.
 ///
 /// This is an abstract representation to be implemented by arbitrary I/O
@@ -353,15 +368,12 @@ fn copy_write<T: IntoBytes>(view: Self::View<'_, T>, value: T) {
 ///
 /// - The valid `Base` to operate on. For most registers, this should be [`Region`].
 /// - The offset to access (returned by [`IoLoc::offset`]),
-/// - The width of the access (determined by [`IoLoc::IoType`]),
-/// - The type `T` in which the raw data is returned or provided.
+/// - The type `T` in which the data is returned or provided.
 ///
-/// `T` and `IoLoc::IoType` may differ: for instance, a typed register has `T` = the register type
-/// with its bitfields, and `IoType` = its backing primitive (e.g. `u32`).
+/// `T` is not necessarily the type for underlying I/O operation. Methods that take `IoLoc` have `T:
+/// AsRepr` bound and the `<T as AsRepr>::Repr` type would be used to perform I/O and converted to
+/// `T` instead.
 pub trait IoLoc<Base: ?Sized, T> {
-    /// Size ([`u8`], [`u16`], etc) of the I/O performed on the returned [`offset`](IoLoc::offset).
-    type IoType: Into<T> + From<T>;
-
     /// Consumes `self` and returns the offset of this location.
     fn offset(self) -> usize;
 }
@@ -372,8 +384,6 @@ macro_rules! impl_usize_ioloc {
     ($($ty:ty),*) => {
         $(
             impl<const SIZE: usize> IoLoc<Region<SIZE>, $ty> for usize {
-                type IoType = $ty;
-
                 #[inline(always)]
                 fn offset(self) -> usize {
                     self
@@ -498,10 +508,10 @@ fn try_cast<U>(self) -> Result<<Self::Backend as IoBackend>::View<'a, U>>
     #[inline]
     fn read_val(self) -> Self::Target
     where
-        Self::Backend: IoCapable<Self::Target>,
-        Self::Target: Sized,
+        Self::Target: AsReprMut,
+        Self::Backend: IoCapable<<Self::Target as AsRepr>::Repr>,
     {
-        Self::Backend::io_read(self.as_view())
+        Self::Target::from_repr(Self::Backend::io_read(io_view_as_repr(self)))
     }
 
     /// Write a value to I/O.
@@ -520,10 +530,10 @@ fn read_val(self) -> Self::Target
     #[inline]
     fn write_val(self, value: Self::Target)
     where
-        Self::Backend: IoCapable<Self::Target>,
-        Self::Target: Sized,
+        Self::Target: AsRepr,
+        Self::Backend: IoCapable<<Self::Target as AsRepr>::Repr>,
     {
-        Self::Backend::io_write(self.as_view(), value)
+        Self::Backend::io_write(io_view_as_repr(self), Self::Target::into_repr(value))
     }
 
