[PATCH 6/7] rust: io: register: add `#[ro]` and `#[wo]` attributes

[Daniel Almeida <daniel.almeida@collabora.com>]

`register!` gives every register a location with the `ReadWrite` access
mode, so any register can be read, written and updated, including the
ones the hardware defines as read-only or write-only.

Add the `#[ro]` and `#[wo]` attributes, which make the macro use the
`ReadOnly` or `WriteOnly` mode instead: in the register's `OffsetLoc`
for fixed registers, and as `RegisterArray::Access` for arrays. Reading
a write-only register, or writing or updating a read-only one, then does
not compile. An alias does not inherit the mode of the register it
aliases. Add an example to the `register!` documentation.

The tyr driver uses the attributes in a following patch.

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
---
 rust/kernel/io/register.rs | 53 ++++++++++++++++++++++++++++++++++++++++++++++
 rust/macros/io/register.rs | 35 ++++++++++++++++++++++++------
 2 files changed, 82 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/io/register.rs b/rust/kernel/io/register.rs
index 9218dc66017b..5c0188947879 100644
--- a/rust/kernel/io/register.rs
+++ b/rust/kernel/io/register.rs
@@ -147,6 +147,13 @@ impl<Base: ?Sized, T> OffsetLoc<Base, T> {
     pub const fn new(offset: usize) -> Self {
         Self(offset, PhantomData)
     }
+}
+
+impl<Base: ?Sized, T, A> OffsetLoc<Base, T, A> {
+    #[inline]
+    pub const fn with_access(offset: usize) -> Self {
+        Self(offset, PhantomData)
+    }
 
     #[inline]
     pub const fn const_offset(self) -> usize {
@@ -596,6 +603,52 @@ pub const fn element_alias_offset<Base: ?Sized, Alias: RegisterArray<Base = Base
 /// io_project!(io, build: CPU0).write_reg(CPU_CTL::zeroed());
 /// # }
 /// ```
+///
+/// ## Access modes
+///
+/// Registers are read-write unless declared with the `#[ro]` (read-only) or `#[wo]` (write-only)
+/// attribute. Reading a write-only register, or writing or updating a read-only one, does not
+/// compile:
+///
+/// ```no_run
+/// use kernel::io::{
+///     register,
+///     Io,
+///     Region,
+/// };
+/// # use kernel::io::Mmio;
+///
+/// register! {
+///     base: Region<0x1000>;
+///
+///     #[ro]
+///     pub R1(u32) @ 0x0 {
+///         31:0 value;
+///     }
+///
+///     #[wo]
+///     pub R2(u32) @ 0x4 {
+///         31:0 value;
+///     }
+///
+///     #[ro]
+///     pub R3: u8 @ 0x8;
+/// }
+///
+/// # fn test(io: Mmio<'_, Region<0x1000>>) {
+/// let r1 = io.read(R1);
+/// let r3 = io.read(R3);
+/// io.write_reg(R2::zeroed());
+///
+/// // Neither of these compiles.
+/// // io.write_reg(r1);
+/// // io.read(R2);
+/// # }
+/// ```
+///
+/// The mode applies to accesses through the register's location only. An alias is read-write
+/// unless declared `#[ro]` or `#[wo]` itself, and the register's offset can still be accessed
+/// directly through the I/O region.
 #[macro_export]
 macro_rules! register {
     ($($tt:tt)*) => {
diff --git a/rust/macros/io/register.rs b/rust/macros/io/register.rs
index 2d92c7f41447..171a4c1f96b4 100644
--- a/rust/macros/io/register.rs
+++ b/rust/macros/io/register.rs
@@ -63,6 +63,7 @@ struct Reg {
     attrs: Vec<Attribute>,
     vis: Visibility,
     name: Ident,
+    access: Type,
     unique: bool,
     ty: Type,
     array: Option<RegArrayDef>,
@@ -72,7 +73,26 @@ struct Reg {
 
 impl Parse for Reg {
     fn parse(input: syn::parse::ParseStream<'_>) -> Result<Self> {
-        let attrs = input.call(Attribute::parse_outer)?;
+        let mut attrs = input.call(Attribute::parse_outer)?;
+        let modes = attrs
+            .extract_if(.., |attr| {
+                attr.path().is_ident("ro") || attr.path().is_ident("wo")
+            })
+            .collect::<Vec<_>>();
+        if let Some(attr) = modes.get(1) {
+            Err(Error::new_spanned(
+                attr,
+                "only one of `#[ro]` and `#[wo]` can be specified",
+            ))?
+        }
+        let access = match modes.first() {
+            None => parse_quote!(::kernel::io::access::ReadWrite),
+            Some(attr) if attr.path().is_ident("ro") => {
+                parse_quote!(::kernel::io::access::ReadOnly)
+            }
+            Some(_) => parse_quote!(::kernel::io::access::WriteOnly),
+        };
+
         let vis = input.parse()?;
         let name = input.parse()?;
 
@@ -161,6 +181,7 @@ fn parse(input: syn::parse::ParseStream<'_>) -> Result<Self> {
             attrs,
             vis,
             name,
+            access,
             unique,
             ty,
             array,
@@ -205,6 +226,7 @@ pub(crate) fn register(def: RegDef) -> Result<TokenStream> {
             attrs,
             vis,
             name,
+            access,
             unique,
             ty,
             array,
@@ -220,7 +242,7 @@ pub(crate) fn register(def: RegDef) -> Result<TokenStream> {
             RegOffset::Fixed { offset } => quote!(#offset),
             RegOffset::Alias { alias } => {
                 quote_spanned!(alias.span().resolved_at(span) =>
-                    ::kernel::io::register::OffsetLoc::<#base, _>::const_offset(#alias)
+                    ::kernel::io::register::OffsetLoc::<#base, _, _>::const_offset(#alias)
                 )
             }
             RegOffset::ElementAlias { alias, idx } => {
@@ -246,15 +268,16 @@ pub(crate) fn register(def: RegDef) -> Result<TokenStream> {
                 if unique {
                     outputs.extend(quote!(
                         impl ::kernel::io::register::FixedIoLoc<#base> for #ty {
-                            type Location = ::kernel::io::register::OffsetLoc<#base, #ty>;
+                            type Location = ::kernel::io::register::OffsetLoc<#base, #ty, #access>;
                             const LOCATION: Self::Location = #name;
                         }
                     ))
                 }
 
                 outputs.extend(quote_spanned!(span =>
-                    #(#attrs)* #vis const #name: ::kernel::io::register::OffsetLoc<#base, #ty> =
-                        ::kernel::io::register::OffsetLoc::new(#offset);
+                    #(#attrs)* #vis const #name:
+                        ::kernel::io::register::OffsetLoc<#base, #ty, #access> =
+                        ::kernel::io::register::OffsetLoc::with_access(#offset);
                 ));
             }
 
@@ -283,7 +306,7 @@ impl ::kernel::io::register::Array for #name {}
 
                     impl ::kernel::io::register::RegisterArray for #name {
                         type Base = #base;
-                        type Access = ::kernel::io::access::ReadWrite;
+                        type Access = #access;
                         const OFFSET: usize = #offset;
                         const SIZE: usize = #size;
                         const STRIDE: usize = #stride;

-- 
2.56.0