Re: [PATCH 4/7] gpu: nova-core: export Rust symbols for dependent modules

[Joel Fernandes <joelagnelf@nvidia.com>]

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
One comment below:

On 4/30/2026 10:55 AM, Alexandre Courbot wrote:
> Export `nova-core`'s Rust symbols so that other loadable modules,
> particularly `nova-drm`, can resolve references to `nova-core` at
> runtime.
> 
> This is done by generating declarations and `EXPORT_SYMBOL_GPL()` calls
> for Rust global text symbols using `nm` and compiling them into the
> module as `nova_core_exports.o`.
> 
> This is a workaround until the build system supports Rust cross-crate
> dependencies natively.
> 
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  drivers/gpu/nova-core/Makefile            | 22 +++++++++++++++++++++-
>  drivers/gpu/nova-core/nova_core_exports.c | 11 +++++++++++
>  2 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/nova-core/Makefile b/drivers/gpu/nova-core/Makefile
> index 1f794baadc86..f9aaf19f2477 100644
> --- a/drivers/gpu/nova-core/Makefile
> +++ b/drivers/gpu/nova-core/Makefile
> @@ -2,4 +2,24 @@
>  
>  obj-$(CONFIG_NOVA_CORE) += nova-core.o
>  
> -nova-core-y := nova_core.o
> +nova-core-y := nova_core.o nova_core_exports.o
> +
> +# Export Rust symbols so dependent modules can use them at runtime.
> +#
> +# This is a workaround until the build system supports Rust cross-module
> +# dependencies natively.
> +# Kbuild uses Rust v0 mangling, whose symbols start with "_R".
> +rust_exports = \
> +	$(NM) -p --defined-only $(1) | \
> +	awk '$$2 == "T" && $$3 ~ /^_R/ { \
> +		printf "extern void %s(void); EXPORT_SYMBOL_GPL(%s);\n", $$3, $$3 \
> +	}'

I am curious (in a fun way) how this (Or Miguel's later approach) will work with
generics. I don't think we have such use cases but suppose a module has a
function foo<T> which is expected to be called externally outside the module.
Suppose the module itself does not call foo internally.

How does the rust compiler know that this function can be called externally if
it resolves to no callers within the translation unit?

Further, as a result, how does the above extern get emitted then if foo never
ended up in the object?

Probably Miguel's future infrastructure will address this? If so, probably the
limitation of the above approach should be called out in the code comments and
commit message.

thanks,

--
Joel Fernandes

> +
> +define filechk_nova_core_exports
> +	$(call rust_exports,$(obj)/nova_core.o)
> +endef
> +
> +$(obj)/exports_nova_core_generated.h: $(obj)/nova_core.o FORCE
> +	$(call filechk,nova_core_exports)
> +
> +$(obj)/nova_core_exports.o: $(obj)/exports_nova_core_generated.h
> diff --git a/drivers/gpu/nova-core/nova_core_exports.c b/drivers/gpu/nova-core/nova_core_exports.c
> new file mode 100644
> index 000000000000..1d3f1544fe8d
> --- /dev/null
> +++ b/drivers/gpu/nova-core/nova_core_exports.c
> @@ -0,0 +1,11 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Exports Rust symbols from the `nova_core` crate for use by dependent modules.
> + *
> + * This is a workaround until the build system supports Rust cross-module
> + * dependencies natively.
> + */
> +
> +#include <linux/export.h>
> +
> +#include "exports_nova_core_generated.h"
> 


I
-- 
Joel Fernandes