     /// Copy-read from I/O memory.
@@ -645,7 +655,7 @@ fn copy_to_slice(self, data: &mut [u8])
     #[inline(always)]
     fn try_read8(self, offset: usize) -> Result<u8>
     where
-        usize: IoLoc<Self::Target, u8, IoType = u8>,
+        usize: IoLoc<Self::Target, u8>,
         Self::Backend: IoCapable<u8>,
     {
         self.try_read(offset)
@@ -655,7 +665,7 @@ fn try_read8(self, offset: usize) -> Result<u8>
     #[inline(always)]
     fn try_read16(self, offset: usize) -> Result<u16>
     where
-        usize: IoLoc<Self::Target, u16, IoType = u16>,
+        usize: IoLoc<Self::Target, u16>,
         Self::Backend: IoCapable<u16>,
     {
         self.try_read(offset)
@@ -665,7 +675,7 @@ fn try_read16(self, offset: usize) -> Result<u16>
     #[inline(always)]
     fn try_read32(self, offset: usize) -> Result<u32>
     where
-        usize: IoLoc<Self::Target, u32, IoType = u32>,
+        usize: IoLoc<Self::Target, u32>,
         Self::Backend: IoCapable<u32>,
     {
         self.try_read(offset)
@@ -675,7 +685,7 @@ fn try_read32(self, offset: usize) -> Result<u32>
     #[inline(always)]
     fn try_read64(self, offset: usize) -> Result<u64>
     where
-        usize: IoLoc<Self::Target, u64, IoType = u64>,
+        usize: IoLoc<Self::Target, u64>,
         Self::Backend: IoCapable<u64>,
     {
         self.try_read(offset)
@@ -685,7 +695,7 @@ fn try_read64(self, offset: usize) -> Result<u64>
     #[inline(always)]
     fn try_write8(self, value: u8, offset: usize) -> Result
     where
-        usize: IoLoc<Self::Target, u8, IoType = u8>,
+        usize: IoLoc<Self::Target, u8>,
         Self::Backend: IoCapable<u8>,
     {
         self.try_write(offset, value)
@@ -695,7 +705,7 @@ fn try_write8(self, value: u8, offset: usize) -> Result
     #[inline(always)]
     fn try_write16(self, value: u16, offset: usize) -> Result
     where
-        usize: IoLoc<Self::Target, u16, IoType = u16>,
+        usize: IoLoc<Self::Target, u16>,
         Self::Backend: IoCapable<u16>,
     {
         self.try_write(offset, value)
@@ -705,7 +715,7 @@ fn try_write16(self, value: u16, offset: usize) -> Result
     #[inline(always)]
     fn try_write32(self, value: u32, offset: usize) -> Result
     where
-        usize: IoLoc<Self::Target, u32, IoType = u32>,
+        usize: IoLoc<Self::Target, u32>,
         Self::Backend: IoCapable<u32>,
     {
         self.try_write(offset, value)
@@ -715,7 +725,7 @@ fn try_write32(self, value: u32, offset: usize) -> Result
     #[inline(always)]
     fn try_write64(self, value: u64, offset: usize) -> Result
     where
-        usize: IoLoc<Self::Target, u64, IoType = u64>,
+        usize: IoLoc<Self::Target, u64>,
         Self::Backend: IoCapable<u64>,
     {
         self.try_write(offset, value)
@@ -727,7 +737,7 @@ fn try_write64(self, value: u64, offset: usize) -> Result
     #[inline(always)]
     fn read8(self, offset: usize) -> u8
     where
-        usize: IoLoc<Self::Target, u8, IoType = u8>,
+        usize: IoLoc<Self::Target, u8>,
         Self::Backend: IoCapable<u8>,
     {
         self.read(offset)
@@ -739,7 +749,7 @@ fn read8(self, offset: usize) -> u8
     #[inline(always)]
     fn read16(self, offset: usize) -> u16
     where
-        usize: IoLoc<Self::Target, u16, IoType = u16>,
+        usize: IoLoc<Self::Target, u16>,
         Self::Backend: IoCapable<u16>,
     {
         self.read(offset)
@@ -751,7 +761,7 @@ fn read16(self, offset: usize) -> u16
     #[inline(always)]
     fn read32(self, offset: usize) -> u32
     where
-        usize: IoLoc<Self::Target, u32, IoType = u32>,
+        usize: IoLoc<Self::Target, u32>,
         Self::Backend: IoCapable<u32>,
     {
         self.read(offset)
@@ -763,7 +773,7 @@ fn read32(self, offset: usize) -> u32
     #[inline(always)]
     fn read64(self, offset: usize) -> u64
     where
-        usize: IoLoc<Self::Target, u64, IoType = u64>,
+        usize: IoLoc<Self::Target, u64>,
         Self::Backend: IoCapable<u64>,
     {
         self.read(offset)
@@ -775,7 +785,7 @@ fn read64(self, offset: usize) -> u64
     #[inline(always)]
     fn write8(self, value: u8, offset: usize)
     where
-        usize: IoLoc<Self::Target, u8, IoType = u8>,
+        usize: IoLoc<Self::Target, u8>,
         Self::Backend: IoCapable<u8>,
     {
         self.write(offset, value)
@@ -787,7 +797,7 @@ fn write8(self, value: u8, offset: usize)
     #[inline(always)]
     fn write16(self, value: u16, offset: usize)
     where
-        usize: IoLoc<Self::Target, u16, IoType = u16>,
+        usize: IoLoc<Self::Target, u16>,
         Self::Backend: IoCapable<u16>,
     {
         self.write(offset, value)
@@ -799,7 +809,7 @@ fn write16(self, value: u16, offset: usize)
     #[inline(always)]
     fn write32(self, value: u32, offset: usize)
     where
-        usize: IoLoc<Self::Target, u32, IoType = u32>,
+        usize: IoLoc<Self::Target, u32>,
         Self::Backend: IoCapable<u32>,
     {
         self.write(offset, value)
@@ -811,7 +821,7 @@ fn write32(self, value: u32, offset: usize)
     #[inline(always)]
     fn write64(self, value: u64, offset: usize)
     where
-        usize: IoLoc<Self::Target, u64, IoType = u64>,
+        usize: IoLoc<Self::Target, u64>,
         Self::Backend: IoCapable<u64>,
     {
         self.write(offset, value)
@@ -843,11 +853,12 @@ fn write64(self, value: u64, offset: usize)
     #[inline(always)]
     fn try_read<T, L>(self, location: L) -> Result<T>
     where
+        T: AsReprMut,
         L: IoLoc<Self::Target, T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
     {
-        let view = io_view::<Self, L::IoType>(self, location.offset())?;
-        Ok(Self::Backend::io_read(view).into())
+        let view = io_view::<Self, T>(self, location.offset())?;
+        Ok(view.read_val())
     }
 
