drm/i915: Use BUILD_BUG if possible in the i915 WARN_ON

Faster feedback to errors is always better. This is inspired by the
addition to WARN_ONs to mask/enable helpers for registers to make sure
callers have the arguments ordered correctly: Pretty much always the
arguments are static.

We use WARN_ON(1) a lot in default switch statements though where we
should always handle all cases. So add a new macro specifically for
that.

The idea to use __builtin_constant_p is from Chris Wilson.

v2: Use the ({}) gcc-ism to avoid the static inline, suggested by
Dave. My first attempt used __cond as the temp var, which is the same
used by BUILD_BUG_ON, but with inverted sense. Hilarity ensued, so
sprinkle i915 into the name.

Also use a temporary variable to only evaluate the condition once,
suggested by Damien.

v3: It's crazy but apparently 32bit gcc can't compile out the
BUILD_BUG_ON in a lot of cases and just falls over. I have no idea
why, but until clue grows just disable this nifty idea on 32bit
builds. Reported by 0-day builder.

v4: Got it all wrong, apparently its the gcc version. We need 4.9+.
Now reported by Imre.

v5: Chris suggested to add the case to MISSING_CASE for speedier
debug.

v6: Even some gcc 4.9 versions don't see through the maze, so give up
for now. Keep the skeleton and MISSING_CASE stuff though.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index ac03a38..ce4e46c 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -132,7 +132,7 @@ static gen6_gtt_pte_t snb_pte_encode(dma_addr_t addr,
 		pte |= GEN6_PTE_UNCACHED;
 		break;
 	default:
-		WARN_ON(1);
+		MISSING_CASE(level);
 	}
 
 	return pte;
@@ -156,7 +156,7 @@ static gen6_gtt_pte_t ivb_pte_encode(dma_addr_t addr,
 		pte |= GEN6_PTE_UNCACHED;
 		break;
 	default:
-		WARN_ON(1);
+		MISSING_CASE(level);
 	}
 
 	return pte;
@@ -1146,7 +1146,7 @@ int i915_ppgtt_init_hw(struct drm_device *dev)
 	else if (INTEL_INFO(dev)->gen >= 8)
 		gen8_ppgtt_enable(dev);
 	else
-		WARN_ON(1);
+		MISSING_CASE(INTEL_INFO(dev)->gen);
 
 	if (ppgtt) {
 		for_each_ring(ring, dev_priv, i) {