drm/i915: Mark the end of intel_ring_begin() and check in intel_ring_advance()

It is required that the caller declare the exact number of dwords they
wish to write into the ring. This is required for two reasons, we need
to allocate sufficient space for the entire command packet and we need
to be sure that the contents are completely written to avoid executing
stale data. The current interface requires for any bug to be caught in
review, the reader has to carefully count the number of
intel_ring_emit() between intel_ring_begin() and intel_ring_advance().
If we record the end of the packet of each intel_ring_begin() we can
also have CI check for us.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170206170502.30944-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.h

@@ -28,9 +28,18 @@
 #ifdef CONFIG_DRM_I915_DEBUG_GEM
 #define GEM_BUG_ON(expr) BUG_ON(expr)
 #define GEM_WARN_ON(expr) WARN_ON(expr)
+
+#define GEM_BUG_ONLY(expr) expr
+#define GEM_BUG_ONLY_DECLARE(var) var
+#define GEM_BUG_ONLY_ON(expr) GEM_BUG_ON(expr)
+
 #else
 #define GEM_BUG_ON(expr) BUILD_BUG_ON_INVALID(expr)
 #define GEM_WARN_ON(expr) (BUILD_BUG_ON_INVALID(expr), 0)
+
+#define GEM_BUG_ONLY(expr) do { } while (0)
+#define GEM_BUG_ONLY_DECLARE(var)
+#define GEM_BUG_ONLY_ON(expr)
 #endif
 
 #define I915_NUM_ENGINES 5

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -2238,6 +2238,7 @@ int intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
 
 	ring->space -= bytes;
 	GEM_BUG_ON(ring->space < 0);
+	GEM_BUG_ONLY(ring->advance = ring->tail + bytes);
 	return 0;
 }
 

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -144,6 +144,8 @@ struct intel_ring {
 
 	u32 head;
 	u32 tail;
+	GEM_BUG_ONLY_DECLARE(u32 advance);
+
 	int space;
 	int size;
 	int effective_size;
@@ -516,6 +518,7 @@ static inline void intel_ring_advance(struct intel_ring *ring)
 	 * reserved for the command packet (i.e. the value passed to
 	 * intel_ring_begin()).
 	 */
+	GEM_BUG_ONLY_ON(ring->tail != ring->advance);
 }
 
 static inline u32 intel_ring_offset(struct intel_ring *ring, void *addr)