rust: firmware: introduce `firmware::ModInfoBuilder`

The `firmware` field of the `module!` only accepts literal strings,
which is due to the fact that it is implemented as a proc macro.

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

The `firmware::ModInfoBuilder` is a helper component to flexibly compose
firmware path strings for the .modinfo section in const context.

It is meant to be used in combination with `kernel::module_firmware!`.

Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250306222336.23482-3-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/firmware.rs

@@ -115,3 +115,130 @@ unsafe impl Send for Firmware {}
 // SAFETY: `Firmware` only holds a pointer to a C `struct firmware`, references to which are safe to
 // be used from any thread.
 unsafe impl Sync for Firmware {}
+
+/// Builder for firmware module info.
+///
+/// [`ModInfoBuilder`] is a helper component to flexibly compose firmware paths strings for the
+/// .modinfo section in const context.
+///
+/// Therefore the [`ModInfoBuilder`] provides the methods [`ModInfoBuilder::new_entry`] and
+/// [`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!`.
+///
+/// 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.
+pub struct ModInfoBuilder<const N: usize> {
+    buf: [u8; N],
+    n: usize,
+    module_name: &'static CStr,
+}
+
+impl<const N: usize> ModInfoBuilder<N> {
+    /// Create an empty builder instance.
+    pub const fn new(module_name: &'static CStr) -> Self {
+        Self {
+            buf: [0; N],
+            n: 0,
+            module_name,
+        }
+    }
+
+    const fn push_internal(mut self, bytes: &[u8]) -> Self {
+        let mut j = 0;
+
+        if N == 0 {
+            self.n += bytes.len();
+            return self;
+        }
+
+        while j < bytes.len() {
+            if self.n < N {
+                self.buf[self.n] = bytes[j];
+            }
+            self.n += 1;
+            j += 1;
+        }
+        self
+    }
+
+    /// Push an additional path component.
+    ///
+    /// Append path components to the [`ModInfoBuilder`] instance. Paths need to be separated
+    /// with [`ModInfoBuilder::new_entry`].
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use kernel::firmware::ModInfoBuilder;
+    ///
+    /// # const DIR: &str = "vendor/chip/";
+    /// # const fn no_run<const N: usize>(builder: ModInfoBuilder<N>) {
+    /// let builder = builder.new_entry()
+    ///     .push(DIR)
+    ///     .push("foo.bin")
+    ///     .new_entry()
+    ///     .push(DIR)
+    ///     .push("bar.bin");
+    /// # }
+    /// ```
+    pub const fn push(self, s: &str) -> Self {
+        // Check whether there has been an initial call to `next_entry()`.
+        if N != 0 && self.n == 0 {
+            crate::build_error!("Must call next_entry() before push().");
+        }
+
+        self.push_internal(s.as_bytes())
+    }
+
+    const fn push_module_name(self) -> Self {
+        let mut this = self;
+        let module_name = this.module_name;
+
+        if !this.module_name.is_empty() {
+            this = this.push_internal(module_name.as_bytes_with_nul());
+
+            if N != 0 {
+                // Re-use the space taken by the NULL terminator and swap it with the '.' separator.
+                this.buf[this.n - 1] = b'.';
+            }
+        }
+
+        this
+    }
+
+    /// Prepare the [`ModInfoBuilder`] for the next entry.
+    ///
+    /// This method acts as a separator between module firmware path entries.
+    ///
+    /// Must be called before constructing a new entry with subsequent calls to
+    /// [`ModInfoBuilder::push`].
+    ///
+    /// See [`ModInfoBuilder::push`] for an example.
+    pub const fn new_entry(self) -> Self {
+        self.push_internal(b"\0")
+            .push_module_name()
+            .push_internal(b"firmware=")
+    }
+
+    /// Build the byte array.
+    pub const fn build(self) -> [u8; N] {
+        // Add the final NULL terminator.
+        let this = self.push_internal(b"\0");
+
+        if this.n == N {
+            this.buf
+        } else {
+            crate::build_error!("Length mismatch.");
+        }
+    }
+}
+
+impl ModInfoBuilder<0> {
+    /// Return the length of the byte array to build.
+    pub const fn build_length(self) -> usize {
+        // Compensate for the NULL terminator added by `build`.
+        self.n + 1
+    }
+}