drm/i915: Assert that the request->tail is always qword aligned

The hardware requires that the tail pointer only advance in qword units,
so assert that the value we write is aligned to qwords, and similarly
enforce this restriction onto the request->tail.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170217163833.731-1-chris@chris-wilson.co.uk
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -784,6 +784,7 @@ static void i9xx_submit_request(struct drm_i915_gem_request *request)
 
 	i915_gem_request_submit(request);
 
+	GEM_BUG_ON(!IS_ALIGNED(request->tail, 8));
 	I915_WRITE_TAIL(request->engine, request->tail);
 }
 
@@ -795,6 +796,7 @@ static void i9xx_emit_breadcrumb(struct drm_i915_gem_request *req, u32 *cs)
 	*cs++ = MI_USER_INTERRUPT;
 
 	req->tail = intel_ring_offset(req, cs);
+	GEM_BUG_ON(!IS_ALIGNED(req->tail, 8));
 }
 
 static const int i9xx_emit_breadcrumb_sz = 4;
@@ -833,6 +835,7 @@ static void gen8_render_emit_breadcrumb(struct drm_i915_gem_request *req,
 	*cs++ = MI_NOOP;
 
 	req->tail = intel_ring_offset(req, cs);
+	GEM_BUG_ON(!IS_ALIGNED(req->tail, 8));
 }
 
 static const int gen8_render_emit_breadcrumb_sz = 8;