rust: macros: improve `#[vtable]` documentation
Traits marked with `#[vtable]` need to provide default implementations
for optional functions. The C side represents these with `NULL` in the
vtable, so the default functions are never actually called. We do not
want to replicate the default behavior from C in Rust, because that is
not maintainable. Therefore we should use `build_error` in those default
implementations. The error message for that is provided at
`kernel::error::VTABLE_DEFAULT_ERROR`.
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Finn Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/20231026201855.1497680-1-benno.lossin@proton.me
[ Wrapped paragraph to 80 as requested and capitalized sentence. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 032b645..376280b 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -335,3 +335,7 @@ pub(crate) fn from_result<T, F>(f: F) -> T
Err(e) => T::from(e.to_errno() as i16),
}
}
+
+/// Error message for calling a default function of a [`#[vtable]`](macros::vtable) trait.
+pub const VTABLE_DEFAULT_ERROR: &str =
+ "This function must not be called, see the #[vtable] documentation.";
diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index 125f5ea..d18a3e7 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -87,27 +87,49 @@ pub fn module(ts: TokenStream) -> TokenStream {
/// implementation could just return `Error::EINVAL`); Linux typically use C
/// `NULL` pointers to represent these functions.
///
-/// This attribute is intended to close the gap. Traits can be declared and
-/// implemented with the `#[vtable]` attribute, and a `HAS_*` associated constant
-/// will be generated for each method in the trait, indicating if the implementor
-/// has overridden a method.
+/// This attribute closes that gap. A trait can be annotated with the
+/// `#[vtable]` attribute. Implementers of the trait will then also have to
+/// annotate the trait with `#[vtable]`. This attribute generates a `HAS_*`
+/// associated constant bool for each method in the trait that is set to true if
+/// the implementer has overridden the associated method.
///
-/// This attribute is not needed if all methods are required.
+/// For a trait method to be optional, it must have a default implementation.
+/// This is also the case for traits annotated with `#[vtable]`, but in this
+/// case the default implementation will never be executed. The reason for this
+/// is that the functions will be called through function pointers installed in
+/// C side vtables. When an optional method is not implemented on a `#[vtable]`
+/// trait, a NULL entry is installed in the vtable. Thus the default
+/// implementation is never called. Since these traits are not designed to be
+/// used on the Rust side, it should not be possible to call the default
+/// implementation. This is done to ensure that we call the vtable methods
+/// through the C vtable, and not through the Rust vtable. Therefore, the
+/// default implementation should call `kernel::build_error`, which prevents
+/// calls to this function at compile time:
+///
+/// ```compile_fail
+/// # use kernel::error::VTABLE_DEFAULT_ERROR;
+/// kernel::build_error(VTABLE_DEFAULT_ERROR)
+/// ```
+///
+/// Note that you might need to import [`kernel::error::VTABLE_DEFAULT_ERROR`].
+///
+/// This macro should not be used when all functions are required.
///
/// # Examples
///
/// ```ignore
+/// use kernel::error::VTABLE_DEFAULT_ERROR;
/// use kernel::prelude::*;
///
/// // Declares a `#[vtable]` trait
/// #[vtable]
/// pub trait Operations: Send + Sync + Sized {
/// fn foo(&self) -> Result<()> {
-/// Err(EINVAL)
+/// kernel::build_error(VTABLE_DEFAULT_ERROR)
/// }
///
/// fn bar(&self) -> Result<()> {
-/// Err(EINVAL)
+/// kernel::build_error(VTABLE_DEFAULT_ERROR)
/// }
/// }
///
@@ -125,6 +147,8 @@ pub fn module(ts: TokenStream) -> TokenStream {
/// assert_eq!(<Foo as Operations>::HAS_FOO, true);
/// assert_eq!(<Foo as Operations>::HAS_BAR, false);
/// ```
+///
+/// [`kernel::error::VTABLE_DEFAULT_ERROR`]: ../kernel/error/constant.VTABLE_DEFAULT_ERROR.html
#[proc_macro_attribute]
pub fn vtable(attr: TokenStream, ts: TokenStream) -> TokenStream {
vtable::vtable(attr, ts)
