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

[Alexandre Courbot <acourbot@nvidia.com>]

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 \
+	}'
+
+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"

-- 
2.54.0