[Intel-gfx] [RFC] drm/i915: Introduce GEM_MAYBE_BUILD_BUG_ON

[Michal Wajdeczko]

This is slightly weaker version of MAYBE_BUILD_BUG_ON that allows
us to avoid adding implicit BUG but still detect as much as possible
during the build. With this new macro we can fix the problem with
GCC 4.4.7 that wrongly triggers build break in wait_for_atomic()
when invoked with non-const parameter.

Fixes: 1d1a9774 ("drm/i915: Extend intel_wait_for_register_fw() with fast timeout")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko at intel.com>
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.h  | 9 +++++++++
 drivers/gpu/drm/i915/intel_drv.h | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.h b/drivers/gpu/drm/i915/i915_gem.h
index 5a49487..ce23cf1 100644
--- a/drivers/gpu/drm/i915/i915_gem.h
+++ b/drivers/gpu/drm/i915/i915_gem.h
@@ -42,6 +42,15 @@
 #define GEM_DEBUG_BUG_ON(expr)
 #endif
 
+#define GEM_MAYBE_BUILD_BUG_ON(expr) \
+	do { \
+		if (__builtin_constant_p((expr))) \
+			BUILD_BUG_ON(expr); \
+		else \
+			GEM_BUG_ON(expr); \
+	} while (0)
+
+
 #define I915_NUM_ENGINES 5
 
 #endif /* __I915_GEM_H__ */
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 7bc0c25..ce50ec4 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -88,7 +88,7 @@
 	int cpu, ret, timeout = (US) * 1000; \
 	u64 base; \
 	_WAIT_FOR_ATOMIC_CHECK(ATOMIC); \
-	BUILD_BUG_ON((US) > 50000); \
+	GEM_MAYBE_BUILD_BUG_ON((US) > 50000); \
 	if (!(ATOMIC)) { \
 		preempt_disable(); \
 		cpu = smp_processor_id(); \
-- 
2.7.4