linux-kernel - Re: [PATCH v5 3/5] rust: firmware: add `module

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Message-ID: <Z8j0otfkVtnMXIRQ@pollux>
Date: Thu, 6 Mar 2025 02:04:34 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: airlied@...il.com, simona@...ll.ch, corbet@....net,
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Subject: Re: [PATCH v5 3/5] rust: firmware: add `module_firmware!` macro

On Thu, Mar 06, 2025 at 12:31:14AM +0000, Benno Lossin wrote:
> On Tue Mar 4, 2025 at 6:34 PM CET, Danilo Krummrich wrote:
> 
> > +#[macro_export]
> > +macro_rules! module_firmware {
> > +    ($($builder:tt)*) => {
> 
> This should probably be `$builder:expr` instead.

That doesn't work, the compiler then complains, since it's not an expression:

193  |         static __MODULE_FIRMWARE: [u8; $builder::create(__module_name()).build_length()] =
     |                                                ^^ expected one of `.`, `?`, `]`, or an operator

`ty` doesn't work either, since then the compiler expects the caller to add the
const generic, which we want the macro to figure out instead.

> 
> > +
> > +        #[cfg(not(MODULE))]
> > +        const fn __module_name() -> &'static kernel::str::CStr {
> > +            <LocalModule as kernel::ModuleMetadata>::NAME
> 
> Please either use `::kernel::` or `$crate::` instead of `kernel::`.

Good catch, thanks.

> 
> Hmm, I am not 100% comfortable with the `LocalModule` way of accessing
> the current module for some reason, no idea if there is a rational
> argument behind that, but it just doesn't sit right with me.
> 
> Essentially you're doing this for convenience, right? So you don't want
> to have to repeat the name of the module type every time?

No, it's really that I can't know the type name here, please see the previous
patch commit message that introduces `LocalModule` for explanation.

> 
> > +        }
> > +
> > +        #[cfg(MODULE)]
> > +        const fn __module_name() -> &'static kernel::str::CStr {
> > +            kernel::c_str!("")
> 
> Ditto.
> 
> > +        }
> 
> Are these two functions used outside of the `static` below? If no, then
> you can just move them into the static? You can also probably use a
> `const` instead of a function, that way you only have 4 lines instead
> of 8.

Is this what you're proposing?

	#[macro_export]
	macro_rules! module_firmware {
	    ($($builder:tt)*) => {
	        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();
	    };
	}