Re: [PATCH v5 2/5] rust: firmware: introduce `firmware::ModInfoBuilder`

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Mar 04, 2025 at 06:34:49PM +0100, Danilo Krummrich wrote:
> 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.

Ditto.

> 
> 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..6e6972d94597 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, s: &str) -> Self {
> +        if N != 0 && self.n == 0 {
> +            crate::build_error!("Must call prepare() before push().");
> +        }
> +
> +        self.push_internal(s.as_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
> 
> 

BR, Jarkko