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Inside an FPU compilation unit DC_FP_START() and DC_FP_END() are defined as BUILD_BUG(), so using them there fails the build. That was done on purpose by commita574f53ed5("drm/amd/display: Permit DC_FP_START/END only in non-FP compilation units"). DC_RUN_WITH_PREEMPTION_ENABLED() was added later by commit3539437f35("drm/amd/display: Move FPU Guards From DML To DC - Part 1") and defined as a plain pass-through in that same branch instead. A wrap placed inside an FPU compilation unit therefore compiles cleanly, reads as correct during review, and does nothing at all. This is not hypothetical. While chasing a "scheduling while atomic" splat in dc_create_plane_state() on PREEMPT_RT, an attempt to place the guard further up the call chain, in dml21_add_phantom_plane() in dc/dml2_0/dml21/dml21_utils.c, had no effect for exactly this reason: dc/dml2_0/Makefile applies CC_FLAGS_FPU to every object under that directory, and the top level Makefile adds -D_LINUX_FPU_COMPILATION_UNIT to CC_FLAGS_FPU. Define the macro as BUILD_BUG() there as well, so that the mistake is a compile error rather than a guard that silently does nothing. The code argument is kept in the expansion so the BUILD_BUG() failure is not accompanied by set-but-unused diagnostics for variables assigned inside it. No current user is affected. dc/core/dc_stream.c and dc/resource/dcn32/dcn32_resource.c are outside the dml directories, and dc/dml2_0/dml2_wrapper.c and dc/dml2_0/dml21/dml21_wrapper.c are built without the FPU flags because dc/dml2_0/Makefile replaces their CFLAGS with CC_FLAGS_NO_FPU and removes CC_FLAGS_FPU. Link: https://lore.kernel.org/all/1ead313022bc62dce1f42af9f855727eb9074443.camel@web.de/ Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com> Reviewed-by: Tom Chung <chiahsuan.chung@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>