[PATCH v4 3/6] rust: firmware: introduce `firmware::ModInfoBuilder`

[Danilo Krummrich <dakr@kernel.org>]

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!`,
which is introduced in a subsequent patch.

Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/firmware.rs | 98 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)

diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
index c5162fdc95ff..0f27dc212d6e 100644
--- a/rust/kernel/firmware.rs
+++ b/rust/kernel/firmware.rs
@@ -115,3 +115,101 @@ 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.
+///
+/// It is meant to be used in combination with [`kernel::module_firmware!`].
+///
+/// For more details and an example, see [`kernel::module_firmware!`].
+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.
+    ///
+    /// After a new [`ModInfoBuilder`] instance has been created, [`ModInfoBuilder::prepare`] must
+    /// be called before adding path components.
+    pub const fn push(self, bytes: &[u8]) -> Self {
+        if N != 0 && self.n == 0 {
+            crate::build_error!("Must call prepare() before push().");
+        }
+
+        self.push_internal(bytes)
+    }
+
+    const fn prepare_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.push_internal(b"firmware=")
+    }
+
+    /// Prepare for the next module info entry.
+    ///
+    /// Must be called before [`ModInfoBuilder::push`] can be called.
+    pub const fn prepare(self) -> Self {
+        self.push_internal(b"\0").prepare_module_name()
+    }
+
+    /// 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
+    }
+}
-- 
2.48.1