drm/amd/display: make DC_RUN_WITH_PREEMPTION_ENABLED misuse a build error

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
commit a574f53ed52e ("drm/amd/display: Permit DC_FP_START/END only in non-FP compilation units").

DC_RUN_WITH_PREEMPTION_ENABLED() was added later by
commit 3539437f354b ("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>

0 files changed, 0 insertions, 0 deletions