     /// Generic fallible write with runtime bounds check.
@@ -876,12 +887,12 @@ fn try_read<T, L>(self, location: L) -> Result<T>
     #[inline(always)]
     fn try_write<T, L>(self, location: L, value: T) -> Result
     where
+        T: AsRepr,
         L: IoLoc<Self::Target, T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
     {
-        let view = io_view::<Self, L::IoType>(self, location.offset())?;
-        let io_value = value.into();
-        Self::Backend::io_write(view, io_value);
+        let view = io_view::<Self, T>(self, location.offset())?;
+        view.write_val(value);
         Ok(())
     }
 
@@ -920,9 +931,10 @@ fn try_write<T, L>(self, location: L, value: T) -> Result
     #[inline(always)]
     fn try_write_reg<T, L, V>(self, value: V) -> Result
     where
+        T: AsRepr,
         L: IoLoc<Self::Target, T>,
         V: LocatedRegister<Self::Target, Location = L, Value = T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
     {
         let (location, value) = value.into_io_op();
 
@@ -954,16 +966,13 @@ fn try_write_reg<T, L, V>(self, value: V) -> Result
     #[inline(always)]
     fn try_update<T, L, F>(self, location: L, f: F) -> Result
     where
+        T: AsReprMut,
         L: IoLoc<Self::Target, T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
         F: FnOnce(T) -> T,
     {
-        let view = io_view::<Self, L::IoType>(self, location.offset())?;
-
-        let value: T = Self::Backend::io_read(view).into();
-        let io_value = f(value).into();
-        Self::Backend::io_write(view, io_value);
-
+        let view = io_view::<Self, T>(self, location.offset())?;
+        view.write_val(f(view.read_val()));
         Ok(())
     }
 
@@ -991,11 +1000,12 @@ fn try_update<T, L, F>(self, location: L, f: F) -> Result
     #[inline(always)]
     fn read<T, L>(self, location: L) -> T
     where
+        T: AsReprMut,
         L: IoLoc<Self::Target, T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
     {
-        let view = io_view_assert::<Self, L::IoType>(self, location.offset());
-        Self::Backend::io_read(view).into()
+        let view = io_view_assert::<Self, T>(self, location.offset());
+        view.read_val()
     }
 
     /// Generic infallible write with compile-time bounds check.
@@ -1022,12 +1032,12 @@ fn read<T, L>(self, location: L) -> T
     #[inline(always)]
     fn write<T, L>(self, location: L, value: T)
     where
+        T: AsRepr,
         L: IoLoc<Self::Target, T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
     {
-        let view = io_view_assert::<Self, L::IoType>(self, location.offset());
-        let io_value = value.into();
-        Self::Backend::io_write(view, io_value);
+        let view = io_view_assert::<Self, T>(self, location.offset());
+        view.write_val(value)
     }
 
