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>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2347,7 +2347,7 @@ static const char *power_domain_str(enum intel_display_power_domain domain)
 	case POWER_DOMAIN_INIT:
 		return "INIT";
 	default:
-		WARN_ON(1);
+		MISSING_CASE(domain);
 		return "?";
 	}
 }

drivers/gpu/drm/i915/i915_drv.h

@@ -58,7 +58,19 @@
 #define DRIVER_DATE		"20141205"
 
 #undef WARN_ON
-#define WARN_ON(x)		WARN(x, "WARN_ON(" #x ")")
+/* Many gcc seem to no see through this and fall over :( */
+#if 0
+#define WARN_ON(x) ({ \
+	bool __i915_warn_cond = (x); \
+	if (__builtin_constant_p(__i915_warn_cond)) \
+		BUILD_BUG_ON(__i915_warn_cond); \
+	WARN(__i915_warn_cond, "WARN_ON(" #x ")"); })
+#else
+#define WARN_ON(x) WARN((x), "WARN_ON(" #x ")")
+#endif
+
+#define MISSING_CASE(x) WARN(1, "Missing switch case (%lu) in %s\n", \
+			     (long) (x), __func__);
 
 enum pipe {
 	INVALID_PIPE = -1,

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) {

drivers/gpu/drm/i915/intel_display.c

@@ -4847,7 +4847,7 @@ static void cherryview_set_cdclk(struct drm_device *dev, int cdclk)
 		cmd = 0;
 		break;
 	default:
-		WARN_ON(1);
+		MISSING_CASE(cdclk);
 		return;
 	}
 
@@ -8224,7 +8224,7 @@ static void i9xx_update_cursor(struct drm_crtc *crtc, u32 base)
 				cntl |= CURSOR_MODE_256_ARGB_AX;
 				break;
 			default:
-				WARN_ON(1);
+				MISSING_CASE(intel_crtc->cursor_width);
 				return;
 		}
 		cntl |= pipe << 28; /* Connect to correct pipe */

drivers/gpu/drm/i915/intel_uncore.c

@@ -1202,7 +1202,7 @@ void intel_uncore_init(struct drm_device *dev)
 
 	switch (INTEL_INFO(dev)->gen) {
 	default:
-		WARN_ON(1);
+		MISSING_CASE(INTEL_INFO(dev)->gen);
 		return;
 	case 9:
 		ASSIGN_WRITE_MMIO_VFUNCS(gen9);
@@ -1300,7 +1300,7 @@ int i915_reg_read_ioctl(struct drm_device *dev,
 		reg->val = I915_READ8(reg->offset);
 		break;
 	default:
-		WARN_ON(1);
+		MISSING_CASE(entry->size);
 		ret = -EINVAL;
 		goto out;
 	}