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

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Mar 05, 2025 at 10:30:31PM +0000, Benno Lossin wrote:
> On Tue Mar 4, 2025 at 6:34 PM CET, 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.
> >
> > 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().");
> 
> This will only prevent the first `prepare` call being missed, right?

Correct, unfortunately there's no way to detect subsequent ones.

> 
> > +        }
> > +
> > +        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.
> 
> If you always have to call this before `push`, why not inline it there?

You can push() multiple times to compose the firmware path string (which is the
whole purpose :).

Example from nova-core:

	pub(crate) struct ModInfoBuilder<const N: usize>(firmware::ModInfoBuilder<N>);
	
	impl<const N: usize> ModInfoBuilder<N> {
	    const fn make_entry_file(self, chipset: &str, fw: &str) -> Self {
	        let version = "535.113.01";
	
	        ModInfoBuilder(
	            self.0
	                .prepare()
	                .push("nvidia/")
	                .push(chipset)
	                .push("/gsp/")
	                .push(fw)
	                .push("-")
	                .push(version)
	                .push(".bin"),
	        )
	    }
	
	    const fn make_entry_chipset(self, chipset: &str) -> Self {
	        self.make_entry_file(chipset, "booter_load")
	            .make_entry_file(chipset, "booter_unload")
	            .make_entry_file(chipset, "bootloader")
	            .make_entry_file(chipset, "gsp")
	    }
	
	    pub(crate) const fn create(
	        module_name: &'static kernel::str::CStr,
	    ) -> firmware::ModInfoBuilder<N> {
	        let mut this = Self(firmware::ModInfoBuilder::new(module_name));
	        let mut i = 0;
	
	        while i < gpu::Chipset::NAMES.len() {
	            this = this.make_entry_chipset(gpu::Chipset::NAMES[i]);
	            i += 1;
	        }
	
	        this.0
	    }
	}