rust: firmware: add `module_firmware!` macro

Analogous to the `module!` macro `module_firmware!` adds additional
firmware path strings to the .modinfo section.

In contrast to `module!`, where path strings need to be string literals,
path strings can be composed with the `firmware::ModInfoBuilder`.

Some drivers require a lot of firmware files (such as nova-core) and
hence benefit from more flexibility composing firmware path strings.

Acked-by: Jarkko Sakkinen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Danilo Krummrich <[e-mail]>

rust/kernel/firmware.rs

···
 // be used from any thread.
 unsafe impl Sync for Firmware {}
 
+/// Create firmware .modinfo entries.
+///
+/// This macro is the counterpart of the C macro `MODULE_FIRMWARE()`, but instead of taking a
+/// simple string literals, which is already covered by the `firmware` field of
+/// [`crate::prelude::module!`], it allows the caller to pass a builder type, based on the
+/// [`ModInfoBuilder`], which can create the firmware modinfo strings in a more flexible way.
+///
+/// Drivers should extend the [`ModInfoBuilder`] with their own driver specific builder type.
+///
+/// The `builder` argument must be a type which implements the following function.
+///
+/// `const fn create(module_name: &'static CStr) -> ModInfoBuilder`
+///
+/// `create` should pass the `module_name` to the [`ModInfoBuilder`] and, with the help of
+/// it construct the corresponding firmware modinfo.
+///
+/// Typically, such contracts would be enforced by a trait, however traits do not (yet) support
+/// const functions.
+///
+/// # Example
+///
+/// ```
+/// # mod module_firmware_test {
+/// # use kernel::firmware;
+/// # use kernel::prelude::*;
+/// #
+/// # struct MyModule;
+/// #
+/// # impl kernel::Module for MyModule {
+/// #     fn init(_module: &'static ThisModule) -> Result<Self> {
+/// #         Ok(Self)
+/// #     }
+/// # }
+/// #
+/// #
+/// struct Builder<const N: usize>;
+///
+/// impl<const N: usize> Builder<N> {
+///     const DIR: &'static str = "vendor/chip/";
+///     const FILES: [&'static str; 3] = [ "foo", "bar", "baz" ];
+///
+///     const fn create(module_name: &'static kernel::str::CStr) -> firmware::ModInfoBuilder<N> {
+///         let mut builder = firmware::ModInfoBuilder::new(module_name);
+///
+///         let mut i = 0;
+///         while i < Self::FILES.len() {
+///             builder = builder.new_entry()
+///                 .push(Self::DIR)
+///                 .push(Self::FILES[i])
+///                 .push(".bin");
+///
+///                 i += 1;
+///         }
+///
+///         builder
+///      }
+/// }
+///
+/// module! {
+///    type: MyModule,
+///    name: "module_firmware_test",
+///    author: "Rust for Linux",
+///    description: "module_firmware! test module",
+///    license: "GPL",
+/// }
+///
+/// kernel::module_firmware!(Builder);
+/// # }
+/// ```
+#[macro_export]
+macro_rules! module_firmware {
+    // The argument is the builder type without the const generic, since it's deferred from within
+    // this macro. Hence, we can neither use `expr` nor `ty`.
+    ($($builder:tt)*) => {
+        const _: () = {
+            const __MODULE_FIRMWARE_PREFIX: &'static $crate::str::CStr = if cfg!(MODULE) {
+                $crate::c_str!("")
+            } else {
+                <LocalModule as $crate::ModuleMetadata>::NAME
+            };
+
+            #[link_section = ".modinfo"]
+            #[used]
+            static __MODULE_FIRMWARE: [u8; $($builder)*::create(__MODULE_FIRMWARE_PREFIX)
+                .build_length()] = $($builder)*::create(__MODULE_FIRMWARE_PREFIX).build();
+        };
+    };
+}
+
 /// Builder for firmware module info.
 ///
 /// [`ModInfoBuilder`] is a helper component to flexibly compose firmware paths strings for the
···
 /// [`ModInfoBuilder::push`], where the latter is used to push path components and the former to
 /// mark the beginning of a new path string.
 ///
-/// [`ModInfoBuilder`] is meant to be used in combination with `kernel::module_firmware!`.
+/// [`ModInfoBuilder`] is meant to be used in combination with [`kernel::module_firmware!`].
 ///
 /// The const generic `N` as well as the `module_name` parameter of [`ModInfoBuilder::new`] is an
 /// internal implementation detail and supplied through the above macro.