[PATCH v5 2/5] gpu: nova-core: export Rust symbols for nova-drm

[Alexandre Courbot]

Export nova-core's Rust symbols so nova-drm can resolve references to it
when loaded as a module.

This is done by generating declarations and EXPORT_SYMBOL_RUST_GPL()
calls for Rust symbols referenced by nova-drm, and compiling them into
the module as `nova_core_exports.o`.

`nova_core_exports.o` declares every Rust symbol as `extern int`.
Running `gendwarfksyms` on it would compute CRCs from those placeholder
types instead of the real Rust ones, so make MODVERSIONS use this shim
only for the export list, and derive CRCs from `nova_core.o`.

This patch is intended to be a workaround until the build system
supports Rust cross-crate dependencies natively.

Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
---
 drivers/gpu/Makefile                      | 40 ++++++++++++++++++++++++++++++-
 drivers/gpu/nova-core/.gitignore          |  1 +
 drivers/gpu/nova-core/nova_core_exports.c | 15 ++++++++++++
 3 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/Makefile b/drivers/gpu/Makefile
index 45e0941324fb..67d51b7f3f55 100644
--- a/drivers/gpu/Makefile
+++ b/drivers/gpu/Makefile
@@ -14,7 +14,45 @@ obj-$(CONFIG_TRACE_GPU_MEM)		+= trace/
 # system supports cross-crate dependencies natively.
 
 obj-$(CONFIG_NOVA_CORE) += nova-core.o
-nova-core-y := nova-core/nova_core.o
+nova-core-y := nova-core/nova_core.o nova-core/nova_core_exports.o
 
 obj-$(CONFIG_DRM_NOVA) += nova-drm.o
 nova-drm-y := drm/nova/nova.o
+
+# Export Rust symbols from nova-core only if nova-drm actually references them.
+nova-core-export-deps := $(if $(CONFIG_DRM_NOVA),$(obj)/drm/nova/nova.o)
+
+rust_needed_exports = \
+	{ $(if $(strip $(2)),$(NM) -u $(2);,) echo "__DEFINED_RUST_SYMBOLS__"; \
+	  $(NM) -p --defined-only $(1); } | \
+	awk -v fmt='$(3)' ' \
+		/^__DEFINED_RUST_SYMBOLS__$$/ { defs = 1; next } \
+		!defs { if ($$NF ~ /^_R/) needed[$$NF] = 1; next } \
+		defs && $$2 ~ /(T|R|D|B)/ && $$3 ~ /^_R/ && \
+			$$3 !~ /_(init|cleanup)_module$$/ && \
+			$$3 !~ /__(pfx|cfi|odr_asan)/ && \
+			$$3 in needed { printf fmt, $$3 } \
+	'
+
+quiet_cmd_exports = EXPORTS $@
+      cmd_exports = \
+	$(call rust_needed_exports,$<,$(nova-core-export-deps),EXPORT_SYMBOL_RUST_GPL(%s);\n) > $@
+
+$(obj)/nova-core/exports_nova_core_generated.h: $(obj)/nova-core/nova_core.o $(nova-core-export-deps) FORCE
+	$(call if_changed,exports)
+
+targets += nova-core/exports_nova_core_generated.h
+
+$(obj)/nova-core/nova_core_exports.o: $(obj)/nova-core/exports_nova_core_generated.h
+CFLAGS_nova-core/nova_core_exports.o := -I $(objtree)/$(obj)/nova-core
+
+ifdef CONFIG_MODVERSIONS
+# The C export shim declares Rust symbols as `extern int`, so reuse its export
+# list but generate symbol CRCs from the Rust object instead of the shim's DWARF.
+$(obj)/nova-core/nova_core_exports.o: private cmd_gensymtypes_c = \
+	$(call getexportsymbols,\1) | \
+	$(objtree)/scripts/gendwarfksyms/gendwarfksyms \
+		$(if $(KBUILD_GENDWARFKSYMS_STABLE), --stable) \
+		$(if $(KBUILD_SYMTYPES), --symtypes $(@:.o=.symtypes),) \
+		$(obj)/nova-core/nova_core.o
+endif
diff --git a/drivers/gpu/nova-core/.gitignore b/drivers/gpu/nova-core/.gitignore
new file mode 100644
index 000000000000..7cc8318c76b1
--- /dev/null
+++ b/drivers/gpu/nova-core/.gitignore
@@ -0,0 +1 @@
+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..6e80ca9792ee
--- /dev/null
+++ b/drivers/gpu/nova-core/nova_core_exports.c
@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+
+/*
+ * 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>
+
+#define EXPORT_SYMBOL_RUST_GPL(sym) extern int sym; EXPORT_SYMBOL_GPL(sym)
+
+#include "exports_nova_core_generated.h"

-- 
2.54.0