When I enable UBSAN and compile this driver with clang I get the
following build error:

drivers/gpu/drm/i915/intel_engine_cs.o: In function 
drivers/gpu/drm/i915/intel_engine_cs.c:411: undefined reference to 

from what I can figure out, the compiler can't optimize
execlists_num_ports() sufficiently enough at compile time to figure out
that the 'execlists->port_mask = 1' assignment one line above the
BUILD_BUG_ON_NOT_POWER_OF_2 check will make execlists_num_ports() return
2. Most likely that's because UBSAN is going to check the load inside
execlists_num_ports() and that check isn't omitted so the optimizer
can't optimize away the whole function.

So let's just change this check to cause a build error when the maximum
number of ports isn't a power of two. It looks like this is similar to
what's being checked here so this might work well enough.

Cc: Masahiro Yamada <yamada.masah...@socionext.com>
Cc: Michal Marek <michal.l...@markovi.net>
Cc: Andrey Ryabinin <aryabi...@virtuozzo.com>
Signed-off-by: Stephen Boyd <swb...@chromium.org>
 drivers/gpu/drm/i915/intel_engine_cs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c 
index 217ed3ee1cab..bdf75628ed83 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -463,7 +463,7 @@ static void intel_engine_init_execlist(struct 
intel_engine_cs *engine)
        struct intel_engine_execlists * const execlists = &engine->execlists;
        execlists->port_mask = 1;
-       BUILD_BUG_ON_NOT_POWER_OF_2(execlists_num_ports(execlists));
        GEM_BUG_ON(execlists_num_ports(execlists) > EXECLIST_MAX_PORTS);
        execlists->queue_priority = INT_MIN;
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