     /// Generic infallible write of a fully-located register value.
@@ -1064,9 +1074,10 @@ fn write<T, L>(self, location: L, value: T)
     #[inline(always)]
     fn write_reg<T, L, V>(self, value: V)
     where
+        T: AsRepr,
         L: IoLoc<Self::Target, T>,
         V: LocatedRegister<Self::Target, Location = L, Value = T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
     {
         let (location, value) = value.into_io_op();
 
@@ -1098,14 +1109,13 @@ fn write_reg<T, L, V>(self, value: V)
     #[inline(always)]
     fn update<T, L, F>(self, location: L, f: F)
     where
+        T: AsReprMut,
         L: IoLoc<Self::Target, T>,
-        Self::Backend: IoCapable<L::IoType>,
+        Self::Backend: IoCapable<<T as AsRepr>::Repr>,
         F: FnOnce(T) -> T,
     {
-        let view = io_view_assert::<Self, L::IoType>(self, location.offset());
-        let value: T = Self::Backend::io_read(view).into();
-        let io_value = f(value).into();
-        Self::Backend::io_write(view, io_value);
+        let view = io_view_assert::<Self, T>(self, location.offset());
+        view.write_val(f(view.read_val()));
     }
 }
 
diff --git a/rust/kernel/io/register.rs b/rust/kernel/io/register.rs
index 6a19552ffb95..45d2e73a92e7 100644
--- a/rust/kernel/io/register.rs
+++ b/rust/kernel/io/register.rs
@@ -117,9 +117,6 @@
 
 /// Trait implemented by all registers.
 pub trait Register: Sized {
-    /// Backing primitive type of the register.
-    type Storage: Into<Self> + From<Self>;
-
     /// Start offset of the register.
     ///
     /// The interpretation of this offset depends on the type of the register.
@@ -135,8 +132,6 @@ impl<const SIZE: usize, T> IoLoc<Region<SIZE>, T> for ()
 where
     T: FixedRegister,
 {
-    type IoType = T::Storage;
-
     #[inline(always)]
     fn offset(self) -> usize {
         T::OFFSET
@@ -149,8 +144,6 @@ impl<const SIZE: usize, T> IoLoc<Region<SIZE>, T> for T
 where
     T: FixedRegister,
 {
-    type IoType = T::Storage;
-
     #[inline(always)]
     fn offset(self) -> usize {
         T::OFFSET
@@ -174,8 +167,6 @@ impl<const SIZE: usize, T> IoLoc<Region<SIZE>, T> for FixedRegisterLoc<T>
 where
     T: FixedRegister,
 {
-    type IoType = T::Storage;
-
     #[inline(always)]
     fn offset(self) -> usize {
         T::OFFSET
@@ -246,8 +237,6 @@ impl<const SIZE: usize, T, B> IoLoc<Region<SIZE>, T> for RelativeRegisterLoc<T,
     T: RelativeRegister,
     B: RegisterBase<T::BaseFamily> + ?Sized,
 {
-    type IoType = T::Storage;
-
     #[inline(always)]
     fn offset(self) -> usize {
         RelativeRegisterLoc::offset(self)
@@ -289,8 +278,6 @@ impl<const SIZE: usize, T> IoLoc<Region<SIZE>, T> for RegisterArrayLoc<T>
 where
     T: RegisterArray,
 {
-    type IoType = T::Storage;
-
     #[inline(always)]
     fn offset(self) -> usize {
         T::OFFSET + self.0 * T::STRIDE
@@ -377,8 +364,6 @@ impl<const SIZE: usize, T, B> IoLoc<Region<SIZE>, T> for RelativeRegisterArrayLo
     T: RelativeRegisterArray,
     B: RegisterBase<T::BaseFamily> + ?Sized,
 {
-    type IoType = T::Storage;
-
     #[inline(always)]
     fn offset(self) -> usize {
         self.0.offset() + self.1 * T::STRIDE
diff --git a/rust/macros/io/register.rs b/rust/macros/io/register.rs
index 83ba4dd75ab3..9b92c8099edd 100644
--- a/rust/macros/io/register.rs
+++ b/rust/macros/io/register.rs
@@ -213,8 +213,6 @@ pub(crate) fn register(def: RegDef) -> Result<TokenStream> {
             );
 
             impl ::kernel::io::register::Register for #name {
-                type Storage = #storage;
-
                 const OFFSET: usize = #offset;
             }
         ));

-- 
2.54.0