[PATCH v3 3/5] gpu: nova-core: emit Rust metadata for dependent modules

[Alexandre Courbot <acourbot@nvidia.com>]

Emit nova-core's `.rmeta` crate metadata so dependent Rust modules can
resolve nova-core's types and functions via `--extern`.

The metadata is exposed as an explicit Kbuild target instead of relying
on an untracked side effect of the object build. This lets dependent
modules ask for the metadata directly, including single-target builds
where `nova-core`'s object may already be up to date.

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 | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/gpu/nova-core/Makefile b/drivers/gpu/nova-core/Makefile
index 59ac25488896..b8ccea78b2d2 100644
--- a/drivers/gpu/nova-core/Makefile
+++ b/drivers/gpu/nova-core/Makefile
@@ -24,3 +24,27 @@ $(obj)/exports_nova_core_generated.h: $(obj)/nova_core.o FORCE
 targets += exports_nova_core_generated.h
 
 $(obj)/nova_core_exports.o: $(obj)/exports_nova_core_generated.h
+
+# Dependent Rust drivers need `nova-core`'s crate metadata.
+# Expose it as an explicit target so they can request it directly.
+nova_core_rmeta := libnova_core.rmeta
+nova_core_rmeta_private := .nova_core_rmeta/$(nova_core_rmeta)
+targets += $(nova_core_rmeta) $(nova_core_rmeta_private)
+
+# Build in a private directory to avoid racing with `nova_core.o` on rustc's
+# intermediate object names during parallel builds.
+quiet_cmd_rustc_rmeta_rs = RUSTC RMETA $@
+      cmd_rustc_rmeta_rs = mkdir -p $(dir $@); \
+		$(rust_common_cmd) --emit=metadata=$@ \
+		--emit=obj=$(basename $@).o $<; \
+		rm -f $(basename $@).o
+
+$(obj)/$(nova_core_rmeta_private): private __modname := nova-core
+$(obj)/$(nova_core_rmeta_private): private part-of-builtin := $(if $(filter y,$(CONFIG_NOVA_CORE)),y)
+$(obj)/$(nova_core_rmeta_private): private part-of-module := $(if $(filter m,$(CONFIG_NOVA_CORE)),y)
+$(obj)/$(nova_core_rmeta_private): $(obj)/nova_core.rs FORCE
+	+$(call if_changed_dep,rustc_rmeta_rs)
+
+# Keep the metadata available at a stable path for consumers.
+$(obj)/$(nova_core_rmeta): $(obj)/$(nova_core_rmeta_private) FORCE
+	$(call if_changed,copy)

-- 
2.54.0