RING_START
I915_WRITE(RING_START(base), 0);
I915_READ(RING_START(engine->mmio_base)));
CTX_REG(regs, CTX_RING_BUFFER_START, RING_START(base), 0);
#define I915_READ_START(engine) I915_READ(RING_START((engine)->mmio_base))
#define I915_WRITE_START(engine, val) I915_WRITE(RING_START((engine)->mmio_base), val)